Document:

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                                                                    EXHIBIT 4.10

                             PUBLIC SERVICE COMPANY
                                   OF COLORADO

                                       TO

                      U.S. BANK TRUST NATIONAL ASSOCIATION,

                                                            AS TRUSTEE

                              ---------------------

                      FORM OF SUPPLEMENTAL INDENTURE NO. 13

                          Dated as of [______________]

                          Supplemental to the Indenture
                           dated as of October 1, 1993

                              ---------------------

                  Establishing the Securities of Series No. 12,
             designated First Collateral Trust Bonds, Series No. 12

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         SUPPLEMENTAL INDENTURE NO. 13, dated as of [______________], between
PUBLIC SERVICE COMPANY OF COLORADO, a corporation duly organized and existing
under the laws of the State of Colorado (hereinafter sometimes called the
"Company"), and U.S. BANK TRUST NATIONAL ASSOCIATION (FORMERLY FIRST TRUST OF
NEW YORK, NATIONAL ASSOCIATION), a national banking association, as successor
trustee (hereinafter sometimes called the "Trustee") to Morgan Guaranty Trust
Company of New York under the Indenture, dated as of October 1, 1993
(hereinafter called the "Original Indenture"), as previously supplemented and as
further supplemented by this Supplemental Indenture No. 13. The Original
Indenture and any and all indentures and all other instruments supplemental
thereto are hereinafter sometimes collectively called the "Indenture".

                             RECITALS OF THE COMPANY

         The Original Indenture was authorized, executed and delivered by the
Company to provide for the issuance from time to time of its Securities (such
term and all other capitalized terms used herein without definition having the
meanings assigned to them in the Original Indenture), to be issued in one or
more series as contemplated therein, and to provide security for the payment of
the principal of and premium, if any, and interest, if any, on the Securities.

         The Company has heretofore executed and delivered to the Trustee the
Supplemental Indentures referred to in Schedule A hereto for the purpose of
establishing a series of bonds and appointing the successor Trustee.

         The Company desires to establish a series of Securities to be
designated "First Collateral Trust Bonds, Series No. 12", such series of
Securities to be hereinafter sometimes called "Series No. 12".

         The Company has duly authorized the execution and delivery of this
Supplemental Indenture No. 13 to establish the Securities of Series No. 12 and
has duly authorized the issuance of such Securities; and all acts necessary to
make this Supplemental Indenture No. 13 a valid agreement of the Company, and to
make the Securities of Series No. 12 valid obligations of the Company, have been
performed.

                                GRANTING CLAUSES

         NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 13 WITNESSETH, that, in
consideration of the premises and of the purchase of the Securities by the
Holders thereof, and in order to secure the payment of the principal of and
premium, if any, and interest, if any, on all Securities from time to time
Outstanding and the performance of the covenants contained therein and in the
Indenture and to declare the terms and conditions on which such Securities are
secured, the Company hereby grants, bargains, sells, releases, conveys, assigns,
transfers, mortgages, pledges, sets over and confirms to the Trustee, and grants
to the Trustee a security interest in, the following:

                              GRANTING CLAUSE FIRST

         All right, title and interest of the Company, as of the date of the
         execution and delivery of this Supplemental Indenture No. 13, in and to
         property (other than Excepted Property), real, personal and mixed and
         wherever situated, in any case used or to be used in or in connection
         with the Electric Utility Business (whether or not such use is the sole
         use of such property), including without limitation (a) all lands,
         easements, servitudes, licenses, permits, rights of way and other
         rights and interests in or relating to real property used or to be used
         in or in connection with the Electric Utility Business or relating to
         the occupancy or use of such real property, subject however, to the
         exceptions and exclusions set forth in clause (a) of Granting Clause
         First of the Original Indenture; (b) all plants,

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         generators, turbines, engines, boilers, fuel handling and
         transportation facilities, air and water pollution control and sewage
         and solid waste disposal facilities and other machinery and facilities
         for the generation of electric energy; (c) all switchyards, lines,
         towers, substations, transformers and other machinery and facilities
         for the transmission of electric energy; (d) all lines, poles,
         conduits, conductors, meters, regulators and other machinery and
         facilities for the distribution of electric energy; (e) all buildings,
         offices, warehouses and other structures used or to be used in or in
         connection with the Electric Utility Business; (f) all pipes, cables,
         insulators, ducts, tools, computers and other data processing and/or
         storage equipment and other equipment, apparatus and facilities used or
         to be used in or in connection with the Electric Utility Business; (g)
         any or all of the foregoing properties in the process of construction;
         and (h) all other property, of whatever kind and nature, ancillary to
         or otherwise used or to be used in conjunction with any or all of the
         foregoing or otherwise, directly or indirectly, in furtherance of the
         Electric Utility Business;

                             GRANTING CLAUSE SECOND

         Subject to the applicable exceptions permitted by Section 810(c),
         Section 1303 and Section 1305 of the Original Indenture, all property
         (other than Excepted Property) of the kind and nature described in
         Granting Clause First which may be hereafter acquired by the Company,
         it being the intention of the Company that all such property acquired
         by the Company after the date of the execution and delivery of this
         Supplemental Indenture No. 13 shall be as fully embraced within and
         subjected to the Lien hereof as if such property were owned by the
         Company as of the date of the execution and delivery of this
         Supplemental Indenture No. 13;

                             GRANTING CLAUSE FOURTH

         All other property of whatever kind and nature subjected or required to
         be subjected to the Lien of the Indenture by any of the provisions
         thereof;

                                EXCEPTED PROPERTY

         Expressly excepting and excluding, however, from the Lien and operation
         of the Indenture all Excepted Property of the Company, whether now
         owned or hereafter acquired;

         TO HAVE AND TO HOLD all such property, real, personal and mixed, unto
the Trustee, its successors in trust and their assigns forever;

         SUBJECT, HOWEVER, to (a) Liens existing at the date of the execution
and delivery of the Original Indenture (including, but not limited to, the Lien
of the PSCO 1939 Mortgage), (b) as to property acquired by the Company after the
date of the execution and delivery of the Original Indenture, Liens existing or
placed thereon at the time of the acquisition thereof (including, but not
limited to, the Lien of any Class A Mortgage and purchase money Liens), (c)
Retained Interests and (d) any other Permitted Liens, it being understood that,
with respect to any property which was at the date of execution and delivery of
the Original Indenture or thereafter became or hereafter becomes subject to the
Lien of any Class A Mortgage, the Lien of the Indenture shall at all times be
junior, subject and subordinate to the Lien of such Class A Mortgage;

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         IN TRUST, NEVERTHELESS, for the equal and proportionate benefit and
security of the Holders from time to time of all Outstanding Securities without
any priority of any such Security over any other such Security;

         PROVIDED, HOWEVER, that the right, title and interest of the Trustee in
and to the Mortgaged Property shall cease, terminate and become void in
accordance with, and subject to the conditions set forth in, Article Nine of the
Original Indenture, and if, thereafter, the principal of and premium, if any,
and interest, if any, on the Securities shall have been paid to the Holders
thereof, or shall have been paid to the Company pursuant to Section 603 of the
Original Indenture, then and in that case the Indenture shall terminate, and the
Trustee shall execute and deliver to the Company such instruments as the Company
shall require to evidence such termination; otherwise the Indenture, and the
estate and rights thereby granted shall be and remain in full force and effect;
and

         THE PARTIES HEREBY FURTHER COVENANT AND AGREE as follows:

                                   ARTICLE ONE

                           SECURITIES OF SERIES NO. 12

         There are hereby established the Securities of Series No. 12, which
shall have the terms and characteristics set forth below (the lettered
subdivisions set forth below corresponding to the lettered subdivisions of
Section 301 of the Original Indenture):

         (a) the title of the Securities of such series shall be "First
         Collateral Trust Bonds, Series No. 12"; provided, however, that, at any
         time after the PSCO 1939 Mortgage shall have been satisfied and
         discharged, the Company shall have the right, without any consent or
         other action by the Holders of such Securities, to change such title in
         such manner as shall be deemed by the Company to be appropriate to
         reflect such satisfaction and discharge, such change to be evidenced in
         an Officer's Certificate;

         (b) the Securities of Series No. 12 shall be initially authenticated
         and delivered in exchange for, and upon presentation for surrender of,
         Securities designated First Collateral Trust Bonds, Series No. 11 (the
         "Original Bonds"), in an aggregate principal amount not to exceed
         $250,000,000 (less the principal amount of Original Bonds not presented
         for exchange);

         (c) interest on the Securities of Series No. 12 shall be payable to the
         Persons in whose names such Securities are registered at the close of
         business on the Regular Record Date for such interest, except as
         otherwise expressly provided in the form of such Securities attached as
         Exhibit A hereto;

         (d) the principal of the Securities of Series No. 12 shall be payable
         on March 1, 2013, the Stated Maturity.

         (e) the Securities of Series No. 12 shall bear interest at a rate of
         4.875% per annum; interest shall accrue on the Securities of Series No.
         12 from [______________], or the most recent date to which interest has
         been paid or duly provided for; the Interest Payment Dates for such
         Securities shall be March 1 and September 1 in each year, commencing
         [______________], and the Regular Record Dates with respect to the
         Interest Payment Dates for such Securities shall be February 15 and
         August 15 in each year, respectively (whether or not a Business Day);

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         (f) the Corporate Trust Office of U.S. Bank Trust National Association
         in New York, New York shall be the place at which (i) the principal of,
         premium, if any, and interest, if any, on the Securities of Series No.
         12 shall be payable, (ii) registration of transfer of such Securities
         may be effected, (iii) exchanges of such Securities may be effected and
         (iv) notices and demands to or upon the Company in respect of such
         Securities and the Indenture may be served; and U.S. Bank Trust
         National Association shall be the Security Registrar for such
         Securities; provided, however, that the Company reserves the right to
         change, by one or more Officer's Certificates, any such place or the
         Security Registrar; and provided, further, that the Company reserves
         the right to designate, by one or more Officer's Certificates, its
         principal office in Denver, Colorado as any such place or itself as the
         Security Registrar;

         (g) the Securities of Series No. 12 shall be redeemable at the option
         of the Company at any time prior to maturity, in whole or in part, at a
         redemption price equal to the greater of (i) 100% of the principal
         amount thereof to be redeemed, or (ii) the sum of the present values of
         the remaining scheduled payments of principal and interest on such
         Securities to be redeemed, discounted to the Redemption Date on a
         semi-annual basis (assuming a 360-day year consisting of twelve 30-day
         months) at the Treasury Yield plus 20 basis points, plus in each case,
         accrued and unpaid interest to the Redemption Date. For purposes
         hereof, the following defined terms shall have the meaning ascribed to
         them.

                  "Treasury Yield" means, for any Redemption Date, the rate per
         annum equal to the semi-annual equivalent yield to maturity of the
         Comparable Treasury Issue, assuming a price for the Comparable Treasury
         Issue (expressed as a percentage of its principal amount) equal to the
         Comparable Treasury Price for the Redemption Date.

                  "Comparable Treasury Issue" means the United States Treasury
         security selected by an Independent Investment Banker as having a
         maturity comparable to the remaining term of the Securities of Series
         No. 12 that would be utilized, at the time of selection and in
         accordance with customary financial practice, in pricing new issues of
         corporate debt securities of comparable maturity to the remaining term
         of the Securities of Series No. 12.

                  "Comparable Treasury Price" means, for any Redemption Date,
         (i) the average of the bid and asked prices for the Comparable Treasury
         Issue (expressed in each case as a percentage of its principal amount)
         on the third Business Day preceding the Redemption Date, as set forth
         in the daily statistical release (or any successor release) published
         by the Federal Reserve Bank of New York and designated "Composite 3:30
         p.m. Quotations for U.S. Government Securities" or (ii) if that release
         (or any successor release) is not published or does not contain those
         prices on that Business Day, (A) the average of the Reference Treasury
         Dealer Quotations for the Redemption Date, after excluding the highest
         and lowest Reference Treasury Dealer Quotations for the Redemption
         Date, or (B) if the Trustee obtains fewer than four Reference Treasury
         Dealer Quotations, the average of all of the Quotations.

                  "Independent Investment Banker" means Banc One Capital
         Markets, Inc. or its successor or, if such firm or its successor is
         unwilling or unable to select the Comparable Treasury Issue, one of the
         remaining Reference Treasury Dealers appointed by the Trustee after
         consultation with the Company.

                  "Reference Treasury Dealer" means (i) each of Banc One Capital
         Markets, Inc., UBS Warburg LLC and any other Primary Treasury Dealer
         designated by, and not affiliated with, Banc One Capital Markets, Inc.,
         UBS Warburg LLC and their respective successors, provided, however,
         that if either of the foregoing or any of their designees ceases to be
         a Primary Treasury

                                       4

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         Dealer, the Company will appoint another Primary Treasury Dealer as a
         substitute and (ii) any other Primary Treasury Dealer selected by the
         Company.

                  "Primary Treasury Dealer" means any U.S. Government securities
         dealer in the United States.

                  "Reference Treasury Dealer Quotations" means, for each
         Reference Treasury Dealer and any Redemption Date, the average, as
         determined by the Independent Investment Banker, of the bid and asked
         prices for the Comparable Treasury Issue (expressed in each case as a
         percentage of its principal amount) quoted in writing to the
         Independent Investment Banker by the Reference Treasury Dealer at 5:00
         p.m. on the third Business Day preceding the Redemption Date.

         (h) not applicable;

         (i) not applicable;

         (j) not applicable;

         (k) not applicable;

         (l) not applicable;

         (m) not applicable;

         (n) not applicable;

         (o) not applicable;

         (p) not applicable;

         (q) the Securities of Series No. 12 are to be initially registered in
         the name of Cede & Co., as nominee for The Depository Trust Company
         (the "Depositary"). Such Securities shall not be transferable or
         exchangeable, nor shall any purported transfer be registered, except as
         follows:

                  (i)    such Securities may be transferred in whole, and
                         appropriate registration of transfer effected, if such
                         transfer is by such nominee to the Depositary, or by
                         the Depositary to another nominee thereof, or by any
                         nominee of the Depositary to any other nominee thereof,
                         or by the Depositary or any nominee thereof to any
                         successor securities depositary or any nominee thereof;
                         and

                  (ii)   such Securities may be exchanged for definitive
                         Securities registered in the respective names of the
                         beneficial holders thereof, and thereafter shall be
                         transferable without restriction, if:

                         (A)    the Depositary, or any successor securities
                                depositary, shall have notified the Company and
                                the Trustee that it is unwilling or unable to
                                continue to act as securities depositary with
                                respect to such Securities or the Depository has
                                ceased to be a clearing agency registered under
                                the Securities Exchange Act of 1934, as amended,
                                and the Trustee shall not have been notified by
                                the Company within ninety (90) days of the

                                       5

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                                identity of a successor securities depositary
                                with respect to such Securities;

                         (B)    the Company shall have delivered to the Trustee
                                a Company Order to the effect that such
                                Securities shall be so exchangeable on and after
                                a date specified therein; or

                         (C)    (1) an Event of Default shall have occurred and
                                be continuing, (2) the Trustee shall have given
                                notice of such Event of Default pursuant to
                                Section 1102 of the Original Indenture and (3)
                                there shall have been delivered to the Company
                                and the Trustee an Opinion of Counsel to the
                                effect that the interests of the beneficial
                                owners of such Securities in respect thereof
                                will be materially impaired unless such owners
                                become Holders of definitive Securities;

         (r) not applicable;

         (s) no service charge shall be made for the registration of transfer or
         exchange of the Securities of Series No. 12; provided, however, that
         the Company may require payment of a sum sufficient to cover any tax or
         other governmental charge payable in connection with the exchange or
         transfer;

         (t) not applicable;

         (u) (i) If the Company shall have caused the Company's indebtedness in
                 respect of any Securities of Series No. 12 to have been
                 satisfied and discharged prior to the Maturity of such
                 Securities, as provided in Section 901 of the Original
                 Indenture, the Company shall, promptly after the date of such
                 satisfaction and discharge, give a notice to each Person who
                 was a Holder of any of such Securities on such date stating
                 (A)(1) the aggregate principal amount of such Securities and
                 (2) the aggregate amount of any money (other than amounts, if
                 any, deposited in respect of accrued interest on such
                 Securities) and the aggregate principal amount of, the rate or
                 rates of interest on, and the aggregate fair market value of,
                 any Eligible Obligations deposited pursuant to Section 901 of
                 the Original Indenture with respect to such Securities and (B)
                 that the Company will provide (and the Company shall promptly
                 so provide) to such Person, or any beneficial owner of such
                 Securities holding through such Person (upon written request to
                 the Company sent to an address specified in such notice), such
                 other information as such Person or beneficial owner, as the
                 case may be, reasonably may request in order to enable it to
                 determine the federal income tax consequences to it resulting
                 from the satisfaction and discharge of the Company's
                 indebtedness in respect of such Securities. Thereafter, the
                 Company shall, within forty-five (45) days after the end of
                 each calendar year, give to each Person who at any time during
                 such calendar year was a Holder of such Securities a notice
                 containing (X) such information as may be necessary to enable
                 such Person to report its income, gain or loss for federal
                 income tax purposes with respect to such Securities or the
                 assets held on deposit in respect thereof during such calendar
                 year or the portion thereof during which such Person was a
                 Holder of such Securities, as the case may be (such information
                 to be set forth for such calendar year as a whole and for each
                 month during such year) and (Y) a statement to the effect that
                 the Company will provide (and the Company shall promptly so
                 provide) to such Person, or any beneficial owner of such
                 Securities holding through such Person (upon written request to
                 the Company sent to an address specified in such notice), such
                 other information as such Person or

                                       6

<PAGE>

                 beneficial owner, as the case may be, reasonably may request in
                 order to enable it to determine its income, gain or loss for
                 federal income tax purposes with respect to such Securities or
                 such assets for such year or portion thereof, as the case may
                 be. The obligation of the Company to provide or cause to be
                 provided information for purposes of income tax reporting by
                 any Person as described in the first two sentences of this
                 paragraph shall be deemed to have been satisfied to the extent
                 that the Company has provided or caused to be provided
                 substantially comparable information pursuant to any
                 requirements of the Internal Revenue Code of 1986, as amended
                 from time to time (the "Code") and United States Treasury
                 regulations thereunder.

                 (ii)   Notwithstanding the provisions of subparagraph (i)
                 above, the Company shall not be required to give any notice
                 specified in such subparagraph or to otherwise furnish any of
                 the information contemplated therein if the Company shall have
                 delivered to the Trustee an Opinion of Counsel to the effect
                 that the Holders of such Securities will not recognize income,
                 gain or loss for federal income tax purposes as a result of the
                 satisfaction and discharge of the Company's indebtedness in
                 respect of such Securities and such Holders will be subject to
                 federal income taxation on the same amounts and in the same
                 manner and at the same times as if such satisfaction and
                 discharge had not occurred.

                 (iii)   Anything in this clause (u) to the contrary
                 notwithstanding, the Company shall not be required to give any
                 notice specified in subparagraph (i) or to otherwise furnish
                 the information contemplated therein or to deliver any Opinion
                 of Counsel contemplated by subparagraph (ii) if the Company
                 shall have caused Securities of Series No. 12 to be deemed to
                 have been paid for purposes of the Indenture, as provided in
                 Section 901 of the Original Indenture, but shall not have
                 effected the satisfaction and discharge of its indebtedness in
                 respect of such Securities pursuant to such Section.

         (v) The Securities of Series No. 12 shall be substantially in the form
         attached hereto as Exhibit A and shall have such further terms as are
         set forth in such form.

                                   ARTICLE TWO

                            MISCELLANEOUS PROVISIONS

         This Supplemental Indenture No. 13 is a supplement to the Original
Indenture. As previously supplemented and further supplemented by this
Supplemental Indenture No. 13, the Original Indenture is in all respects
ratified, approved and confirmed, and the Original Indenture, all previous
supplements thereto and this Supplemental Indenture No. 13 shall together
constitute one and the same instrument.

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                  IN WITNESS WHEREOF, the parties hereto have caused this
Supplemental Indenture No. 13 to be duly executed as of the day and year first
above written.

                                           PUBLIC SERVICE COMPANY OF COLORADO

                                           By: ______________________________

                                               Name:
                                               Title:

STATE OF [MINNESOTA]                )
                                    ) ss.:
COUNTY OF [MINNEAPOLIS]             )

         On the [_____] day of [________, ______], before me personally came
[__________] to me known, who, being by me duly sworn, did depose and say that
he is a [Vice President] of Public Service Company of Colorado, one of the
corporations described in and which executed the foregoing instrument; and that
he signed his name thereto by authority of the Board of Directors of said
corporation.

                                           ___________________________________
                                           Name:
                                           Notary Public, State of [Minnesota]

                                           Commission Expires:

                                       8

<PAGE>

                                           U.S. BANK TRUST NATIONAL ASSOCIATION,
                                                                    Trustee

                                           By: ______________________________

                                               Name:
                                               Title:

STATE OF NEW YORK                   )
                                    ) ss.:
CITY AND COUNTY OF NEW YORK         )

         On the [_____] day of [__________, ____], before me personally came
[__________], to me known, who, being by me duly sworn, did depose and say that
he is a [Vice President] of U.S. Bank Trust National Association, the banking
association described in and which executed the foregoing instrument; and that
he signed his name thereto by authority of the Board of Directors of said
banking association.

                                           ___________________________________
                                           Name:
                                           Notary Public, State of New York

                                           Commission Expires:

                                       9

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                                                                       EXHIBIT A

                                FORM OF SECURITY

                   (See legend at the end of this Security for
                            restrictions on transfer)

                       PUBLIC SERVICE COMPANY OF COLORADO
                   First Collateral Trust Bond, Series No. 12

       Original Interest Accrual Date:       [__________________________]
                        Interest Rate:       4.875%
                      Stated Maturity:       March 1, 2013
               Interest Payment Dates:       March 1 and September 1
                 Regular Record Dates:       February 15 and August 15

                    This Security is not a Discount Security
              within the meaning of the within-mentioned Indenture

                    -----------------------------------------

Principal Amount                                           Registered No.
$                                                          CUSIP

         PUBLIC SERVICE COMPANY OF COLORADO, a corporation duly organized and
existing under the laws of the State of Colorado (herein called the "Company,"
which term includes any successor corporation under the Indenture referred to
below), for value received, hereby promises to pay to [________________________
____________________________________________________], or registered assigns,
the principal sum of [_____ __________________________________________] Dollars
on the Stated Maturity specified above, and to pay interest thereon from the
Original Interest Accrual Date specified above or from the most recent Interest
Payment Date to which interest has been paid or duly provided for, semi-annually
in arrears on the Interest Payment Dates specified above in each year,
commencing [______________], and at Maturity, at the Interest Rate per annum
specified above, until the principal hereof is paid or duly provided for. The
interest so payable, and paid or duly provided for, on any Interest Payment Date
shall, as provided in such Indenture, be paid to the Person in whose name this
Security (or one or more Predecessor Securities) is registered at the close of
business on the Regular Record Date specified above (whether or not a Business
Day) next preceding such Interest Payment Date. Notwithstanding the foregoing,
interest payable at Maturity shall be paid to the Person to whom principal shall
be paid. Except as otherwise provided in said Indenture, any such interest not
so paid or duly provided for shall forthwith cease to be payable to the Holder
on such Regular Record Date and may either be paid to the Person in whose name
this Security (or one or more Predecessor Securities) is registered at the close
of business on a Special Record Date for the payment of such Defaulted Interest
to be fixed by the Trustee, notice of which shall be given to Holders of
Securities of this series not less than 15 days prior to such Special Record
Date, or be paid in such other manner as permitted by the Indenture.

                                  Exhibit A-1

<PAGE>

         Payment of the principal of this Security and interest hereon at
Maturity shall be made upon presentation of this Security at the Corporate Trust
Office of U.S. Bank Trust National Association, in New York, New York or at such
other office or agency as may be designated for such purpose by the Company from
time to time. Payment of interest on this Security (other than interest at
Maturity) shall be made by check mailed to the address of the Person entitled
thereto as such address shall appear in the Security Register, except that if
such Person shall be a securities depositary, such payment may be made by such
other means in lieu of check as shall be agreed upon by the Company, the Trustee
and such Person. Payment of the principal of and interest on this Security, as
aforesaid, shall be made in such coin or currency of the United States of
America as at the time of payment shall be legal tender for the payment of
public and private debts.

         This Security is one of a duly authorized issue of securities of the
Company (herein called the "Securities"), issued and issuable in one or more
series under and equally secured by an Indenture, dated as of October 1, 1993
(such Indenture as originally executed and delivered and as supplemented or
amended from time to time thereafter, together with any constituent instruments
establishing the terms of particular Securities, being herein called the
"Indenture"), between the Company and U.S. Bank Trust National Association
(formerly First Trust of New York, National Association) as successor trustee
(herein called the "Trustee," which term includes any successor trustee under
the Indenture), to which Indenture and all indentures supplemental thereto
reference is hereby made for a description of the property mortgaged, pledged
and held in trust, the nature and extent of the security and the respective
rights, limitations of rights, duties and immunities of the Company, the Trustee
and the Holders of the Securities thereunder and of the terms and conditions
upon which the Securities are, and are to be, authenticated and delivered and
secured. The acceptance of this Security shall be deemed to constitute the
consent and agreement by the Holder hereof to all of the terms and provisions of
the Indenture. This Security is one of the series designated above.

         If any Interest Payment Date or the Stated Maturity shall not be a
Business Day (as hereinafter defined), payment of the amounts due on this
Security on such date may be made on the next succeeding Business Day; and, if
such payment is made or duly provided for on such Business Day, no interest
shall accrue on such amounts for the period from and after such Interest Payment
Date or Stated Maturity, as the case may be, to such Business Day.

         This Security shall be redeemable at the option of the Company at any
time prior to maturity, in whole or in part, at a redemption price equal to the
greater of (i) 100% of the principal amount hereof to be redeemed, or (ii) the
sum of the present values of the remaining scheduled payments of principal and
interest on this Security, discounted to the Redemption Date on a semi-annual
basis (assuming a 360-day year consisting of twelve 30-day months) at the
Treasury Yield plus 20 basis points, plus in each case, accrued and unpaid
interest to the Redemption Date. For purposes hereof, the following defined
terms shall have the meaning ascribed to them:

         "Treasury Yield" means, for any Redemption Date, the rate per annum
equal to the semi-annual equivalent yield to maturity of the Comparable Treasury
Issue, assuming a price for the Comparable Treasury Issue (expressed as a
percentage of its principal amount) equal to the Comparable Treasury Price for
the Redemption Date.

         "Comparable Treasury Issue" means the United States Treasury security
selected by an Independent Investment Banker as having a maturity comparable to
the remaining term of the Securities of Series No. 12 that would be utilized, at
the time of selection and in accordance with customary financial practice, in
pricing new issues of corporate debt securities of comparable maturity to the
remaining term of the Securities of Series No. 12.

                                  Exhibit A-2

<PAGE>

         "Comparable Treasury Price" means, for any Redemption Date, (i) the
average of the bid and asked prices for the Comparable Treasury Issue (expressed
in each case as a percentage of its principal amount) on the third Business Day
preceding the Redemption Date, as set forth in the daily statistical release (or
any successor release) published by the Federal Reserve Bank of New York and
designated "Composite 3:30 p.m. Quotations for U.S. Government Securities" or
(ii) if that release (or any successor release) is not published or does not
contain those prices on that Business Day, (A) the average of the Reference
Treasury Dealer Quotations for the Redemption Date, after excluding the highest
and lowest Reference Treasury Dealer Quotations for the Redemption Date, or (B)
if the Trustee obtains fewer than four Reference Treasury Dealer Quotations, the
average of all of the Quotations.

         "Independent Investment Banker" means Banc One Capital Markets, Inc. or
its successor or, if such firm or its successor is unwilling or unable to select
the Comparable Treasury Issue, one of the remaining Reference Treasury Dealers
appointed by the Trustee after consultation with the Company.

         "Reference Treasury Dealer" means (i) each of Banc One Capital Markets,
Inc., UBS Warburg LLC and any other Primary Treasury Dealer designated by, and
not affiliated with, Banc One Capital Markets, Inc., UBS Warburg LLC and their
respective successors, provided, however, that if either of the foregoing or any
of their designees ceases to be a Primary Treasury Dealer, the Company will
appoint another Primary Treasury Dealer as a substitute and (ii) any other
Primary Treasury Dealer selected by the Company.

         "Primary Treasury Dealer" means any U.S. Government securities dealer
in the United States.

         "Reference Treasury Dealer Quotations" means, for each Reference
Treasury Dealer and any Redemption Date, the average, as determined by the
Independent Investment Banker, of the bid and asked prices for the Comparable
Treasury Issue (expressed in each case as a percentage of its principal amount)
quoted in writing to the Independent Investment Banker by the Reference Treasury
Dealer at 5:00 p.m. on the third Business Day preceding the Redemption Date.

         If an Event of Default shall occur and be continuing, the principal of
this Security may be declared due and payable in the manner and with the effect
provided in the Indenture.

         The Indenture permits, with certain exceptions as therein provided, the
Trustee to enter into one or more supplemental indentures for the purpose of
adding any provisions to, or changing in any manner or eliminating any of the
provisions of, the Indenture with the consent of the Holders of not less than a
majority in aggregate principal amount of the Securities of all series then
Outstanding under the Indenture, considered as one class; provided, however,
that if there shall be Securities of more than one series Outstanding under the
Indenture and if a proposed supplemental indenture shall directly affect the
rights of the Holders of Securities of one or more, but less than all, of such
series, then the consent only of the Holders of a majority in aggregate
principal amount of the Outstanding Securities of all series so directly
affected, considered as one class, shall be required; and provided, further,
that if the Securities of any series shall have been issued in more than one
Tranche and if the proposed supplemental indenture shall directly affect the
rights of the Holders of Securities of one or more, but less than all, of such
Tranches, then the consent only of the Holders of a majority in aggregate
principal amount of the Outstanding Securities of all Tranches so directly
affected, considered as one class, shall be required; and provided, further,
that the Indenture permits the Trustee to enter into one or more supplemental
indentures for limited purposes without the consent of any Holders of
Securities. The Indenture also contains provisions permitting the Holders of a
majority in principal amount of the Securities then Outstanding, on behalf of
the Holders of all Securities, to waive compliance by the Company with certain
provisions of the Indenture and certain past defaults under the Indenture and
their consequences. Any such consent or waiver by the Holder of this Security
shall be conclusive and binding upon such Holder and upon all

                                  Exhibit A-3

<PAGE>

future Holders of this Security and of any Security issued upon the registration
of transfer hereof or in exchange therefor or in lieu hereof, whether or not
notation of such consent or waiver is made upon this Security.

         As provided in the Indenture and subject to certain limitations therein
set forth, this Security or any portion of the principal amount hereof will be
deemed to have been paid for all purposes of the Indenture and to be no longer
Outstanding thereunder, and, at the election of the Company, the Company's
entire indebtedness in respect thereof will be satisfied and discharged, if
there has been irrevocably deposited with the Trustee or any Paying Agent (other
than the Company), in trust, money in an amount which will be sufficient and/or
Eligible Obligations, the principal of and interest on which when due, without
regard to any reinvestment thereof, will provide moneys which, together with
moneys so deposited, will be sufficient, to pay when due the principal of and
interest on this Security when due.

         As provided in the Indenture and subject to certain limitations therein
set forth, the transfer of this Security is registrable in the Security
Register, upon surrender of this Security for registration of transfer at the
office of U.S. Bank Trust National Association, in New York, New York or such
other office or agency as may be designated by the Company from time to time,
duly endorsed by, or accompanied by a written instrument of transfer in form
satisfactory to the Company and the Security Registrar duly executed by, the
Holder hereof or his attorney duly authorized in writing, and thereupon one or
more new Securities of this series of authorized denominations and of like tenor
and aggregate principal amount, will be issued to the designated transferee or
transferees.

         The Securities of this series are issuable only as registered
Securities, without coupons, and in denominations of $1,000 and integral
multiples thereof. As provided in the Indenture and subject to certain
limitations therein set forth, Securities of this series are exchangeable for a
like aggregate principal amount of Securities of the same series, of any
authorized denominations, as requested by the Holder surrendering the same, and
of like tenor upon surrender of the Security or Securities to be exchanged at
the office of U.S. Bank Trust National Association, in New York, New York or
such other office or agency as may be designated by the Company from time to
time.

         No service charge shall be made for any such registration of transfer
or exchange, but the Company may require payment of a sum sufficient to cover
any tax or other governmental charge payable in connection therewith.

         Prior to due presentment of this Security for registration of transfer,
the Company, the Trustee and any agent of the Company or the Trustee may treat
the Person in whose name this Security is registered as the absolute owner
hereof for all purposes, whether or not this Security be overdue, and neither
the Company, the Trustee nor any such agent shall be affected by notice to the
contrary.

         The Indenture and the Securities shall be governed by and construed in
accordance with the laws of the State of New York.

         As used herein "Business Day" means any day, other than a Saturday or
Sunday, which is not a day on which banking institutions or trust companies in
The City of New York, New York or other city in which is located any office or
agency maintained for the payment of principal or interest on this Security, are
authorized or required by law, regulation or executive order to remain closed.
All other terms used in this Security which are defined in the Indenture shall
have the meanings assigned to them in the Indenture.

         As provided in the Indenture, no recourse shall be had for the payment
of the principal of or interest on any Securities, or any part thereof, or for
any claim based thereon or otherwise in respect

                                  Exhibit A-4

<PAGE>

thereof, or of the indebtedness represented thereby, or upon any obligation,
covenant or agreement under the Indenture, against, and no personal liability
whatsoever shall attach to, or be incurred by, any incorporator, shareholder,
officer or director, as such, past, present or future of the Company or of any
predecessor or successor corporation (either directly or through the Company or
a predecessor or successor corporation), whether by virtue of any constitutional
provision, statute or rule of law, or by the enforcement of any assessment or
penalty or otherwise; it being expressly agreed and understood that the
Indenture and all the Securities are solely corporate obligations and that any
such personal liability is hereby expressly waived and released as a condition
of, and as part of the consideration for, the execution of the Indenture and the
issuance of the Securities.

         Unless the certificate of authentication hereon has been executed by
the Trustee or an Authenticating Agent by manual signature, this Security shall
not be entitled to any benefit under the Indenture or be valid or obligatory for
any purpose.

                                  Exhibit A-5

<PAGE>

IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed
and its corporate seal to be hereunto affixed and attested.

                                      PUBLIC SERVICE COMPANY OF COLORADO

                                      By:_________________________________
                                             [Title]

Attest:_____________________________
                 [Title]

                          CERTIFICATE OF AUTHENTICATION

         This is one of the Securities of the series designated therein referred
to in the within-mentioned Indenture.

Dated:____________________________

            U.S. BANK TRUST            OR            U.S. BANK TRUST
         NATIONAL ASSOCIATION,                    NATIONAL ASSOCIATION,
              as Trustee                               as Trustee

By:_______________________________           By:________________________________
         Authorized Officer                      as Authenticating Agent

                                             By:________________________________
                                                        Authorized Officer

         UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF
THE DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION ("DTC"), TO THE COMPANY OR
ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY
CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME
AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE
TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

         THIS SECURITY MAY NOT BE TRANSFERRED OR EXCHANGED, NOR MAY ANY
PURPORTED TRANSFER BE REGISTERED, EXCEPT (i) THIS SECURITY MAY BE TRANSFERRED IN
WHOLE, AND APPROPRIATE REGISTRATION OF TRANSFER EFFECTED, IF SUCH TRANSFER IS BY
CEDE & CO., AS NOMINEE FOR THE DEPOSITORY TRUST COMPANY (THE "DEPOSITARY"), TO
THE DEPOSITARY, OR BY THE DEPOSITARY TO ANOTHER NOMINEE THEREOF, OR BY ANY
NOMINEE OF THE DEPOSITARY TO ANY OTHER NOMINEE THEREOF, OR BY THE DEPOSITARY OR
ANY NOMINEE THEREOF TO ANY SUCCESSOR SECURITIES DEPOSITARY OR ANY NOMINEE
THEREOF; AND (ii) THIS SECURITY MAY BE EXCHANGED FOR DEFINITIVE SECURITIES
REGISTERED IN THE RESPECTIVE NAMES OF THE BENEFICIAL HOLDERS HEREOF, AND
THEREAFTER SHALL BE TRANSFERABLE WITHOUT RESTRICTIONS IF: (A) THE DEPOSITARY, OR
ANY SUCCESSOR SECURITIES DEPOSITARY, SHALL HAVE NOTIFIED THE COMPANY AND THE
TRUSTEE THAT IT IS UNWILLING OR UNABLE TO CONTINUE TO ACT AS SECURITIES
DEPOSITARY WITH RESPECT TO THE SECURITIES AND THE TRUSTEE SHALL NOT HAVE BEEN
NOTIFIED BY THE COMPANY WITHIN NINETY (90) DAYS OF THE IDENTITY OF A SUCCESSOR

                                  Exhibit A-6

<PAGE>

SECURITIES DEPOSITARY WITH RESPECT TO THE SECURITIES; (B) THE COMPANY SHALL HAVE
DELIVERED TO THE TRUSTEE A COMPANY ORDER TO THE EFFECT THAT THE SECURITIES SHALL
BE SO EXCHANGEABLE ON AND AFTER A DATE SPECIFIED THEREIN; OR (C)(1) AN EVENT OF
DEFAULT SHALL HAVE OCCURRED AND BE CONTINUING, (2) THE TRUSTEE SHALL HAVE GIVEN
NOTICE OF SUCH EVENT OF DEFAULT PURSUANT TO SECTION 1102 OF THE INDENTURE AND
(3) THERE SHALL HAVE BEEN DELIVERED TO THE COMPANY AND THE TRUSTEE AN OPINION OF
COUNSEL TO THE EFFECT THAT THE INTERESTS OF THE BENEFICIAL OWNERS OF THE
SECURITIES IN RESPECT THEREOF WILL BE MATERIALLY IMPAIRED UNLESS SUCH OWNERS
BECOME HOLDERS OF DEFINITIVE SECURITIES.

                            ------------------------

                                  Exhibit A-7

<PAGE>

         FOR VALUE RECEIVED the undersigned hereby sells, assigns and transfers
unto
         _______________________________________________________________________

         [please insert social security or other identifying number of assignee]

         _______________________________________________________________________
               [please print or typewrite name and address of assignee]

         _______________________________________________________________________

the within Security of PUBLIC SERVICE COMPANY OF COLORADO and does hereby
irrevocably constitute and appoint __________________________________ ,
Attorney, to transfer said Security on the books of the within-mentioned
Company, with full power of substitution in the premises.

Dated:____________________________

                   ___________________________________________

         Notice: The signature to this assignment must correspond with the name
         as written upon the face of the Security in every particular without
         alteration or enlargement or any change whatsoever.

                                  Exhibit A-8

<PAGE>

                                                                      SCHEDULE A

                             SUPPLEMENTAL INDENTURES

<TABLE>
<CAPTION>
     DATE OF                                                           PRINCIPAL
  SUPPLEMENTAL                                      PRINCIPAL            AMOUNT
    INDENTURE             SERIES OF BONDS         AMOUNT ISSUED       OUTSTANDING
     ---------            ---------------         -------------       -----------
<S>                    <C>                        <C>                 <C>
November 1, 1993            Series No. 1          $134,500,000        $134,500,000
January 1, 1994        Series No. 2 due 2001      $102,667,000            None
                                and
                       Series No. 2 due 2024      $110,000,000        $110,000,000
September 2, 1994               None                  None                None
(Appointment of
Successor Trustee)
May 1, 1996                Series No.  3          $125,000,000        $125,000,000
November 1, 1996           Series No.  4          $250,000,000        $175,000,000
February 1, 1997           Series No.  5          $150,000,000            None
April 1, 1998              Series No.  6          $250,000,000            None
August 15, 2002            Series No.  7          $ 48,750,000        $ 48,750,000
September 1, 2002          Series No.  8          $600,000,000            None
September 15, 2002         Series No.  9          $530,000,000            None
April 1, 2003              Series No. 10          $600,000,000        $600,000,000
March 1, 2003              Series No. 11          $250,000,000        $250,000,000
May 1, 2003                Series No. 13          $350,000,000        $350,000,000
</TABLE>

                                  Schedule A-1<PAGE>
                                                                     Exhibit 4.1

                                 ANTIGENICS INC.

                               AMENDMENT NO. 1 TO
                           1999 EQUITY INCENTIVE PLAN

     Pursuant to Section 10(d) of the Antigenics Inc. 1999 Equity Incentive Plan
(the "Plan"), the Plan is hereby amended as follows:

     1.   Section 5(a) of the Plan is hereby amended and restated in its
entirety to read as follows:

          "(a) Subject to adjustment under subsection (b), Awards may be made
     under the Plan for up to 6,000,000 shares of Common Stock. If any Award in
     respect of shares of Common Stock expires or is terminated unexercised or
     is forfeited without the Participant having had the benefits of ownership
     (other than voting rights), the shares subject to such Award, to the extent
     of such expiration, termination or forfeiture, shall again be available for
     award under the Plan. Common Stock issued through the assumption or
     substitution of outstanding grants from an acquired company shall not
     reduce the shares available for Awards under the Plan. Shares issued under
     the Plan may consist in whole or in part of authorized but unissued shares
     or treasury shares."

     IN WITNESS WHEREOF, Antigenics Inc. has caused this instrument of amendment
to be executed by its duly authorized officer as of the 11th day of June, 2003.

                                        ANTIGENICS INC.

                                            /s/ Garo H. Armen, Ph.D.
                                        By:________________________________
                                           Name:  Garo H. Armen, Ph.D.
                                           Title: Chief Executive Officer

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