Document:

<PAGE>

                                                                Exhibit 10.21(a)

UPON RECORDING, BE RETURNED TO:

Teachers Insurance and Annuity Association
   of America
c/o Beverly J. Quail, Esq.
Ballard Spahr Andrews & Ingersoll, LLP
1225 17th Street, Suite 2300
Denver, Colorado 80202-5596
Tel: (303) 292-2400

--------------------------------------------------------------------------------
                                                             TIAA Appl. #AAA1280
                                                            TIAA Mtge #M-0005296

               CROSS-COLLATERALIZATION AND CROSS-DEFAULT AGREEMENT

     THIS CROSS-COLLATERALIZATION AND CROSS-DEFAULT AGREEMENT (this "Agreement")
is made as of December 23, 2002, by and between AMERIVEST PROPERTIES INC., a
Maryland corporation ("AmeriVest"), AMERIVEST SHERIDAN CENTER INC., a Colorado
corporation ("Sheridan Borrower"), AMERIVEST KELLOGG INC., a Colorado
corporation ("Kellogg Borrower"), and AMERIVEST ARROWHEAD INC., an Arizona
corporation ("Arrowhead Borrower")(collectively, the "AmeriVest Affiliate
Borrowers"), each having an address at c/o AmeriVest Properties, Inc., 1780 S.
Bellaire Street, Suite 515, Denver, Colorado 80222, and TEACHERS INSURANCE AND
ANNUITY ASSOCIATION OF AMERICA, a New York corporation, its successors and
assigns (hereinafter called "Lender"), having an address at 730 Third Avenue,
New York, New York 10017.

                                    RECITALS

     A. Pursuant to that Loan Application and Commitment Agreement dated May 24,
2002 (the "Commitment") by and between Lender and AmeriVest, Lender agreed to
make three separate loans, one to each of the AmeriVest Affiliate Borrowers, in
the aggregate amount of $29,700,000.00 (collectively, the "Loan") including (i)
a loan in the amount of $10,100,000 to Sheridan Borrower to be secured by
property known as Sheridan Center in Denver, Colorado, as more particularly
described on Exhibit A to this Agreement and as set forth in that Deed of Trust,
Assignment of Leases and Rents, Security Agreement and Fixture Filing Statement
by Borrower in favor of Lender as recorded in the official records of Denver
County, Colorado contemporaneously herewith; (ii) a loan in the amount of
$9,900,000 to Kellogg Borrower to be secured by property known as the Kellogg
Building in Littleton, Colorado, as more particularly described on Exhibit B to
this Agreement and as set forth in that Deed of Trust, Assignment of Leases and
Rents, Security Agreement and Fixture Filing Statement by Borrower in favor of
Lender as recorded in the official records of Arapahoe County, Colorado
contemporaneously herewith; and (iii) a loan in the amount of $9,700,000 to
Arrowhead Borrower to be secured by

<PAGE>

property known as Arrowhead Fountains, in Peoria, Arizona, as more particularly
described in Exhibit C to this Agreement and as set forth in that Deed of Trust,
Assignment of Leases and Rents, Security Agreement and Fixture Filing Statement
in favor of Lender as recorded in the official records of Maricopa County,
Arizona contemporaneously herewith. The loans described in (i)-(iii) of this
Section are collectively referred to as the "Related Loans", and the deeds of
trust described in (i) - (iii) of this Section are collectively referred to as
the "Related Deeds of Trust." The property secured by the Loans is collectively
referred to as the "Property" and each portion of the Property encumbered by
each Related Deed of trust is referred to as a "Related Property."

     B. The AmeriVest Affiliate Borrowers acknowledge that Lender is unwilling
to make the Related Loans unless the AmeriVest Affiliate Borrowers agree to the
terms of this Agreement.

                                    AGREEMENT

     NOW, THEREFORE, in consideration of the foregoing recitals, to induce
Lender to make the Loan, and other good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, Borrower agrees with Lender,
and Lender agrees with Borrower, as follows:

     1. DEFINED TERMS. Capitalized terms used and not specifically defined in
this Agreement are defined in Exhibit B of each Related Deed of Trust. In
addition, the following definitions apply to this Agreement:

          A. "Agreement" is defined in the introductory paragraph.

          B. "Closing" means the date that Lender funds the amount of the Loan.

          C. "Commitment" is defined in Recital A.

          D. "Debt Service Coverage Ratio" means the numerical result of
dividing net operating income from the Related Property by the annual sum of
regularly scheduled monthly installments of principal and interest on the
relevant Related Loan, as calculated by Lender based on Borrower's financial
statements.

          E. "Loan" is defined in Recital A.

          F. "Property" is defined in Recital A.

          G. "Related Deeds of Trust" is defined in Recital A.

          H. "Related Loans" is defined in Recital A.

          I. "Related Loan Documents" means the Related Deeds of Trust and all
other loan documents executed in connection with the Related Loan.

          J. "Related Property" is defined in Recital A.

                                       2

<PAGE>

          K. "Release" is defined in Section 4.

          L. "Release Conditions" is defined in Section 4.

          M. "Release Date" is defined in Section 4.

          N. "Release Parcel" is defined in Section 4.

          O. "Substitution Property" is defined in Section 4.

     2. CROSS-DEFAULT. The Related Loans are hereby cross-defaulted with one
another and the AmeriVest Affiliate Borrowers hereby consent to and agree that
the occurrence of an Event of Default as defined in, and pursuant to any of the
Related Loan Documents, which is not cured within applicable grace or curative
periods set forth therein, shall constitute an immediate Event of Default
(without need of notice or the expiration of any additional cure period other
than as specified in such Related Loan Document) under each and every other of
the Related Loan Documents. Upon the occurrence of an Event of Default under any
of the Related Loan Documents, and subject to Section 5 hereof, Lender shall
have the right, in its sole and absolute discretion, to exercise and perfect any
and all rights in and under any of the Related Loan Documents with regard to any
or all of the other Related Properties, including, but not limited to, an
acceleration of one or all of the notes evidencing the Related Loans, the sale
of one or all of the Related Properties in accordance with the terms of the
respective Related Loan Document, and the disposition of any defeasance
collateral or any additional collateral in accordance with the respective
Related Loan Document. No notice, except as may be required by the respective
Related Loan Documents, shall be required to be given to any AmeriVest Affiliate
Borrower in connection with Lender's exercise of any and all of its rights after
an Event of Default has occurred.

     3. CROSS-COLLATERALIZATION. Borrower hereby agrees that, as security for
payment of the Loan, each of the Related Properties, including any Substitution
Property and future defeasance collateral or additional collateral, shall
constitute collateral for all of the Related Loans. Each AmeriVest Affiliate
Borrower agrees that the collateral described in each of the Related Deeds of
Trust shall secure: (1) the payment of the indebtedness evidenced by the Note
with Interest at the rates set forth in the Related Loan Documents, together
with all renewals, modifications, consolidations and extensions of the Note, all
additional advances or fundings made by Lender, and any other amounts required
to be paid by Borrower under any of the Related Loan Documents, and (2) the full
performance by Borrower of all of the terms, covenants and obligations set forth
in any of the Related Loan Documents (collectively, the "Obligations").

     4. RELEASE. Notwithstanding anything to the contrary in the Related Loan
Documents, on or after January 1, 2004, at Borrower's request, Lender will
release no more than one of the Related Properties (the "Release Parcel") from
the lien of the Related Deed of Trust pertaining to such Related Property and
from the terms of this Agreement ("Release"), provided that all of the
conditions stated below ("Release Conditions") are met to Lender's satisfaction:

          A. the Release is solely for the purpose of a transfer of the Release
Parcel from the relevant AmeriVest Affiliate Borrower to an unaffiliated bona
fide purchaser;

                                       3

<PAGE>

          B. not less than 90 days prior to the date of the Release (the
"Release Date"), the relevant AmeriVest Affiliate Borrower delivers to Lender a
notice setting forth: (i) the proposed Release Date; (ii) the name of the
proposed transferee; and (iii) any other information reasonably necessary for
Lender to analyze the terms of the Release, including a copy of the contract of
sale and information reasonably necessary for Lender to analyze the terms of the
proposed Substitute Property, as hereinafter defined;

          C. there is no default under the Related Loan Documents on either the
notice date or the Release Date;

          D. the relevant AmeriVest Affiliate Borrower delivers to Lender
evidence satisfactory to Lender that such AmeriVest Affiliate Borrower has
complied with any requirements of the applicable Property Documents or the
Leases applicable to the Release, the Release does not violate any of the
provisions of the applicable Property Documents or the Leases and, to the extent
necessary to comply with the applicable Property Documents or the Leases, the
transferee has assumed all of the relevant AmeriVest Affiliate Borrower's
obligations relating to the Release Parcel under the applicable Property
Documents by an instrument reasonably satisfactory to Lender;

          E. the relevant AmeriVest Affiliate Borrower delivers to Lender an
endorsement to Lender's title insurance policy satisfactory to the Lender that
(i) extends the effective date of the policy to the effective date of the
Release; (ii) confirms that there is no change in the priority of the lien of
Lender's Related Deeds of Trust on the balance of the Related Properties or in
the amount of coverage; (iii) consents to the Release; (iv) waives any defense
resulting from the Release; (v) to the extent of the value of the Release
parcel, waives any right of subrogation; and (vi) confirms that the Release
Parcel and the balance of the Related Properties constitute separate tax lots;

          F. the relevant AmeriVest Affiliate Borrower pays all out-of-pocket
expenses relating to the Release, including Lender's reasonable attorneys' fees;

          G. the relevant AmeriVest Affiliate Borrower delivers to Lender copies
of the approved, final executed documents evidencing the transfer of the Release
Parcel;

          H. the relevant AmeriVest Affiliate Borrower delivers to Lender any
other information, approvals and documents reasonably required by Lender
relating to the Release; and

          I. the remaining Related Properties each have occupancy of at least
93%;

          J. the relevant AmeriVest Affiliate Borrower provides to Lender, as
security for the Loan, the replacement and substitution of one or more new
properties for the Release Parcel, to become part of the Property (the
"Substitution Property"). The substitution and the Substitution Property must be
satisfactory to Lender, in Lender's sole and absolute discretion, provided that
Lender's approval of the substitution shall not be unreasonably withheld if the
Substitution Property is of comparable quality to the Release Parcel and if the
relevant AmeriVest Affiliate Borrower demonstrates to Lender's reasonable
satisfaction that the Substitution Property has a fair market value of not less
than the fair market value of the Release

                                       4

<PAGE>

Parcel immediately prior to the Release. In any event, the following conditions
must be met before a Substitution Property will be approved and a Release
granted:

               (i) the substitution is subject to Lender's internal approval
process and is on terms and conditions reasonably acceptable to Lender;

               (ii) the Property, when including the Substitution Property
rather than the Release Parcel, has a maximum loan-to-value ratio of 75% and has
a minimum Debt Service Coverage Ratio of 1.50;

               (iii) the Substitution Property has a Debt Service Coverage Ratio
that is equal to or greater than the Release Parcel, with a Lease expiration
profile reasonably acceptable to Lender;

               (iv) the substitution will not, as determined by Lender in
Lender's reasonable discretion, negatively impact the Loan with regard to
geographic diversity, credit risk, leasing pro formas, tenant quality, lease
expiration risk, the ability to cross collateralize and cross default the
instrument encumbering the Substitution Property with the balance of the
Property, and other similar factors;

               (v) The relevant AmeriVest Affiliate Borrower delivers to Lender
such other information and documentation as Lender may reasonably require in
order to evaluate the substitution or the Substitution Property; and

               (vi) the Substitution Property has a value that is not less than
90% of the value of the Release Parcel. If such value is at least 90% but is
less than 100% of the value of the Release Parcel at the time of the Release,
the relevant AmeriVest Affiliate Borrower shall pay to Lender the differential
between the loan to value ratio allocable to the Release Parcel and the loan to
value ratio allocable to the Substitution Property, and such payment shall be a
prepayment to be applied in accordance with the applicable Related Note and
shall be subject to the Prepayment Premium set forth in the applicable Related
Note.

          K. In connection with providing the Substitution Property, the
relevant AmeriVest Affiliate Borrower shall, at its sole cost and expense,
execute and deliver to Lender loan documents, including an additional deed of
trust or mortgage, an assignment of leases, a UCC-1 financing statement, and any
other document, including title policy, that Lender may require, on forms
consistent with and modeled upon the Related Loan Documents and in content
acceptable to Lender in Lender's reasonable discretion, necessary in order to
add the Substitution Property as collateral for the Loan. The relevant AmeriVest
Affiliate Borrower shall also provide for the Substitution Property all items
required for the Property by the Commitment.

          L. The relevant AmeriVest Affiliate Borrower shall pay all
out-of-pocket costs and expenses incurred by Lender in connection with the
proposed substitution and Release, including but not limited to reasonable
attorneys fees, title insurance premiums, and the cost of engineering,
environmental and appraisal reports. In addition, the AmeriVest Affiliate
Borrowers shall pay to Lender a processing fee in the amount of $20,000.00.

                                       5

<PAGE>

     5. EVENTS OF DEFAULT. Each of the following events shall constitute an
"Event of Default" under this Agreement:

          A. Subject to the grace and cure periods set forth in Paragraph
14.1(j) of the Deed of Trust for non-monetary defaults, a default or breach by
any AmeriVest Affiliate Borrower of any provision of this Agreement.

          B. A default or breach by any AmeriVest Affiliate Borrower under any
of the Related Loan Documents beyond any notice, grace or cure period set forth
in that Related Loan Document; or

          C. Any event or condition defined as an "Event of Default" under any
Related Loan Document.

     6. REMEDIES. Upon the occurrence of an Event of Default, Lender, in its
sole discretion, may, but shall not be obligated to, exercise any or all of its
remedies under the Related Loan Documents and applicable law. Lender, in its
sole discretion, may, but shall not be obligated to, exercise any or all of the
following remedies:

          A. declare immediately due and payable any Related Loan whether or not
the Lender exercises its right to declare immediately due and payable the
Related Loan corresponding to a particular Related Deed of Trust or Related Loan
Document under which the Event of Default may have occurred; and

          B. exercise any or all of its rights and remedies under this
Agreement, any Related Loan Document, or applicable law.

          Lender may exercise its remedies in one or more proceedings, whether
contemporaneous or consecutive or a combination of both, to be determined by
Lender in its sole discretion. Lender may enforce its rights against any one or
more Related Property or portions of Related Properties, in such order and
manner as it may elect in its sole discretion. The enforcement of any one
Related Deed of Trust or other Related Loan Document shall not constitute an
election of remedies, and shall not limit or preclude the enforcement of any
other Related Deed of Trust or other Related Loan Document, through one or more
additional proceedings. Lender may bring any action or proceeding, including but
not limited to judicial or non-judicial foreclosure proceedings, without regard
to the fact that one or more other proceedings may have been commenced elsewhere
with respect to the same Related Property or Related Properties or any portion
of them. Each of the AmeriVest Affiliate Borrowers waives any rights it may
have, whether at law or in equity, to require Lender to enforce or exercise any
of its rights or remedies under this Agreement or under any Related Loan
Document in any particular manner or order or in any particular state or county.

     7. APPLICATION OF PROCEEDS. Notwithstanding anything to the contrary set
forth herein, proceeds of the enforcement or foreclosure of any Related Deed of
Trust shall be applied first to the repayment of the "Indebtedness" as defined
in that Related Deed of Trust. Any funds remaining after such application shall
be applied to the payment of the remaining Obligations in such order as Lender
may determine in its sole discretion.

                                       6

<PAGE>

     8. EXERCISE OF LENDER'S RIGHTS. Each of the AmeriVest Affiliate Borrowers
agrees that Lender may, without incurring responsibility to such AmeriVest
Affiliate Borrower, and without impairing or releasing the Obligations of such
AmeriVest Affiliate Borrower, upon or without any terms or conditions and in
whole or in part, exercise or refrain from exercising any rights against any of
the Related Properties.

     9. WAIVER OF PRESENTMENT, MARSHALLING, ETC. In addition to and not in lieu
of the waivers set forth in the Related Loan Documents:

          A. With respect to its obligations under this Agreement, each of the
AmeriVest Affiliate Borrowers waives presentment, demand, notice of dishonor,
protest, notice of acceleration, notice of intent to demand or accelerate
payment or maturity, presentment for payment, notice of nonpayment, grace, and
diligence in collecting such obligations, except to the extent Lender is
expressly required to provide notice to any of the AmeriVest Affiliate Borrowers
pursuant to the Related Loan Documents.

          B. Notwithstanding the existence of any other security interests in
any Property held by Lender or by any other party, Lender shall have the right
to determine in its discretion the order in which any or all of the Related
Properties or portions of any of the Related Properties shall be subjected to
the remedies provided in this Agreement and the Related Loan Documents or
applicable law. Lender shall have the right to determine in its discretion the
order in which any or all portions of the Obligations are satisfied from the
proceeds realized upon the exercise of such remedies. Each AmeriVest Affiliate
Borrower and any party who now or in the future acquires a security interest in
any of the Related Properties and who has actual or constructive notice of this
Agreement waives any and all right to require the marshalling of assets or to
require that any of the Related Properties or portions of any of the Related
Properties be sold in the inverse order of alienation or in parcels or as an
entirety in connection with the exercise of any such remedies.

     10. NOTICES. All notices to an AmeriVest Affiliate Borrower under this
Agreement shall be in writing and shall be given in the manner provided in the
Related Deeds of Trust. All notices to Lender under this Agreement shall be in
writing and shall be given in the manner provided in the Related Deeds of Trust.

     11. CAPTIONS, CROSS REFERENCES AND EXHIBITS. The captions assigned to
provisions of this Agreement are for convenience only and shall be disregarded
in construing this Agreement. Any reference in this Agreement to a "Section," a
"Subsection," or an "Exhibit" shall, unless otherwise explicitly provided, be
construed as referring to a section of this Agreement, to a subsection of the
section of this Agreement in which the reference appears or to an Exhibit
attached to this Agreement. All Exhibits referred to in this Agreement are
hereby incorporated by reference.

     12. NUMBER AND GENDER. Use of the singular in this Agreement includes the
plural, use of the plural includes the singular, and use of one gender includes
all other genders, as the context may require.

                                       7

<PAGE>

     13. STATUTES AND REGULATIONS. Any reference in this Agreement to a statute
or regulation shall include all amendments to and successors to such statute or
regulation, whether adopted before or after the date of this Agreement.

     14. NO PARTNERSHIP. This Agreement is not intended to, and shall not,
create a partnership or joint venture among the parties, and no party to this
Agreement shall have the power or authority to bind any other party except as
explicitly provided in this Agreement.

     15. RECORDING OR DOCUMENTARY STAMP TAXES. Upon recordation of the Related
Deeds of Trust constituting part of the Related Loan Documents, all necessary
mortgage recording, intangible, or documentary stamp taxes will be duly paid by
the AmeriVest Affiliate Borrowers. This Agreement is being given as additional
collateral to secure the Obligations of the AmeriVest Affiliate Borrower under
the Related Loan Documents.

     16. NO ORAL AGREEMENTS. The Related Loan Documents represent the final
agreement between the parties and may not be contradicted by evidence of prior,
contemporaneous, or subsequent oral agreements of the parties. There are no
unwritten oral agreements between the parties.

     17. SEVERABILITY. The invalidity or unenforceability of any provision of
this Agreement shall not affect the validity of any other provision, and all
other provisions shall remain in full force and effect.

     18. ENTIRE AGREEMENT. This Agreement, together with the Related Loan
Documents, contains the entire agreement among the parties as to the rights
granted and the obligations assumed in this Agreement.

     19. WAIVER; NO REMEDY EXCLUSIVE. Any forbearance by a party to this
Agreement in exercising any right or remedy given under this Agreement or
existing at law or in equity shall not constitute a waiver of or preclude the
exercise of that or any other right or remedy. Unless otherwise explicitly
provided, no remedy under this Agreement is intended to be exclusive of any
other available remedy, but each remedy shall be cumulative and shall be in
addition to other remedies given under this Agreement or existing at law or in
equity.

     20. THIRD PARTY BENEFICIARIES. Neither any creditor of any party to this
Agreement, nor any other person, is intended to be a third party beneficiary of
this Agreement.

     21. COURSE OF DEALING. No course of dealing among the parties to this
Agreement shall operate as a waiver of any rights of any party under this
Agreement.

     22. NO PARTY DEEMED DRAFTER. No party shall be deemed the drafter of this
Agreement, and this Agreement shall not be construed against either party as the
drafter of the Agreement.

     23. CONTROLLING LAW. Except as provided to the contrary below, this
Agreement shall be governed by and construed in accordance with the internal
laws of the State of Colorado applicable to contracts made and to be performed
in such state (without regard to principles of conflicts of law applicable under
Colorado law) and applicable laws of the United

                                       8

<PAGE>

States of America; provided, however, that with respect to the provisions hereof
which relate to title or the creation, perfection, priority, enforcement or
foreclosure of liens on and security interests in or assignments of any of the
Related Property, or as otherwise required by the laws of the state in which
such Related Property is located (the "Property Jurisdiction"), this Agreement
shall be governed by the laws of the Property Jurisdiction; it being understood
that, to the fullest extent permitted by the laws of the Property Jurisdiction,
the laws of the State of Colorado shall govern the validity and enforceability
of this Agreement in all instances where the law of the Property Jurisdiction is
not specifically made the law applicable to this Agreement.

     24. WAIVER OF JURY TRIAL. EACH AMERIVEST AFFILIATE BORROWER HEREBY WAIVES
ANY RIGHT IT MAY HAVE TO A TRIAL BY JURY ON ANY CLAIM, COUNTERCLAIM, SETOFF,
DEMAND, ACTION OR CAUSE OF ACTION (A) ARISING OUT OF OR IN ANY WAY RELATED TO
THIS AGREEMENT OR THE RELATED LOANS, OR (B) IN ANY WAY CONNECTED WITH OR
PERTAINING OR RELATED TO OR INCIDENTAL TO ANY DEALINGS OF LENDER AND/OR ANY
AMERIVEST AFFILIATE BORROWER WITH RESPECT TO THE RELATED LOAN DOCUMENTS OR IN
CONNECTION WITH THIS AGREEMENT OR THE EXERCISE OF ANY PARTY'S RIGHTS AND
REMEDIES UNDER THIS AGREEMENT OR OTHERWISE, OR THE CONDUCT OR THE RELATIONSHIP
OF THE PARTIES HERETO, IN ALL OF THE FOREGOING CASES WHETHER NOW EXISTING OR
HEREAFTER ARISING AND WHETHER SOUNDING IN CONTRACT, TORT OR OTHERWISE. EACH
AMERIVEST AFFILIATE BORROWER AGREES THAT LENDER MAY FILE A COPY OF THIS
AGREEMENT WITH ANY COURT AS WRITTEN EVIDENCE OF THE KNOWING, VOLUNTARY, AND
BARGAINED AGREEMENT OF BORROWER TO IRREVOCABLY WAIVE ITS RIGHT TO TRIAL BY JURY
AS AN INDUCEMENT OF LENDER TO MAKE THE LOAN TO BORROWER.

                  [Remainder of Page Intentionally Left Blank]

                                       9

<PAGE>

     IN WITNESS WHEREOF, each of the AmeriVest Affiliate Borrowers and Lender
have caused this Agreement to be properly executed as of the date set forth
above.

                                        BORROWER:

                                        AMERIVEST PROPERTIES INC., a Maryland
                                        corporation

                                           By: John B. Greenman
                                               ---------------------------------
                                           Name: John B. Greenman
                                           Title: Vice President

                                        AMERIVEST SHERIDAN CENTER INC., a
                                        Colorado corporation

                                           By: John B. Greenman
                                               ---------------------------------
                                           Name: John B. Greenman
                                           Title: Vice President

                                        AMERIVEST KELLOGG INC., a Colorado
                                        corporation

                                           By: John B. Greenman
                                               ---------------------------------
                                           Name: John B. Greenman
                                           Title: Vice President

                                        AMERIVEST ARROWHEAD INC., an Arizona
                                        corporation

                                           By: John B. Greenman
                                               ---------------------------------
                                           Name: John B. Greenman
                                           Title: Vice President

                                        LENDER:

                                        TEACHERS INSURANCE AND ANNUITY
                                        ASSOCIATION OF AMERICA, INC., a New York
                                        corporation

                                           By: Faye J. Friedman
                                               ---------------------------------
                                           Name: Faye J. Friedman
                                           Title: Director

                                       10

<PAGE>

                                 ACKNOWLEDGMENTS

STATE OF COLORADO          )

                           )        SS.

COUNTY OF DENVER           )

     The foregoing instrument was acknowledged before me on this 19th day of
December, 2002, by John B. Greenman as Vice President of AmeriVest Properties
Inc., a Maryland corporation.

     WITNESS my hand and official seal.

     My commission expires April 9, 2005

(SEAL)

                                        Jean M. Gonzales
                                        ----------------------------------------

                                        Notary Public

<PAGE>

                                 ACKNOWLEDGMENTS

STATE OF COLORADO          )

                           )        SS.

COUNTY OF DENVER           )

     The foregoing instrument was acknowledged before me on this 19th day of
December, 2002, by John B. Greenman as Vice President of AmeriVest Sheridan
Center Inc., a Colorado corporation.

     WITNESS my hand and official seal.

     My commission expires April 9, 2005

(SEAL)

                                        Jean M. Gonzales
                                        ----------------------------------------

                                        Notary Public

<PAGE>

                                 ACKNOWLEDGMENTS

STATE OF COLORADO          )

                           )        SS.

COUNTY OF DENVER           )

     The foregoing instrument was acknowledged before me on this 19th day of
December, 2002, by John B. Greenman as Vice President of AmeriVest Kellogg Inc.,
a Colorado corporation.

     WITNESS my hand and official seal.

     My commission expires April 9, 2005

(SEAL)

                                        Jean M. Gonzales
                                        ----------------------------------------

                                        Notary Public

<PAGE>

                                 ACKNOWLEDGMENTS

STATE OF COLORADO          )

                           )        SS.

COUNTY OF DENVER           )

     The foregoing instrument was acknowledged before me on this 19th day of
December, 2002, by John B. Greenman as Vice President of AmeriVest Arrowhead
Inc., an Arizona corporation.

     WITNESS my hand and official seal.

     My commission expires April 9, 2005

(SEAL)

                                        Jean M. Gonzales
                                        ----------------------------------------

                                        Notary Public

<PAGE>

                                 ACKNOWLEDGMENTS

STATE OF NEW YORK          )

                           )        SS.

COUNTY OF NEW YORK         )

     The foregoing instrument was acknowledged before me on this 19th day of
December, 2002, by Faye J. Friedman as Director of Teachers Insurance and
Annuity Association of America, Inc., a New York corporation.

     WITNESS my hand and official seal.

     My commission expires
                           -------------------------------------

(SEAL)

                                        Jennifer Pittello
                                        ----------------------------------------

                                        Notary Public<PAGE>

                                                                Exhibit 10.21(b)

                                                        TIAA Appl. #AAA1280
                                                        TIAA Mtge. # M - 0005296

                                 PROMISSORY NOTE

$9,700,000.00                                           Denver, Colorado
                                                        Dated: December 23, 2002

          FOR VALUE RECEIVED, AMERIVEST ARROWHEAD INC., an Arizona corporation
("Borrower") having its principal place of business at 1780 S. Bellaire Street,
Suite 515, Denver, Colorado 80222, promises to pay to TEACHERS INSURANCE AND
ANNUITY ASSOCIATION OF AMERICA ("Lender"), a New York corporation, or order, at
Lender's offices at 730 Third Avenue, New York, New York 10017 or at such other
place as Lender designates in writing, the principal sum of NINE MILLION, SEVEN
HUNDRED THOUSAND AND NO/100s DOLLARS ($9,700,000.00) (the principal sum or so
much of the principal sum as may be advanced and outstanding from time to time,
the "Principal"), in lawful money of the United States of America, with interest
on the Principal from the date of this Promissory Note (this "Note") through and
including January 1, 2013 (the "Maturity Date") at the fixed rate of seven and
four tenths percent (7.40%) per annum (the "Fixed Interest Rate").

          This Note is secured by, among other things, the Deed of Trust,
Assignment of Leases and Rents, Security Agreement, and Fixture Filing Statement
(the "Deed of Trust") dated the date of this Note made by Borrower for the
benefit of Lender as security for the Loan. All capitalized terms not expressly
defined in this Note will have the definitions set forth in the Deed of Trust.

     Section 1. Payments of Principal and Fixed Interest.

          a. Borrower will make monthly installment payments ("Debt Service
Payments") as follows:

               (i) On January 1, 2003, a payment of accrued interest on the
     Principal at the Fixed Interest Rate; and

               (ii) On February 1, 2003 and on the first day of each succeeding
     calendar month through and including January 1, 2013, payments in the
     amount of Seventy-One Thousand Fifty-Two Dollars and Fifty Cents
     ($71,052.50), each of which will be applied first to interest on the
     Principal at the Fixed Interest Rate and then to the Principal.

          b. On the Maturity Date, Borrower will pay the Principal in full
together with accrued interest at the Fixed Interest Rate and all other amounts
due under the Loan Documents.

     Section 2. Prepayment Provisions.

          a. The following definitions apply:

          "Discount Rate" means the yield on a U.S. Treasury issue selected by
Lender, as published in the Wall Street Journal, two weeks prior to prepayment,
having a maturity date

                                       1

<PAGE>

corresponding (or most closely corresponding, if not identical) to the Maturity
Date, and, if applicable, a coupon rate corresponding (or most closely
corresponding, if not identical) to the Fixed Interest Rate.

          "Default Discount Rate" means the Discount Rate less 300 basis points.

          "Discounted Value" means the Discounted Value of a Note Payment based
on the following formula:

               NP
          (1 + R/12)n = Discounted Value

          NP = Amount of Note Payment

          R  = Discount Rate or Default Discount Rate as the case may be.

          n  = The number of months between the date of prepayment and the
               scheduled date of the Note Payment being discounted rounded to
               the nearest integer.

          "Note Payments" means (i) the scheduled Debt Service Payments for the
period from the date of prepayment through the Maturity Date and (ii) the
scheduled repayment of Principal, if any, on the Maturity Date.

          "Prepayment Date Principal" means the Principal on the date of
prepayment.

          b. This Note may not be prepaid in full or in part before January 1,
2008. Commencing on January 1, 2008, provided there is no Event of Default,
Borrower may prepay this Note in full, but not in part, on the first day of any
calendar month, upon 60 days prior notice to Lender and upon payment in full of
the Debt which will include a payment (the "Prepayment Premium") equal to the
greater of (i) an amount equal to the product of 1% times the Prepayment Date
Principal and (ii) the amount by which the sum of the Discounted Values of Note
Payments, calculated at the Discount Rate, exceeds the Prepayment Date
Principal. Provided there is no Event of Default, this Note may be prepaid in
full without payment of the Prepayment Premium during the last 90 days of the
Term. This Note may not be prepaid without simultaneous prepayment in full of
any other notes secured by the Loan Documents. Notwithstanding the foregoing,
this Note may be prepaid in part in accordance with Section 4 of the
Cross-Collateralization and Cross-Default Agreement of even date herewith under
the circumstances set forth therein.

          c. After an Event of Default or upon any prepayment not permitted by
the Loan Documents, any tender of payment of the amount necessary to satisfy all
or any part of the Debt, any decree of foreclosure, any statement of the amount
due at the time of foreclosure (including foreclosure by power of sale) and any
tender of payment during any redemption period after foreclosure, will include
an amount (the "Evasion Premium") equal to the greater of (i) an amount equal to
the product of 1% plus 300 basis points times the Prepayment Date Principal, and
(ii) the amount by which the sum of the Discounted Values of the Note Payments,
calculated at the Default Discount Rate, exceeds the Prepayment Date Principal.

                                       2

<PAGE>

          d. Borrower acknowledges that:

               (i) a prepayment will cause damage to Lender;

               (ii) the Evasion Premium is intended to compensate Lender for the
     loss of its investment and the expense incurred and time and effort
     associated with making the Loan, which will not be fully repaid if the Loan
     is prepaid;

               (iii) it will be extremely difficult and impractical to ascertain
     the extent of Lender's damages caused by a prepayment after an Event of
     Default or any other prepayment not permitted by the Loan Documents; and

               (iv) the Evasion Premium represents Lender and Borrower's
     reasonable estimate of Lender's damages for the prepayment and is not a
     penalty.

     Section 3. Events of Default:

          a. It is an "Event of Default" under this Note:

               (i) if Borrower fails to pay any amount due, as and when
     required, under this Note or any other Loan Document and the failure
     continues for a period of 5 days; or

               (ii) if an Event of Default occurs under any other Loan Document.

          b. If an Event of Default occurs, Lender may declare all or any
portion of the Debt immediately due and payable ("Acceleration") and exercise
any of the other Remedies.

     Section 4. Default Rate. Interest on the Principal will accrue at the
Default Interest Rate from the date an Event of Default occurs.

     Section 5. Late Charges.

          a. If Borrower fails to pay any Debt Service Payment when due and the
failure continues for a period of 5 days or more or fails to pay any amount due
under the Loan Documents on the Maturity Date, Borrower agrees to pay to Lender
an amount (a "Late Charge") equal to five cents ($.05) for each one dollar
($1.00) of the delinquent payment.

          b. Borrower acknowledges that:

               (i) a delinquent payment will cause damage to Lender;

               (ii) the Late Charge is intended to compensate Lender for loss of
     use of the delinquent payment and the expense incurred and time and effort
     associated with recovering the delinquent payment;

               (iii) it will be extremely difficult and impractical to ascertain
     the extent of Lender's damages caused by the delinquency; and

                                       3

<PAGE>

               (iv) the Late Charge represents Lender and Borrower's reasonable
     estimate of Lender's damages from the delinquency and is not a penalty.

     Section 6. Limitation of Liability. This Note is subject to the limitations
on liability set forth in the Article of the Deed of Trust entitled "Limitation
of Liability."

     Section 7. WAIVERS. IN ADDITION TO THE WAIVERS SET FORTH IN THE ARTICLE OF
THE DEED OF TRUST ENTITLED "WAIVERS," BORROWER WAIVES PRESENTMENT FOR PAYMENT,
DEMAND, DISHONOR AND, EXCEPT AS EXPRESSLY SET FORTH IN THE LOAN DOCUMENTS,
NOTICE OF ANY OF THE FOREGOING. BORROWER FURTHER WAIVES ANY PROTEST, LACK OF
DILIGENCE OR DELAY IN COLLECTION OF THE DEBT OR ENFORCEMENT OF THE LOAN
DOCUMENTS. BORROWER AND ALL INDORSERS, SURETIES AND GUARANTORS OF THE
OBLIGATIONS CONSENT TO ANY EXTENSIONS OF TIME, RENEWALS, WAIVERS AND
MODIFICATIONS THAT LENDER MAY GRANT WITH RESPECT TO THE OBLIGATIONS AND TO THE
RELEASE OF ANY SECURITY FOR THIS NOTE AND AGREE THAT ADDITIONAL MAKERS MAY
BECOME PARTIES TO THIS NOTE AND ADDITIONAL INDORSERS, GUARANTORS OR SURETIES MAY
BE ADDED WITHOUT NOTICE AND WITHOUT AFFECTING THE LIABILITY OF THE ORIGINAL
MAKER OR ANY ORIGINAL INDORSER, SURETY OR GUARANTOR.

     Section 8. Commercial Loan. The Loan is made for the purpose of carrying on
a business or commercial activity or acquiring real or personal property as an
investment or carrying on an investment activity and not for personal or
household purposes.

     Section 9. Usury Limitations. Borrower and Lender intend to comply with all
Laws with respect to the charging and receiving of interest. Any amounts charged
or received by Lender for the use or forbearance of the Principal to the extent
permitted by Law, will be amortized and spread throughout the Term until payment
in full so that the rate or amount of interest charged or received by Lender on
account the Principal does not exceed the Maximum Interest Rate. If any amount
charged or received under the Loan Documents that is deemed to be interest is
determined to be in excess of the amount permitted to be charged or received at
the Maximum Interest Rate, the excess will be deemed to be a prepayment of
Principal when paid, without premium, and any portion of the excess not capable
of being so applied will be refunded to Borrower. If during the Term the Maximum
Interest Rate, if any, is eliminated, then for purposes of the Loan, there will
be no Maximum Interest Rate.

     Section 10. Applicable Law. This Note is governed by and will be construed
in accordance with the Laws of the State of Colorado, without regard to conflict
of law provisions, except to the extent that enforcement of the Deed of Trust
requires application of Arizona law to this Note.

     Section 11. Time of the Essence. Time is of the essence with respect to the
payment and performance of the Obligations.

     Section 12. Cross-Default. A default under any other note now or hereafter
secured by the Loan Documents or under any loan document related to such other
note constitutes a

                                       4

<PAGE>

default under this Note and under the other Loan Documents. When the default
under the other note constitutes an Event of Default under that note or the
related loan document, an Event of Default also will exist under this Note and
the other Loan Documents.

     Section 13. Construction. Unless expressly provided otherwise in this Note,
this Note will be construed in accordance with the Exhibit attached to the Deed
of Trust entitled "Rules of Construction."

     Section 14. Deed of Trust Provisions Incorporated. To the extent not
otherwise set forth in this Note, the provisions of the Articles of the Deed of
Trust entitled "Expenses and Duty to Defend," "Waivers," "Notices," and
"Miscellaneous" are applicable to this Note and deemed incorporated by reference
as if set forth at length in this Note.

     Section 15. Joint and Several Liability; Successors and Assigns. If Maker
consists of more than one entity, the obligations and liabilities of each such
entity will be joint and several. Without limiting the generality of the
foregoing, all entities constituting Borrower waive any rights which they might
otherwise have under Section 13ss.50ss.02 or Section 13ss.50ss.103, Colorado
Revised Statutes (or under any corresponding future statute) by reason of any
release of fewer than all of the parties constituting Borrower. This Note binds
Borrower and successors, assigns, heirs, administrators, executors, agents and
representatives and inures to the benefit of Lender and its successors, assigns,
heirs, administrators, executors, agents and representatives.

     Section 16. Absolute Obligation. Except for the Section of this Note
entitled "Limitation of Liability," no reference in this Note to the other Loan
Documents and no other provision of this Note or of the other Loan Documents
will impair or alter the obligation of Borrower, which is absolute and
unconditional, to pay the Principal, interest at the Fixed Interest Rate and any
other amounts due and payable under this Note, as and when required.

          IN WITNESS WHEREOF, Borrower has executed and delivered this Note as
of the date first set forth above.

                                            AMERIVEST ARROWHEAD INC., an Arizona
                                            corporation

                                            By: John B. Greenman
                                                --------------------------------
                                            Name: John B. Greenman
                                            Title: Vice President

                                       5

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