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                                                                  Exhibit 10.122


                      PLEDGE OF CASH COLLATERAL AGREEMENT

            THIS PLEDGE OF CASH COLLATERAL AGREEMENT (this "Agreement") dated as
of February 13, 1997 is made by CSTONE-527 MADISON, INC. , a Delaware
corporation ("Pledgor"), having an address at c/o Cornerstone Properties Inc.,
126 East 56th Street, Suite 600, New York, New York 10022, in favor of BANKERS
TRUST COMPANY, a New York banking corporation ("Lender"), having an office at
280 Park Avenue, New York, New York 10017.

                             W I T N E S S E T H:

            WHEREAS, Pledgor desires to purchase certain improved real property
known as 527 Madison Avenue, New York, New York (the "Property"); and

            WHEREAS, the Property is encumbered by a certain Amended and
Restated Consolidated First Mortgage, Spreader, Modification, Extension,
Consolidation and Security Agreement and Assignment of Rents, dated July 24,
1996, made by 527 Madison Holdings ("527") to Property Equity Corp. (the
"Mortgage") , which Mortgage secures an Amended and Restated Demand Note, dated
July 24, 1996, made by 527 in favor of Property Equity Corp. (as modified,
amended and restated from time to time, the "Note"), which Mortgage and Note
have been assigned to Lender; and

            WHEREAS, pursuant to a certain Note Modification Agreement and
Mortgage Modification Agreement of even date herewith between 527 and Lender,
the Note and the Mortgage have been modified and amended; and

            WHEREAS, a portion of the consideration for the purchase by Pledgor
of the Property is Pledgor's agreement to take title to the Property subject to
the Note and the Mortgage; and

            WHEREAS, it is an Event of Default under the Mortgage and the Note
if the Property is transferred or conveyed without the consent of the holder of
the Mortgage; and

            WHEREAS, Pledgor has requested that Lender consent to the transfer
of the Property to Pledgor without invoking Lender's right to declare a default
under the Mortgage and demand payment in full of the Note; and

            WHEREAS, Lender is unwilling to consent to the transfer of the
Property to Pledgor unless, among other things, Pledgor executes and delivers
this Pledge of Cash Collateral Agreement pursuant to which Pledgor makes a
deposit and opens an account with Lender and pledges and assigns such account to
Lender as additional security for the obligations of 527 under the Note; and

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            WHEREAS, in order to induce Lender to consent to the transfer of the
Property to Pledgor, Pledgor desires to execute this Agreement and to deliver
the same to Lender.

            NOW, THEREFORE, in consideration of the foregoing, the mutual
promises contained herein, Ten Dollars ($10.00) and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
intending to be legally bound, and in order to induce Lender to consent to the
transfer of the Property to Pledgor, Pledgor and Lender hereby agree as follows:

            SECTION 1. Deposit. Immediately following its acquisition of the
Property, Pledgor will open an account with Lender in the name of Pledgor and
made a deposit therein in the sum of THIRTY-SIX MILLION SEVEN HUNDRED FIFTY
THOUSAND DOLLARS ($36,750,000) (the "Deposit"). The Deposit shall be held in an
interest-bearing account which shall be accessible to Pledgor only upon
compliance by Pledgor with the terms and conditions contained in Section 27
hereof and, at the option of Lender, may be invested in short-term financial
instruments having maturities not later than May 13, 1997 and issued by entities
carrying a rating of no lower than A1/P1. All interest accruing on the Deposit
shall be credited by Lender to payment of interest due under the Note.

            SECTION 2. Pledge. Pledgor hereby pledges to Lender and grants to
Lender a security interest in the following collateral (collectively, the
"Pledged Collateral"):

            (a) the Deposit (including all interest accruing thereon and any
renewals, extensions, replacements and reinvestments thereof);

            (b) all certificates, instruments and book entry records, if any,
from time to time representing the Deposit, and all substitutions for or
additions to the same, and all money and property of any nature of Pledgor
hereafter delivered to or held in custody or otherwise by Lender or any of its
branches, subsidiaries or affiliates; and

            (c) to the extent not covered by clauses (a) and (b) above, all
proceeds of any and all of the foregoing (including, without limitation,
proceeds that constitute property of the types described in such clauses) and
any and all investments and reinvestments made with such proceeds.

            SECTION 3. Security for Obligations. This Agreement secures the
payment of all obligations of (i) 527 now or hereafter existing under the Note
and the Mortgage and (ii) the Pledgor now or hereafter existing under this
Agreement, and any other instruments and agreements relating thereto and
executed by 527 or Pledgor in connection therewith, together with all expenses
incurred by Lender in enforcing any rights thereunder or hereunder, including
without limitation, reasonable attorney's fees (all such obligations, whether of
Pledgor or of 527 under the Note and Mortgage, are referred to herein
collectively as the "Obligations"). Without

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limiting the generality of the foregoing, this Agreement secures the payment of
all amounts which constitute part of the Obligations and would be owed to
Lender, but are unenforceable or not allowable due to the existence of a
bankruptcy, reorganization or similar proceeding involving 527.

            SECTION 4. Delivery of Pledged Collateral.

            (a) All funds and instruments representing or evidencing the Pledged
Collateral have been delivered to and are being held by Lender. Pledgor hereby
acknowledges and agrees that Lender shall have the normal rights of a custodian
and depository of such funds and instruments, as well as the rights of a
pledgee, as set forth herein, it being intended that the assets comprising the
Deposit be considered to be in the full possession and control of Lender in its
capacity as pledgee so as to perfect a pledge of the said assets and to permit
Lender to exercise all rights and remedies of a pledgee with respect to such
assets. Pledgor and Lender hereby acknowledge and agree that Lender shall have
no liability hereunder with respect to any of its acts or omissions made in good
faith.

            (b) The instruments delivered to Lender pursuant to Section 2(a)
hereof, and any additional instruments to be delivered from and after the date
hereof are and shall be in suitable form for transfer by delivery, or are to be
accompanied by duly executed instruments of transfer or assignment in blank, all
in form and substance reasonably satisfactory to Lender. Lender shall have the
right, upon the occurrence and during the continuance of an Event of Default (as
hereinafter defined) (unless Lender shall specifically in writing agree
otherwise in lieu of declaring an Event of Default) in its discretion and
without notice to Pledgor, to transfer to or to register in the name of Lender,
or any of its nominees, any or all of the Pledged Collateral.

            (c) Except in accordance with Section 27 hereof, Pledgor shall not
be entitled to withdraw or remove any cash, certificates, instruments or other
collateral comprising the Deposit or the other Pledged Collateral, nor shall
Pledgor be entitled to sell, assign, pledge, transfer or hypothecate all or any
portion of such Pledged Collateral until the payment in full of the Obligations
and the termination of this Agreement.

            (d) It is anticipated that on May 13, 1997, the Note shall be repaid
in full and Lender shall return the Deposit to Pledgor. Upon the occurrence of
such events, the obligations of the parties hereto under this Agreement shall
terminate (except to the extent any obligations therein are expressly stated to
survive). Notwithstanding the foregoing, Lender and Pledgor acknowledge and
agree that if the Obligations are not repaid and this Agreement is not
terminated on May 13, 1997 as anticipated, the Deposit may be reinvested
pursuant to Section 10(b) hereof (or as otherwise provided herein).

            SECTION 5. Pledgor Remains Liable. Anything herein to the contrary
notwithstanding, the exercise by Lender of any of its rights hereunder shall not
release Pledgor

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from any of its duties or obligations under the contracts and agreements, if
any, included in the Pledged Collateral.

            SECTION 6. Representations and Warranties. Pledgor hereby represents
and warrants as follows:

            (a) Pledgor has full legal power and authority to execute, deliver
and perform this Agreement and such execution, delivery and performance does not
contravene any law or any contractual restriction binding on or affecting
Pledgor, and does not result in or require the creation of any lien, security
interest or other charge or encumbrance (other than pursuant hereto) upon or
with respect to any of Pledgor's properties.

            (b) Pledgor is the legal and beneficial owner of the Pledged
Collateral free and clear of any lien, security interest, option or other charge
or encumbrance except for the security interest created by this Agreement or any
other security agreement in favor of Lender or any of its Affiliates. No
effective financing statement or other document similar in effect covering all
or any part of the Pledged Collateral is on file in any recording office, except
as may have been filed in favor of Lender or any of its Affiliates.

            (c) The assignment and pledge of the Deposit and the other Pledged
Collateral and the delivery of such Pledged Collateral to Lender pursuant to
this Agreement creates a valid and perfected first priority security interest in
such portion of the Pledged Collateral, securing the payment of the Obligations.

            (d) No consent of any other person or entity and no authorization,
approval, or other action by, and no notice to or filing with, any governmental
authority or regulatory body is required (i) for the pledge by Pledgor of the
Pledged Collateral pursuant to this Agreement or for the execution, delivery or
performance of this Agreement by Pledgor, (ii) for the perfection or maintenance
of the security interest created hereby (including the first priority nature of
such security interest) or (iii) for the exercise by Lender of the rights
provided for in this Agreement or the remedies in respect of the Pledged
Collateral pursuant to this Agreement.

            (e) The place of business and chief executive office of Pledgor and
the office where Pledgor keeps its records concerning the Pledged Collateral are
located at its address specified in the first paragraph of this Agreement.

            (f) To the best knowledge of Pledgor, there are no conditions
precedent to the effectiveness of this Agreement that have not been satisfied or
waived.

            (g) Pledgor has, independently and without reliance upon Lender, and
based on such documents and information as it has deemed appropriate, made its
own analysis and decision to enter into this Agreement.

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            (h) All financial, net worth and other statements relating to
Pledgor and any of its Affiliates that have been delivered to Lender in
connection with this transaction are true, complete and correct.

            SECTION 7. Covenants and Agreements.

            (a) Pledgor shall not incur direct or contingent liabilities during
the period that this Agreement is in effect other than the indebtedness incurred
in the ordinary course of owning and operating the Property.

            (b) Upon the written request of Lender, Pledgor shall furnish to
Lender updated financial statements of Pledgor substantially in the form
heretofore delivered to Lender by Pledgor in connection with other borrowings
made by affiliates of Pledgor from Lender.

            (c) For so long as this Agreement is in effect and any Obligations
are outstanding, Pledgor shall not engage in any business or activity other than
the ownership and operation of the Property.

            SECTION 8. Further Assurances.

            (a) Pledgor agrees that at any time and from time to time, at the
expense of Pledgor, Pledgor shall promptly execute and deliver all further
instruments and documents, and take all further action, that may be necessary or
desirable, or that Lender may reasonably request, in order to perfect and
protect any security interest granted or purported to be granted hereby or to
enable Lender to exercise and enforce its rights and remedies hereunder with
respect to any Pledged Collateral.

            (b) Pledgor hereby authorizes Lender to file one or more financing
or continuation statements, and amendments thereto, relating to all or any part
of the Pledged Collateral without the signature of Pledgor where permitted by
law. A photocopy or other reproduction of this Agreement or any financing
statement covering the Pledged Collateral or any part thereof shall be
sufficient as a financing statement where permitted by law.

            SECTION 9. Place of Perfection; Records. Pledgor shall keep its
place of business, chief executive office, and the office where it keeps its
records concerning the Pledged Collateral at its address set forth in Section 20
hereof. Pledgor will hold and preserve such records and, upon reasonable, prior
notice from Lender, will permit representatives of Lender at any time during
normal business hours to inspect and make abstracts from such records.

            SECTION 10. Interest; Voting or Other Consensual Rights; Etc.

            (a) So long as this Agreement is in effect:

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            i) Any and all interest, dividends, redemption payments and other
distributions paid in respect of the Pledged Collateral or any portion thereof
shall be held in an account with Lender and shall become part of the Pledged
Collateral.

            (b) In the event the Note is not repaid when due and Lender chooses
not to draw upon the Deposit in payment of the Note, then the Deposit shall be
reinvested in an account or instrument with Lender in such manner and for such
period of time as Lender and Pledgor shall mutually agree, provided that all
necessary steps are taken to insure that Lender retains its perfected security
interest in the Pledged Collateral following such reinvestment. If Pledgor and
Lender are unable to agree on the manner in which the Deposit is to be invested,
Lender shall invest the Deposit overnight each day until an agreement is reached
as to the manner of investment.

            (c) Upon the occurrence and during the continuance of an Event of
Default (unless Lender shall otherwise have agreed in writing):

            i) All rights of Pledgor to exercise or refrain from exercising the
consensual rights which Pledgor would otherwise be entitled to exercise pursuant
to this Section 10 shall cease, and all such rights shall thereupon become
vested in Lender, which shall thereupon have the sole right to receive and hold
as Pledged Collateral such interest payments and to exercise or refrain from
exercising such consensual rights.

            ii) All payments which are received by Pledgor contrary to the
provisions of this Section shall be received in trust for the benefit of Lender,
shall be segregated from other funds of Pledgor and shall be forthwith paid over
to Lender as Pledged Collateral in the same form as so received (with any
necessary endorsement or assignment).

            SECTION 11. Events of Default.

            (a) An "Event of Default" shall occur when:

            i) Pledgor fails to maintain the Deposit in an amount equal to 105%
of the principal balance due under the Note; or

            ii) there is any failure to pay any payments due under the Note, the
Mortgage or this Agreement when the same becomes due and payable after any
applicable grace period; or

            iii) any warranty, representation or other statement made by Pledgor
under or in connection with this Agreement or any related document is false or
misleading in any material respect when made; or

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            iv) there is any failure to perform or observe any term, covenant or
agreement contained in this Agreement or the Note or the Mortgage and any such
failure shall remain unremedied for ten (10) days after written notice thereof
shall have been given to Pledgor by Lender; or

            v) Pledgor shall admit in writing its inability to pay its debts
generally, or shall make a general assignment for the benefit of creditors; or
any proceeding shall be instituted by or against Pledgor seeking to adjudicate
Pledgor as bankrupt or insolvent, or seeking liquidation, winding-up,
reorganization, arrangement, adjustment, protection, relief, or composition of
its debts under any law relating to bankruptcy, insolvency or reorganization or
relief of debtors, or seeking the entry of an order for relief or the
appointment of a receiver, trustee, or other similar official for Pledgor or for
any substantial part of its property, and, in the case of any such proceeding
instituted against it (but not instituted by Pledgor), either such proceedings
shall remain undismissed or unstayed for a period of ninety (90) days, or any of
the actions sought in such proceeding (including, without limitation, the entry
of an order for relief against, or the appointment of a receiver, trustee or
other similar official for, it or for any substantial part of Pledgor's
property) shall occur; or

            vi) any judgment for the payment of money in excess of $250,000
(other than those fully covered by insurance) shall be rendered by a court of
record against Pledgor and Pledgor shall not discharge (by bonding or otherwise)
the same or provide for its discharge in accordance with its terms, or procure a
stay of execution thereof within sixty (60) days from the date of entry thereof
and within such period of sixty (60) days, or such longer period during which
execution of such judgment shall have been stayed, appeal therefrom and cause
the execution thereof to be stayed during such appeal; or

            vii) any provision of this Agreement shall for any reason cease to
be valid and binding on Pledgor, or Pledgor shall so state in writing and such
cessation shall, in Lender's reasonable opinion, impair Lender's security
interest under this Agreement; or

            viii) if for any reason Lender at any time fails or ceases to have a
valid and perfected first priority security interest in any of the Pledged
Collateral purported to be covered hereby; or

            ix) Pledgor shall be in default in respect to any other obligations
(excluding the Obligations described herein) owed to Lender or any of its
Affiliates following the expiration of any applicable notice or grace period.

            (b) Upon the occurrence of an Event of Default, Lender may apply the
proceeds from the Pledged Collateral to the payment of the Obligations in such
order as Lender shall determine in its sole discretion, and if Lender has been
unable immediately to satisfy all of the Obligations by resort to the Pledged
Collateral, then, Lender may consider such default

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                                       8


hereunder to be a default under any other agreement between Pledgor or its
affiliates (or any of them) and Lender and shall have all of the rights granted
under such other agreement upon the occurrence of an Event of Default
thereunder.

            (c) Notice of Default. Pledgor shall furnish to Lender as soon as
possible and in any event within five (5) Business Days after the occurrence of
an Event of Default and each event which, with the giving of notice or lapse of
time or both would constitute an Event of Default, a statement of Pledgor
setting forth (i) the details of such Event of Default or event and the action
which Pledgor have taken and propose to take with respect thereto, and (ii) such
other information respecting the condition or operations, financial or
otherwise, of Pledgor as Lender may from time to time reasonably request.

            SECTION 12. Remedies upon Default. In addition to other rights
granted herein (including, without limitation, the right granted under Section
10(c)) or under law, if any Event of Default shall have occurred and be
continuing:

            (a) Lender may exercise in respect of the Pledged Collateral, in
addition to other rights and remedies provided for herein or otherwise available
to it, all of the rights and remedies of a secured party upon a default under
the Uniform Commercial Code in effect in the State of New York at that time (the
"Code") (whether or not the Code applies to the affected Pledged Collateral),
and may also, without notice except as specified below, sell the Pledged
Collateral or any part thereof in one or more parcels at public or private sale,
at any exchange, broker's board or at any of Lender's offices or elsewhere, for
cash, on credit or for future delivery, and upon such other terms as Lender may
deem commercially reasonable. Pledgor acknowledges and agrees that to the extent
notice of sale shall be required by law, ten (10) days' notice by Lender to
Pledgor of the time and place of any public sale or the time after which any
private sale is to be made shall constitute reasonable notification. Lender
shall not be obligated to make any sale of Pledged Collateral regardless of
notice of sale having been given. Lender may adjourn any public or private sale
from time to time by announcement at the time and place fixed therefor, and such
sale may, without further notice, be made at the time and place to which it was
so adjourned.

            (b) Any cash held by Lender as Pledged Collateral and all cash
proceeds received by Lender in respect of any sale of, collection from, or other
realization upon all or any part of the Pledged Collateral may, in the
discretion of Lender, be held by Lender as collateral for, and/or then or at any
time thereafter be applied (after payment of any amounts payable to Lender
pursuant to Section 17)) against all or any part of the Obligations in such
order as Lender shall elect. Any surplus of such cash or cash proceeds held by
Lender and remaining after payment in full of all the Obligations shall be paid
over to Pledgor or to whomsoever may be lawfully entitled to receive such
surplus.

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                                       9


            SECTION 13. Transfers and Other Liens. Pledgor agrees that it will
not (i) sell, assign (by operation of law or otherwise), pledge or otherwise
dispose of, or grant any option with respect to, any of the Pledged Collateral
or (ii) create or permit to exist any lien, security interest, option, or other
charge or encumbrance upon or with respect to any of the Pledged Collateral,
except for the security interest under this Agreement.

            SECTION 14. Lender Appointed Attorney-in-Fact.  Pledgor hereby
appoints Lender Pledgor's attorney-in-fact, with full authority in the place and
stead of Pledgor and in the name of Pledgor or otherwise, upon the occurrence
and during the continuance of an Event of Default, from time to time in Lender's
discretion to take any action and to execute any instrument which Lender may
deem necessary or advisable to accomplish the purposes of this Agreement,
including, without limitation, to receive, endorse and collect all instruments
made payable to Pledgor representing any interest payment or other distribution
in respect of the Pledged Collateral or any part thereof and to give full
discharge for the same. This appointment shall be irrevocable so long as any
Obligations have not been fully paid, discharged and satisfied.

            SECTION 15. Lender May Perform. If Pledgor fails to perform any
agreement contained herein, Lender may itself perform, or cause performance of,
such agreement, and the expenses of Lender incurred in connection therewith
shall be payable by Pledgor under Section 17.

            SECTION 16. Lender's Duties. The powers conferred on Lender
hereunder are solely to protect its interest in the Pledged Collateral and shall
not impose any duty upon it to exercise such powers. Except for the safe custody
of any Pledged Collateral in its possession and the accounting for moneys
actually received by it hereunder, Lender shall have no duty as to any Pledged
Collateral, as to ascertaining or taking action with respect to calls,
conversions, exchanges, maturities, tenders or other matters relative to any
Pledged Collateral, whether or not Lender has or is deemed to have knowledge of
such matters, or as to the taking of any necessary steps to preserve rights
against any parties or any other rights pertaining to any Pledged Collateral.
Lender shall be deemed to have exercised reasonable care in the custody and
preservation of any Pledged Collateral in its possession if such Pledged
Collateral is accorded treatment substantially equal to that which Lender
accords its own property.

            SECTION 17. Indemnity and Expenses. Pledgor agrees to indemnify
Lender from and against any and all claims, liens and liabilities (including
reasonable attorneys' fees) growing out of or resulting from this Agreement,
except claims, losses or liabilities resulting from the gross negligence or
unlawful misconduct of Lender. Pledgor will within ten (10) days of demand pay
to Lender the amount of any and all reasonable expenses, including the
reasonable fees and expenses of its counsel and of any experts and agents, which
Lender may incur in connection with (a) the administration of this Agreement,
(b) the custody or preservation of, or the sale of, collection from, or other
realization upon, any of the Pledged Collateral, (c) the exercise or enforcement
of any of the rights of Lender hereunder or (d) the failure by Pledgor to

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perform or observe any of the provisions hereof; provided, however, that any
reasonable expenses of Lender incurred in connection with an Event of Default
that has occurred and is continuing shall be paid on demand.

            SECTION 18. Security Interest Absolute. All rights of Lender and
security interests hereunder, and all obligations of Pledgor hereunder, shall be
absolute and unconditional irrespective of:

            (a) any change in the time, manner or place of payment of, or in any
other term of, all or any of the obligations of Pledgor secured hereby, or any
other amendment or waiver of or any consent to any departure from the Note or
any related agreement or instrument, including, without limitation, any increase
in the Obligations resulting from the extension of additional credit to Pledgor
or otherwise;

            (b) any taking, exchange, release or nonperfection of any other
collateral, or any taking, release or amendment or waiver of or consent to
departure from any guaranty, for all or any of the Obligations; or

            (c) any manner of application of collateral, or proceeds thereof, to
all or any of the Obligations, or any manner of sale or other disposition of any
collateral for all or any of the Obligations or any other assets of Pledgor.

            SECTION 19. Amendments, Etc. This Agreement incorporates and
supersedes all prior agreements and undertakings with respect to the subject
matter of this Agreement. No amendment or waiver of any provision of this
Agreement, and no consent to any departure by Pledgor herefrom, shall in any
event be effective unless the same shall be in writing and signed by Lender and
Pledgor, and then such waiver or consent shall be effective only in the specific
instance and for the specific purpose for which given.

            SECTION 20. Notices. All notices and other communications provided
for hereunder shall be in writing (including telecopier, telegraphic, telex or
cable communication) and mailed first class, postage prepaid, sent by certified
or registered mail, return receipt requested, telecopied, telegraphed, telexed,
cabled or delivered by messenger or overnight courier to it, at the following
addresses:

if to Pledgor:    Cstone-527 Madison, Inc.
                  c/o Cornerstone Properties , Inc.
                  126 East 56th Street
                  New York, New York 10022
                  Fax: (212) 605-7199

with a copy to:   Shearman & Sterling

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                                       11


                  599 Lexington Avenue
                  New York, New York 10022
                  Attention: Mason Sleeper, Esq.
                  Fax: (212) 848-7300

if to Lender:     Bankers Trust Company
                  280 Park Avenue
                  New York, New York 10017

                  Attention:  Alexander B.V. Johnson
                              Managing Director
                  Fax: (212) 454-1733

with a copy to:   Willkie Farr & Gallagher
                  One Citicorp Center
                  153 East 53rd Street
                  New York, New York 10022-4677
                  Attention: Monty Davis, Esq.
                  Fax: (212) 821-8111

or, as to any of the parties, at such other address as shall be designated by
such party in a written notice to the other party. All such notices and other
communications shall, when mailed, or telecopied, or delivered by messenger or
overnight courier, be effective three (3) days after deposited in the mails, or
when telecopied, or when delivered (or when proper delivery is refused),
respectively. Lender covenants and agrees to send to Pledgor in the manner
hereinabove provided copies of all notices sent by Lender under the Note and
Mortgage, as modified.

            SECTION 21. Continuing Security Interest; Assignments by Lender.
This Agreement shall create a continuing security interest in the Pledged
Collateral and shall (a) constitute a "security agreement" within the meaning of
the Uniform Commercial Code as in effect in the State of New York from time to
time, (b) remain in full force and effect until the payment in full of the
Obligations and all other amounts payable under this Agreement, (c) be binding
upon Pledgor, and its successors and assigns, and (d) inure to the benefit of,
and be enforceable by, Lender and its successors, transferees and assigns.
Without limiting the generality of the foregoing clause (d), Lender may assign
or otherwise transfer all or any portion of its rights and obligations under
this Agreement, the Note or any other Loan Document to any other person or
entity, and such other person or entity shall thereupon become vested with all
the benefits in respect thereof granted to Lender herein or otherwise. In the
event of any such assignment by Lender, Lender shall provide reasonable notice
thereof to Pledgor. Upon the payment in full of the Obligations and all other
amounts payable under this Agreement, the security interest granted hereby shall
automatically terminate and all rights to the Pledged Collateral shall revert to
Pledgor. Upon any such termination, Lender will, at Pledgor's expense, promptly
return to

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                                       12


Pledgor that portion of the Pledged Collateral as shall not have been sold or
otherwise applied pursuant to the terms hereof and execute and deliver to
Pledgor such documents as Pledgor shall reasonably request to evidence such
termination.

            SECTION 22. Release of Mortgage. If, upon the maturity of the Note
or an Event of Default, Lender exercises its rights to apply the Pledged
Collateral in payment in full of the Obligations, then at the option of Pledgor,
Lender shall release the Mortgage or shall assign the Mortgage without recourse
to Pledgor or an entity designated by Pledgor.

            SECTION 23. Governing Law; Terms. This Agreement shall be governed
by and construed in accordance with the laws of the State of New York, except as
required by mandatory provisions of law and except to the extent that the
validity or perfection of the security interest hereunder, or remedies
hereunder, in respect of any particular Pledged Collateral are governed by the
laws of a jurisdiction other than the State of New York. Unless otherwise
defined herein or in the Note, terms defined in Article 9 of the Code are used
herein as therein defined.

            SECTION 24. Waiver of Jury Trial. PLEDGOR AND LENDER HEREBY WAIVE,
TO THE EXTENT PERMITTED BY LAW, TRIAL BY JURY IN ANY COURT IN ANY SUIT, ACTION
OR PROCEEDING ON ANY MATTER ARISING IN CONNECTION WITH OR IN ANY WAY RELATED TO
THIS AGREEMENT OR THE FINANCING TRANSACTIONS OF WHICH THIS AGREEMENT IS A PART
AND/OR THE DEFENSE OR ENFORCEMENT OF ANY OF LENDER'S RIGHTS AND REMEDIES IN
CONNECTION THEREWITH. PLEDGOR AND LENDER ACKNOWLEDGE THEY MAKE THIS WAIVER
KNOWINGLY, VOLUNTARILY AND ONLY AFTER EXTENSIVE CONSIDERATION OF THE
RAMIFICATIONS OF THIS WAIVER WITH THEIR ATTORNEYS.

            SECTION 25. Severability. In case any one or more of the provisions
contained in this Agreement should be invalid, illegal or unenforceable in any
respect, the validity, legality and enforceability of the remaining provisions
contained herein and therein shall not in any way be affected or impaired
thereby. The parties hereto agree to negotiate in good faith either to replace
any illegal, invalid or unenforceable provision so that, to the extent possible,
the economic bargain of this Agreement shall be preserved, or otherwise to amend
the same to achieve such result.

            SECTION 26. Sale of Property. Lender hereby consents to the sale of
the Property by 527 to Pledgor.

            SECTION 27. Withdrawals by Pledgor.

<PAGE>
                                       13


            (a) Subject to the conditions of this Section, Pledgor shall have
the right from time to time to request a withdrawal of funds from the Deposit to
Pledgor. Each such request shall be in writing and shall be delivered not less
than ten (10) days prior to the date on which Pledgor requires such funds.

            (b) The right of Pledgor to withdraw funds from the Deposit (each
such withdrawal, a "Withdrawal") shall be subject to the following conditions
precedent:

            i) There shall be no Event of Default on the part of Pledgor or 527
      under the Note.

            ii) Prior to the first Withdrawal, Pledgor shall have delivered to
      Lender a full and unconditional guaranty of all amounts due under the Note
      from Cornerstone Properties, Inc., together with an opinion of counsel
      acceptable to Lender in its sole discretion as to the due authorization,
      execution and delivery of such guaranty.

            iii) Prior to the first Withdrawal, Pledgor shall have delivered to
      Lender a current Phase I environmental assessment report of the Property
      prepared by an environmental engineer acceptable to Lender in its sole
      discretion, which report shall indicate the absence of any adverse
      environmental conditions at the Property and shall otherwise be acceptable
      to Lender in its sole discretion.

            iv) Prior to the first Withdrawal, Pledgor shall have delivered to
      Lender an appraisal of the Property prepared by an appraiser acceptable to
      Lender in its sole discretion and in accordance with all applicable
      regulations governing such appraisals, which shall show a fair market
      value for the Property of not less than $70,000,000 and which shall
      otherwise be acceptable to Lender in its sole discretion.

            v) Prior to the first Withdrawal, Pledgor shall have delivered to
      Lender a current structural engineering report covering the Property
      prepared by an engineer or contractor acceptable to Lender in its sole
      discretion, indicating no material adverse conditions at the Property and
      otherwise acceptable to Lender in its sole discretion.

            vi) At the time of each Withdrawal, the ratio of Cash Flow from the
      Property to debt service shall be not less than 4.25:1.

            SECTION 28. Defined Terms.  Unless the context otherwise specifies
or requires, the following terms shall have the meanings herein specified.

            "Affiliate" means, with respect to a Person, any other Person that,
directly, or indirectly through one or more intermediaries, controls, or is
controlled by, or is under common control with, such first Person, which, in the
case of a partnership, shall include each of the direct

<PAGE>
                                       14


and indirect controlling venturers or partners thereof, and in the case of a
corporation, shall include each of the controlling shareholders thereof. For
purposes of this definition only, the term "control" means the possession,
directly or indirectly, of the power (not necessarily the exclusive power) to
direct or cause the direction of the management, business and policies of a
Person, whether by contract or otherwise.

            "Person" means an individual, a corporation, a partnership, an
association, a joint venture, an estate, a trust or any other legal entity.

            "Cash Flow from the Property" means the excess of (i) fixed rent and
additional rents payable under all leases at the Property over (ii) operating
and other expenses of the Property other than debt service on the Note.

            For purposes of this Agreement, whenever the circumstances or the
context of this Agreement so requires, the singular shall be construed as the
plural, the masculine shall be construed as the feminine and/or the neuter and
vice versa, and the phrase "including" shall mean "including without
limitation".

[THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK.]

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                                       15


            IN WITNESS WHEREOF, Pledgor has caused this Agreement to be duly
executed and delivered as of the date first above written.

                                    PLEDGOR:

                                    CSTONE-527 MADISON, INC.

                                    By: /s/ Francis H. Shields, Jr.
                                        -----------------------------------
                                            Name: Francis H. Shields, Jr.
                                            Title: Vice President


                                    By: /s/ Rodney C.D. Mock
                                        -----------------------------------
                                            Name: Rodney C.D. Mock
                                            Title: Executive Vice President


                                    LENDER:

                                    BANKERS TRUST COMPANY

                                    By: /s/ A BV Johnson
                                        -----------------------------------
                                            Name: A BV Johnson
                                            Title: MD