<PAGE>

                                                                   Exhibit 10.80

                            ASSIGNMENT OF COLLATERAL

                           (DIHC BOYLSTON CORPORATION)


      THIS ASSIGNMENT OF COLLATERAL (hereinafter referred to as the
"Assignment") is made as of the 27th day of October, 1997, by STICHTING
PENSIOENFONDS VOOR DE GEZONDHEID, GEESTELIJKE EN MAATSCHAPPELIJKE BELANGEN
(hereinafter referred to as "Assignor") in favor of Cornerstone Properties Inc.
(hereinafter referred to as "Assignee").

                              W I T N E S S E T H :

      WHEREAS, Assignor is the recipient and owner of the promissory note and
collateral security documents described on Exhibit A attached hereto and by this
reference incorporated herein (the "Collateral"); and

      WHEREAS, pursuant to that certain Loan Purchase Agreement dated as of
August 18, 1997, between Assignor and Assignee, Assignor has agreed to transfer
all of its right, title and interest in and to the Collateral to Assignee.

      NOW, THEREFORE, for and in consideration of the exchange of good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged by the parties hereto, the parties hereto covenant and agree as
follows:

      I. Assignor hereby irrevocably transfers, assigns, grants, bargains,
sells, conveys, remises, releases, warrants, set over and confirms unto Assignee
for its benefit and the benefit of its successors and assigns, all of its right,
title and interest in and to the Collateral without recourse or warranty except
as provided for and set forth in that certain Loan Purchase Agreement dated as
of August 18, 1997, executed by and between Assignee and Assignor.

      II. Upon the request of Assignee, Assignor shall execute and deliver to
Assignee such further instruments as Assignee may deem reasonably necessary to
effectuate this Assignment.

      III. This Assignment shall bind and inure to the benefit of the Assignor
and Assignee and their respective heirs, successors and assigns.

      IV. Nothing in this Assignment shall be construed to give to any person
other than Assignee its successors and assigns any legal or equitable right,
remedy or claim under this Assignment, and this Assignment shall be held for the
sole and exclusive benefit of Assignee and its successors and assigns.

      V. This Assignment shall be governed by, construed and enforced in
accordance with the laws of the State of Massachusetts.
<PAGE>
                                       2


      IN WITNESS WHEREOF, Assignor and Assignee have executed or caused to be
executed this Assignment under seal, as of the date and year first above
written.

ASSIGNOR:

STICHTING PENSIOENFONDS VOOR DE
GEZONDHEID, GEESTELIJKE EN
MAATSCHAPPELIJKE BELANGEN


By: /s/ Jan H.W.R. van der Vlist
    ----------------------------------
Name: Jan H.W.R. van der Vlist
     ---------------------------------
Title: Attorney-in Fact
      --------------------------------


By: /s/ Anneke C van de Puttelaar
    ----------------------------------
Name: Anneke C. van de Puttelaar
     ---------------------------------
Title: Attorney-in Facty
      --------------------------------


ASSIGNEE:

CORNERSTONE PROPERTIES INC.


By: /s/ John S. Moody
    ----------------------------------
Name: John S. Moody
     ---------------------------------
Title: President
      --------------------------------


By: /s/ Rodney C. Dimock
    ----------------------------------
Name: Rodney C. Dimock
     ---------------------------------
Title: Executive Vice President
      --------------------------------
<PAGE>

                                    EXHIBIT A

1.    Amended and Restated Promissory Note dated as of July 1, 1994, executed by
      DIHC Boylston Corporation ("Boylston") in favor of Stichting Pensioenfonds
      voor de Gezondheid, Geestelijke en Maatschappelijke Belangen ("PGGM") in
      the principal amount of $18,461,283.03.
<PAGE>

                                     ALLONGE

                               ENDORSEMENT OF NOTE

                              (DIHC BOYLSTON CORP.)


      Pay to the order of CORNERSTONE PROPERTIES INC. (the "Assignee") without
recourse or warranty except as provided for and set forth in that certain Loan
Purchase Agreement dated as of August 18, 1997 executed by and between Stichting
Pensioenfonds voor de Gezondheid, Geestelijke en Maatschappelijke Belangen and
Assignee.


                                          STICHTING PENSIOENFONDS VOOR
                                          DE GEZONDHEID, GEESTELIJKE EN
                                          MAATSCHAPPELIJKE BELANGEN


                                          By: /s/ Jan H. W. R. van der Vlist
                                             -----------------------------------
                                          Name: /s/ Jan H. W. R. van der Vlist
                                               ---------------------------------
                                          Title: Attorney-in Fact
                                                 -------------------------------


                                          By: /s/ Anneke C. van de Puttelaar
                                             -----------------------------------
                                          Name: Anneke C. van de Puttelaar
                                               ---------------------------------
                                          Title: Attorney-in Fact
                                                 -------------------------------

                              DATE: October 27, 1997