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                                                                   Exhibit 10.50


THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER THE SECURITIES LAWS OF CERTAIN
STATES. THESE SECURITIES ARE SUBJECT TO RESTRICTIONS ON TRANSFERABILITY AND
RESALE AND MAY NOT BE TRANSFERRED OR RESOLD EXCEPT AS PERMITTED UNDER THE ACT
AND APPLICABLE STATE SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION
THEREFROM. INVESTORS SHOULD BE AWARE THAT THEY MAY BE REQUIRED TO BEAR THE
FINANCIAL RISKS OF THIS INVESTMENT FOR AN INDEFINITE PERIOD OF TIME. THE ISSUER
OF THESE SECURITIES MAY REQUIRE AN OPINION OF COUNSEL IN FORM AND SUBSTANCE
SATISFACTORY TO THE ISSUER TO THE EFFECT THAT ANY PROPOSED TRANSFER OR RESALE IS
IN COMPLIANCE WITH THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS.


                          HOUSTONSTREET EXCHANGE, INC.

                         SENIOR SECURED PROMISSORY NOTE

                                                                   $8,419,842.00
                                                                  MARCH 30, 2001


                  FOR VALUE RECEIVED, HOUSTONSTREET EXCHANGE, INC. (the
"Company"), a Delaware corporation, hereby promises to pay to the order of
BayCorp Holdings, Ltd., (the "Holder") in lawful money of the United States, the
principal amount of Eight Million, Four Hundred Nineteen Thousand, Eight Hundred
Forty-Two Dollars ($8,419,842.00) plus simple interest thereon in the manner and
at the rate provided herein. This Senior Secured Promissory Note is one of a
duly authorized issue of up to $13,617,015 aggregate principal amount of Notes
(each a "Note" and collectively the "Notes") issued by the Company pursuant to a
certain Senior Secured Note and Warrant Purchase Agreement (the "Purchase
Agreement") among the Company and the Holders, issued commencing March 30, 2001,
and is entitled to the benefits thereof.

                  1. Principal. The principal on this Note is due and payable on
December 31, 2001 (the "Maturity Date"). This Note may be prepaid without
penalty, in whole or in part, at any time. The obligations of the Company to
make payments provided for in this Note are absolute and unconditional and not
subject to any defense, set-off, counterclaim, rescission, recoupment or
adjustment whatsoever. Upon payment in full of all principal and interest
payable hereunder, this Note shall be surrendered to the Company for
cancellation.

                  2. Interest. This Note shall bear interest on the outstanding
principal amount from the date hereof until this Note is paid in full at three
percent (3%) above the rate of interest publicly announced from time to time by
FleetBoston in Boston, Massachusetts as its prime rate ("Prime Rate"). Any
change in the Prime Rate shall take effect on the day specified in the public


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announcement of the change. Accrued interest is payable, at the sole option of
the Holder, in cash or in warrants to purchase shares of Series C Convertible
Preferred Stock ("Warrants"), only at the time payment of the principal is made
or due. In the event that the Holder elects to be paid accrued interest in
Warrants, (i) the Company shall issue to such Holder a Warrant to purchase or
(ii) the shares of Series C Convertible Preferred Stock issuable pursuant to the
outstanding Warrant shall be increased by that number of shares of Series C
Convertible Preferred Stock equal to the quotient of each $1.00 interest divided
by the then current Exercise Price of the Warrants pursuant to the terms of that
certain Series C Preferred Stock Warrant Agreement of even date herewith by and
among the Company and the Holders (which such Exercise Price shall initially be
$0.15 per share).

         In no event shall any interest to be paid hereunder exceed the maximum
rate permitted by law. In any such event, this Note shall automatically be
deemed amended to permit interest charges at an amount equal to, but no greater
than, the maximum rate permitted by law.

                  3. Security. The Company covenants and agrees, and the Holder,
by accepting this Note, also covenants and agrees, that the Notes, as a class,
shall have a first priority security interest among the Holders of the Notes, in
all of the assets of the Company as provided in (i) the Security Agreement (the
"Security Agreement"), dated as of even date herewith, by and between the
Company and United States Trust Company of New York, as secured party and agent
for the Holders of the Notes ("Secured Party Agent") and (ii) the Stock Pledge
Agreement (the "Pledge Agreement"), dated as of even date herewith, by and
between the Company and Secured Party Agent as agent for the Holders of the
Notes.

                  4. Events of Default. An "Event of Default" shall exist if any
of following occurs and is continuing:

                           (a) Failure to make any payment of principal or
interest when due;

                           (b) Default in the payment or performance of any
liability, obligation or agreement of the Company contained in this Note;

                           (c) Failure to observe any covenant of the Company
contained in this Note, the Purchase Agreement, the Security Agreement or the
Pledge Agreement, which remains uncured for a period of ten (10) days (provided,
that if such failure can not be cured in such ten (10) day period, the Company
shall have thirty (30) days so long as the Company is in the process of curing
such failure) after written notice from any Holder as to such failure;

                           (d) The Company willfully takes an action that
requires consent of holders of at least 85% of the principal amount of the Notes
then outstanding without obtaining such consent;

                           (e) The Company files a voluntary petition in
bankruptcy, or shall be adjudicated a bankrupt or insolvent, or shall file any
petition or answer seeking any reorganization arrangement, composition,
readjustment, liquidation, dissolution, or


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similar relief under the present or any future federal bankruptcy code or other
applicable federal, state or similar statute, law or regulation, or shall seek
or consent to or acquiesce in the appointment of any trustee, receiver or
liquidator of the Company or of all or any substantial part of its properties;

                           (f) Within thirty (30) days after the commencement of
any proceedings against the Company seeking any reorganization, arrangement,
composition, readjustment, liquidation, dissolution or similar relief under the
present or any future federal bankruptcy code or other applicable federal, state
or similar statute, law or regulation, such proceeding has not been dismissed or
if, within thirty (30) days after the appointment, without the consent or
acquiescence of the Company, of any trustee, receiver or liquidator of the
Company or of all or any substantial part of its properties, such appointment
has not been vacated;

                           (g) A Change of Control has occurred. "Change of
Control" means the consummation of a transaction, whether in a single
transaction or in a series of related transactions with any other party or
parties in an arm's-length basis, pursuant to which (a) such party or parties,
directly or indirectly, acquire (whether by merger, stock purchase,
recapitalization, reorganization, redemption, issuance of capital stock or
otherwise) more than 40% of the voting stock of the Company, (b) such party or
parties, directly or indirectly, acquire assets constituting all or
substantially all of the assets of the Company or (c) prior to an initial public
offering of the Common Stock pursuant to an offering registered under the
Securities Act, BayCorp and its affiliates cease to have the ability to elect,
directly or indirectly, four members of the Board of Directors of the Company;

                           (h) A default shall have occurred with respect to any
other Indebtedness for borrowed money that is not cured within the applicable
grace period, if any, which results in the modification of the rate of interest,
triggers a penalty payment or accelerates the time which any amount due
thereunder is payable;

                           (i) A court of competent jurisdiction issues a
judgment in favor of Sapient Corporation declaring that the transactions
contemplated by the Purchase Agreement constitute a "Financing" (as such term is
defined in the Letter Agreement, dated as of December 18, 2000 between the
Company and Sapient Corporation); or

                           (j) A court of competent jurisdiction issues a
judgment or judgments against the Company in an amount or amounts exceeding
$125,000 per judgment or $250,000 in the aggregate, provided any such judgment
is not cured prior to the earlier of (i) 30 days after the date of such judgment
or (ii) the date on which such judgment is enforced against the Company.

                  5. Default Remedies. If an Event of Default has occurred the
Notes plus accrued interest shall become immediately due and payable.

                  6. Notices. Any notice, other communication or payment
required or permitted hereunder shall be in writing and shall be deemed to have
been given upon delivery if


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personally delivered or upon deposit if deposited in the United States mail for
mailing by certified mail, postage prepaid, and addressed as follows:

                   To the Company:

                   HoustonStreet Exchange, Inc.
                   222 International Drive, Suite 125
                   Portsmouth, New Hampshire, 03801
                   Attention:  President

                   With copies to (which copies will not constitute notice):

                   Andrew J. Merken, Esq.
                   Mintz Levin Cohn Ferris Glovsky and Popeo, P.C.
                   One Financial Center
                   Boston, MA 02111

                   To the Secured Party Agent:

                   United States Trust Company of New York
                   114 West 47th Street
                   New York, New York 10036
                   Attention:  Mr. John Guiliano
                   Fax:  (212) 852-1626


                   With copies to (which copies will not constitute notice):

                   M. Douglas Dunn, Esq.
                   Milbank, Tweed, Hadley & McCloy LLP
                   One Chase Manhattan Plaza
                   New York, New York 10005
                   Fax:  (212) 530-5219

                   To the Holder:  At the address in the register maintained by
                   the Company for such purpose.

Each of the above addressees may change its address for purposes of this
paragraph by giving to the other addressee notice of such new address in
conformance with this paragraph.

                  7. Restrictions on Transferability. This Note may be
transferred or assigned by the Holder without the prior written consent of the
Company.

                  8. Governing Law. This Note shall be governed and construed in
accordance with the laws of the State of Delaware without giving effect to such
States rules governing conflicts of laws.


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         IN WITNESS WHEREOF, HOUSTONSTREET EXCHANGE, INC. has caused this Note
to be executed and issued on its behalf by officer duly authorized.


                               HOUSTONSTREET EXCHANGE, INC.


                               By:  /s/ Frank W. Getman Jr.
                                   ---------------------------------------------

                               Name: Frank W. Getman Jr.
                                     -------------------------------------------
                               Title:    President and Chief Executive Officer
                                         ---------------------------------------


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