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                                                                      EXHIBIT 4b

                                                                  EXECUTION COPY

         THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
         1933 OR ANY STATE SECURITIES LAWS. THEY MAY NOT BE SOLD OR OFFERED FOR
         SALE EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER SAID
         ACT AND ANY APPLICABLE STATE SECURITIES LAWS OR AN APPLICABLE EXEMPTION
         FROM SUCH REGISTRATION REQUIREMENTS.



         7% CONVERTIBLE DEBENTURE DUE FEBRUARY 26, 1999


         THIS 7% CONVERTIBLE DEBENTURE ("Debenture") is issued by DynaGen,
Inc.,a corporation duly organized and existing under the laws of the State of
Delaware and having its principal address at 840 Memorial Drive, Cambridge,
Massachusetts 02139 (the "Company").

         FOR VALUE RECEIVED, the Company promises to pay to Sovereign Partners,
having an address at Executive Pavilion, 90 Grove Street, Richfield, Connecticut
06877, the holder hereof, or its order (the "Holder"), the principal sum of Five
Hundred Thousand United States Dollars (U.S. $500,000) on February 26, 1999 (the
"Maturity Date") and to pay interest on the principal sum outstanding under this
Debenture, at the rate of 7% per annum due and payable in arrears on the
Maturity Date. Interest shall be calculated based on a 360 day year of twelve
equal months. Accrual of interest shall commence on the date hereof and shall
continue until payment in full of the principal sum has been made. This
Debenture is subordinate to all secured indebtedness of the Company.

         This Debenture is subject to the following additional provisions:

         1. Exchange. The Debenture is exchangeable for an equal aggregate
principal amount of Debentures of different denominations, of not less than
$100,000 (or the total principal amount, if less than $100,000) each as
requested by the Holder surrendering the same. No service charge will be made
for such exchange.

         2. Transfers. This Debenture may be transferred or exchanged in the
United States only in compliance with the Securities Act of 1933, as amended
(the "Securities Act"), and applicable state securities laws and in accordance
with other applicable provisions hereof. Prior to due presentment for transfer
of this Debenture, the Company may treat the person in whose name this Debenture
is duly registered as the owner hereof for the purpose of receiving payment as
herein provided and all other purposes, whether or not this Debenture is then
overdue, and the Company shall not be affected by notice to the contrary.

         3. Definitions. For purposes hereof, the following terms shall have the
following meanings:
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            "Closing Date" shall mean the date of original issuance of this
Debenture.

            "Common Stock" shall mean the Common Stock, par value $.01 per
share, of the Company.

            "Conversion Date" shall have the meaning set forth in Paragraph
4(c)(i).

            "Conversion Date Market Price" shall mean, at any Conversion Date
seventy-five percent (75%) of the average Market Price for Shares of Common
Stock for the five trading days immediately preceding the Conversion Date.

            "Conversion Notice" shall have the meaning set forth in Paragraph
4(c).

            "Conversion Rate" shall have the meaning set forth in Paragraph
4(b).

            "Event of Default" shall have the meaning set forth in Paragraph 16.

            "Market Price for Shares of Common Stock" shall mean the price of
one share of Common Stock determined as follows:

                  (i)   If the Common Stock is listed on the Nasdaq SmallCap
Market, the closing bid price on the date of valuation, as reported by Bloomberg
Financial Markets; or

                  (ii)  If the Common Stock is listed on any other national
securities exchange, the closing bid price on the date of valuation, as reported
by Bloomberg Financial Markets; or

                  (iii) If neither (i) nor (ii) apply, but the Common Stock is
quoted in the over-the-counter market on the pink sheets or bulletin board, the
lowest sales price on the date of valuation, as reported by Bloomberg Financial
Markets; or

                  (iv)  If none of clause (i), (ii) or (iii) above applies, the
market value as determined by an independent nationally recognized investment
banking firm or financial advisor retained in good faith by the Company for such
purpose, taking into consideration, among other factors, the earnings history,
book value and prospects for the Company, and the prices at which shares of
Common Stock recently have been traded. Such determination shall be conclusive
and binding on all persons.

            "Outstanding Amount" shall mean the principal sum outstanding under
this Debenture and all accrued but unpaid interest thereon.

            "Registration Rights Agreement" shall mean that certain Registration
Rights Agreement, dated February 26, 1998, between the Company and Sovereign
Partners.


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         4. Conversion. This Debenture is subject to conversion as follows:

            (a) (i)   Holder's Right to Convert. This Debenture shall be
convertible at any time and from time to time commencing with the 90th day after
the Closing Date and until the Maturity Date, in whole or in part, at the option
of the Holder hereof, into fully paid, validly issued and nonassessable shares
of Common Stock; provided, that, in any 14 calendar day period commencing on
such 90th day after the Closing Date and thereafter, the Holder (or Holders
collectively, of this Debenture shall have been divided into more than one
Debenture) hereof may convert no more than one-fifth of the original principal
amount of the Debenture. The foregoing conversion restrictions shall immediately
terminate, and the Holder shall be permitted to convert all or any part of this
Debenture without regard to the conversion restrictions, upon the occurrence of
any Event of Default, or upon the commencement by any person (other than the
Holder) of any tender offer for shares of Common Stock.

                (ii)  Accrued But Unpaid  Interest.  Notwithstanding  anything
in this Debenture to the contrary, the Outstanding Amount of this Debenture on
any Conversion Date shall include, without limitation, all accrued but unpaid
interest under this Debenture through such date.

            (b) Conversion Price for Converted Shares. The Outstanding Amount
of this Debenture that is converted into shares of Common Stock shall be
convertible into the number of shares of Common Stock determined in accordance
with the following formula:

                                      P + I
                     --------------------------------------
                          Conversion Date Market Price

                P =  principal amount of this Debenture submitted for conversion
                I =  accrued but unpaid interest on the principal amount
                     of this Debenture submitted for conversion plus any
                     unpaid charges or amounts through the Conversion
                     Date.

         The number of shares of Common Stock into which the Outstanding Amount
of this Debenture may be converted pursuant to this paragraph is hereafter
referred to as the "Conversion Rate."

            (c) (i)  Mechanics of Conversion. In order to convert this Debenture
(in whole or in part) into full shares of Common Stock, the Holder shall
surrender this Debenture, duly endorsed, by either overnight courier or two-day
courier, to the Company, and, in case of any conversion pursuant to Section
4(a)(i), shall give written notice in the form of Exhibit A hereto (the
"Conversion Notice") by facsimile (with the original of such notice forwarded
with the foregoing courier) to the Company that the Holder elects to convert all
or the portion of the Outstanding Amount of this Debenture specified therein,
which notice and election shall be irrevocable by the


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Holder, provided, however, that the Company shall not be obligated to issue
certificates evidencing the shares of Common Stock issuable upon conversion
unless this Debenture with evidence of the principal amount hereof to be
converted is delivered to the Company as provided above, or the Holder notifies
the Company that this Debenture has been lost, stolen or destroyed and promptly
executes an agreement reasonably satisfactory to the Company to indemnify the
Company from any loss which may be incurred by it in connection with this
Debenture. The date on which a Conversion Notice is given (the "Conversion
Date") shall be deemed to be the date the Company received by facsimile the
Conversion Notice, as evidenced by a printed confirmation of receipt received by
the Holder. Upon receipt of any Conversion Notice, the Company shall immediately
verify the Holder's calculation of the Conversion Rate.

                (ii) Issuance of Certificates. In the case of any Conversion
Notice given by the Holder, the Company shall use its best efforts to cause the
transfer agent for its Common Stock to issue and deliver as promptly as
practicable and in no event later than three (3) business days after delivery to
the Company of the Debenture, or after receipt of the agreement and
indemnification referred to in Section 4(c),to such Holder or to its designee, a
certificate or certificates for the number of shares of Common Stock to which
the Holder shall be entitled, together with a Debenture for the principal amount
not submitted for conversion. The person or persons entitled to receive the
shares of Common Stock issuable upon conversion shall be treated for all
purposes as the record holder or holders of such shares of Common Stock on the
Conversion Date. If the Company shall not have the requisite number of shares of
Common Stock issuable upon conversion of the Debenture by the Holder, then,
without limiting the Company's obligation to convert the Debenture, such
conversion shall be made for each Holder, pro rata according to the portion of
the total Outstanding Amount of the portion of the Debentures sought to be
converted. At the Holder's option, the request for conversion by the Holder
shall be null and void for any portion of the Debentures for which the Company
does not have shares of Common Stock issuable upon conversion as of the
Conversion Date.

         5. Stock Splits: Dividends, Adjustments, Reorganizations

            (a) Subdivisions, Combinations, etc. If the Company shall subdivide
its outstanding Common Stock, by split-up, spin-off, or otherwise, or combine
its outstanding Common Stock, then the Conversion Rate in effect as of the date
of such subdivision, split-up, spin-off or combination shall be determined with
reference to the Market Price for shares of Common Stock as reported after such
subdivision split up, spin-off or combination without other adjustment.

            (b) Adjustment for Merger, Reorganization; etc. In the event that at
any time or from time to time after the Closing Date, the Common Stock issuable
upon conversion of the Debentures is changed into the same or a different number
of shares of any class or classes of stock, whether in connection with a merger
or consolidation, by recapitalization, reclassification, reorganization or
otherwise (other than a subdivision or combination of shares or stock dividend
or reorganization provided for elsewhere in this Paragraph 5, then and in each
such event the Holder of Debentures shall have the right thereafter to convert
this Debenture into the kind of securities


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receivable upon such merger, recapitalization, reclassification or other change,
all subject to further adjustment as provided herein. In such event, the formula
set forth herein for conversion shall be equitably adjusted to reflect such
change in number of shares or, if shares of a new class of stock are issued, to
reflect the market price of the class or classes of stock (applying the same
factors used in determining the Market Price for Shares of Common Stock) issued
in connection with the above described transaction.

            (c) Certificate as to Adjustments. Upon the occurrence of each
adjustment pursuant to this Section 5, the Company at its expense shall furnish
to the Holder a certificate from its independent auditors or an investment
banking firm setting forth (i) in reasonable detail the facts upon which such
adjustment is based, and (ii) the number of shares of Common Stock and the
amount of other property or securities that after giving effect thereto would be
received by the Holder upon conversion of this Debenture.

            (d) Disputes. In the event of a reasonable, good faith dispute
between the Holder and the Company with respect to the adjustments required by
Paragraphs 5(b), (c) or (c), then, at the option of either the Holder or the
Company, the dispute shall be submitted to the American Arbitration Association
for resolution according to the then applicable rules thereof. The cost of such
proceeding shall be shared 50% by the Holder and 50% by the Company, except that
each party shall bear its own legal and other expenses.

         6. Fractional Shares. No fractional shares of Common Stock or scrip
representing fractional shares of Common Stock shall be issuable hereunder. The
number of shares of Common Stock that are issuable upon any conversion shall be
rounded up or down to the nearest whole share.

         7. Reservation of Stock Issuable Upon Conversion

         The Holder has been informed that (i) as of February __, 1998, the
Company had issued all of its authorized shares of Common Stock and had no
Common Stock available for issuance upon conversion of this Debenture and (ii)
the Company has called a special meeting of its stockholders to be held on March
4, 1998, to approve a reverse split of its outstanding shares of Common Stock
which reverse split would result in the Company having approximately 7,500,000
shares of Common Stock outstanding (immediately after the reverse split) and
75,000,000 authorized shares of Common Stock (the "Reverse Split"). In the event
that the Reverse Split is not effected on or before March 9, 1998, the Holder
may require the Company to repurchase his Debenture for the outstanding
principal amount plus interest accrued thereon by presenting his Debenture for
payment on or before March 12, 1998, and the Company shall pay such amount
within three business days of presentation. Subject to consummation of the
Reverse Split, the Company shall use its best efforts to reserve and keep
available at all times, free of preemptive rights, shares of Common Stock for
the purpose of enabling the Company to satisfy any obligation to issue shares of
its Common Stock upon conversion of the Debenture; provided, however, that the
aggregate number of shares so reserved shall initially be 850,000 shares. The
number of shares so reserved may be reduced by the number of shares actually
delivered pursuant to partial conversion of the Debenture and the number of
shares so


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reserved shall be increased or decreased proportionally to reflect stock splits,
stock dividends and other distributions. In the event that the number of shares
so reserved shall be insufficient for issuance upon conversion of the Debenture
(without giving effect to any applicable conversion restrictions), or if the
Holder would at any time upon conversion thereof be entitled to the issuance of
shares of Common Stock in excess of the limitation in Paragraph 4(d), then upon
receipt by the Company of notice from the Holder, the Company shall use its best
efforts and all due diligence to increase the number of shares so reserved
(without giving effect to any applicable conversion restrictions) to cure such
deficiency and, if necessary, to obtain the approval by its shareholders
therefor, including the authorization of such additional number of shares of
Common Stock as may be required to issue such shares in excess of the number so
reserved (either in the aggregate or as to the Debenture) or in excess of such
limitation, as the case may be.

         8.  No Impairment. The Company shall not intentionally take any action
which would impair the contractual rights and privileges of the Debenture set
forth herein or of the Holder thereof.

         9. Limitations on Holder's Obligation to Convert. Notwithstanding
anything to the contrary contained herein, no Holder shall be required to
convert any part of this Debenture in excess of the portion then convertible
into that number of shares of Common Stock specified in the Holder's
representation to the Company that, after giving effect to the shares of the
Company's Common Stock to be issued pursuant to such Conversion Notice, the
total number of shares of Common Stock deemed beneficially owned by the Holder,
together with all shares of the Company's Common Stock deemed beneficially owned
by the Holder's "affiliates" as defined in Rule 144 of the Act, would exceed
4.9% of the total issued and outstanding shares of the Company's Common Stock.

         10. Obligations Absolute. No provision of this Debenture, other than
conversion as provided herein, shall alter or impair the obligation of the
Company, which is absolute and unconditional, to pay the principal of, and
interest on, this Debenture at the time, place and rate, and in the manner,
herein prescribed.

         11. Waivers of Demand, Etc. The Company hereby expressly waives demand
and presentment for payment, notice of nonpayment, protest, notice of protest,
notice of dishonor, notice of intent to accelerate, prior notice of bringing of
suit and diligence in taking any action to collect amounts called for hereunder
and will be directly and primarily liable for the payments of all sums owing and
to be owing hereon, regardless of and without any notice (except as required by
law), diligence, act or omission as or with respect to the collection of any
amount called for hereunder.

         12. Replacement Debenture. In the event that the Holder notifies the
Company that this Debenture has been lost, stolen or destroyed, a replacement
Debenture identical in all respects to the original Debenture (except for
registration number and Outstanding Amount, if different than that shown on the
original Debenture) shall be issued to the Holder, provided that the Holder
executes and delivers to the Company an agreement reasonably satisfactory to the
Company to indemnity the Company from any loss incurred by it in connection with
the Debenture and provided that the Company is provided a form of Debenture for
such replacement purposes.


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         13. Payment of Expenses. The Company agrees to pay all debts and
expenses, including reasonable attorneys' fees and expenses, which may be
incurred by the Holder in enforcing the provisions of this Debenture and/or
collecting any amount due under this Debenture, the Subscription Agreement or
the Registration Rights Agreement.

         14. Defaults. If one or more of the following events (hereinafter
called "Events of Default") shall occur:

             (a) The Company shall fail to perform or observe any covenant or
agreement in the Registration Rights Agreement or any other covenant, term
provision, condition, agreement or obligation of the Company under this
Debenture, and such failure shall continue uncured for a period of ten (10)
business days after notice from the Holder of such failure, or the Company shall
fail to make any payments upon redemption of this Debenture or fail to issue
shares of Common Stock upon conversion of this Debenture; or

             (b) The Company shall (i) make a general assignment for the benefit
of creditors or commence proceedings for its dissolution; or (ii) apply for or
consent to the appointment of a trustee, liquidator or receiver for it or for a
substantial part of its property or business; or

             (c) A trustee, liquidator or receiver shall be appointed for the
Company or for a substantial part of its property or business without its
consent and shall not be discharged within sixty (60) days after such
appointment; or

             (d) Any governmental agency or any court of competent jurisdiction
shall assume custody or control of the whole or any substantial portion of the
properties or assets of the Company and shall not be dismissed within sixty (60)
days thereafter, or

             (e) Bankruptcy, reorganization, insolvency or liquidation
proceedings or other proceedings, or relief under any bankruptcy law or any law
for the relief of debt shall be instituted by or against the Company and, if
instituted against the Company, shall not be dismissed within sixty (60) days
after such institution, or the Company shall by any action or answer approve of,
consent to, or acquiesce in any such proceedings or admit to any material
allegations of, or default in answering a petition filed in, any such
proceeding;

then, or at any time thereafter prior to the date on which all continuing Events
of Default have been cured, and in each and every such case, unless such Event
of Default shall have been waived in writing by the Holder (which waiver shall
not be deemed to be a waiver of any subsequent default) at the option of the
Holder and in the Holder's sole discretion, the Holder may, by notice to the
Company declare this Debenture immediately due and payable, and the Holder may
immediately, and without expiration of any period of grace, enforce any and all
of the Holder's rights and remedies provided herein or any other rights or
remedies afforded by law. In such event, the Debenture shall be redeemed at a
redemption price per Debenture equal to the Outstanding Amount.


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         15. Savings Clause. In case any provision of this Debenture is held
by a court of competent jurisdiction to be excessive in scope or otherwise
invalid or unenforceable, such provision shall be adjusted rather than voided,
if possible, so that it is enforceable to the maximum extent possible, and the
validity and enforceability of the remaining provisions of this Debenture will
not in any way be affected or impaired thereby.

         16. Entire Agreement. This Debenture and the agreements referred to in
this Debenture constitute the full and entire understanding and agreement
between the Company and the Holder with respect to the subject hereof. Neither
this Debenture nor any term hereof may be amended, waived, discharged or
terminated other than by a written instrument signed by the Company and the
Holder.

         17. Assignment, Etc. The Holder may, transfer or assign this Debenture
or any interest herein (but in no event in an amount less than $100,000 in
Outstanding Amount or, if less than $100,000, the total Outstanding Amount
hereof) and may mortgage, encumber or transfer any of its rights or interest in
and to this Debenture or any part hereof, and each assignee, transferee and
mortgagee (which may include any affiliate of the Holder) shall have the right
to so transfer or assign its interest; provided, however, that before the
Registration Statement contemplated by the Registration Rights Agreement becomes
effective, Holder will furnish the Company with an opinion of counsel to the
effect that such assignment, transfer, mortgage or other encumbrance is exempt
from the registration requirements under the Securities Act and without limiting
the generality of the foregoing will not violate, and will not cause the Company
to be in violation of Rule 152 promulgated under the Securities Act. Each such
assignee, transferee and mortgagee shall have all of the rights and obligations
of the Holder under this Debenture. The Company agrees that, subject to
compliance with the Subscription Agreement, after receipt by the Company of
written notice of assignment from the Holder or from the Holders' assignee, all
principal, interest, and other amounts which are then due and thereafter become
due under this Debenture shall be paid to such assignee at the place of payment
designated in such notice. This Debenture shall be binding upon the Company and
its successors and shall inure to the benefit of the Holder and its successors
and assigns.

         18. No Waiver. No failure on the part of the Holder to exercise, and no
delay in exercising, any right, remedy or power hereunder shall operate as a
waiver thereof, nor shall any single or partial exercise by the Holder of any
right, remedy or power hereunder preclude any other or future exercise of any
other right, remedy or power. Each and every right, remedy or power hereby
granted to the Holder or allowed it by law or other agreement shall be
cumulative and not exclusive of any other, and may be exercised by the Holder
from time to time.

         19. Miscellaneous. Unless otherwise provided herein, any notice or
other communication to a party hereunder shall be deemed to have been duly given
if personally delivered or sent by registered or certified mail, return receipt
requested, postage prepaid with a copy in each case sent on the same day to the
party by facsimile, Federal Express or other such expedited means to said party
at its address set forth herein or such other address as either may designate
for itself in such notice to the other and communications shall be deemed to
have been received when delivered


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personally or, if sent by mail or Federal Express, when actually received by the
party to whom it is addressed. Copies of all notices to the Company shall be
sent to:

                  DynaGen, Inc.
                  840 Memorial Drive
                  Cambridge, MA  02139
                  Attention:  President
                  Telephone:  (617) 491-2527
                  Telecopy:   (617) 354-3902

                  with a copy to:

                  Foley, Hoag & Eliot LLP
                  One Post Office Square
                  Boston, MA 02109
                  Attention:  David A. Broadwin, Esq.
                  Telephone:  (617) 832-1259
                  Telecopy:   (617) 832-7000

                  If to the Investor, to:

                  Sovereign Partners
                  Executive Pavilion
                  90 Grove Street
                  Richfield, CT 06877

Whenever the sense of this Debenture requires, words in the singular shall be
deemed to include the plural and words in the plural shall be deemed to include
the singular. Paragraph headings are for convenience only and shall not affect
the meaning of this document.

         20. Choice of Law and Venue: Waiver of Jury Trial. THIS DEBENTURE SHALL
BE CONSTRUED UNDER THE LAWS OF THE STATE OF DELAWARE, WITHOUT REGARD TO
PRINCIPLES OF CONFLICTS OF LAW OR CHOICE OF LAW THEREOF. The parties hereby (i)
irrevocably submit to the exclusive jurisdiction of the United States District
Court for the Southern District of New York for the purposes of any suit, action
or proceeding arising out of or relating to this Debenture and (ii) waive, and
agree not to assert in any such suit, action or proceeding, any claim that it is
not personally subject to the jurisdiction of such court, that the suit, action
or proceeding is brought in an inconvenient forum or that the venue of the suit,
action or proceeding is improper. The Company consents to process being served
in any such suit, action or proceeding by mailing a copy thereof to the Company
at the address in effect for notices to it under this Debenture and agrees that
such service shall constitute good and sufficient service of process and notice
thereof. Nothing in this paragraph shall affect or limit any right to serve
process in any other manner permitted by law.


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         IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed by an officer thereunto duly authorized.

                                      Dated as of: February 26, 1998



                                      DYNAGEN, INC.



                                      By: /s/ Jay Wadekar
                                      Name:    Dhananjay G. Wadekar
                                      Title:



         The Holder agrees to be bound by all the provisions of this Debenture
applicable to it.

                                      SOVEREIGN PARTNERS


                                      By: /s/ Stephen Hicks
                                      Name:   Stephen Hicks
                                      Title:


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