ARCA BIOPHARMA, INC. WARRANT TO PURCHASE COMMON STOCK

Exhibit 4.19

THIS WARRANT AND THE SECURITIES ISSUABLE UPON EXERCISE HEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”), OR ANY STATE SECURITIES LAW, AND MAY NOT BE SOLD, TRANSFERRED, ASSIGNED, PLEDGED, HYPOTHECATED OR OTHERWISE DISPOSED OF OR EXERCISED UNLESS (I) A REGISTRATION STATEMENT REGISTERING SUCH SECURITIES UNDER THE SECURITIES ACT AND APPLICABLE STATE SECURITIES LAWS SHALL HAVE BECOME EFFECTIVE, OR (II) THE CORPORATION HAS RECEIVED AN OPINION OF COUNSEL SATISFACTORY TO IT THAT SUCH TRANSFER MAY LAWFULLY BE MADE WITHOUT REGISTRATION UNDER THE SECURITIES ACT OR QUALIFICATION UNDER APPLICABLE STATE SECURITIES LAWS, OR (III) SUCH SECURITIES ARE SOLD PURSUANT TO RULE 144.

AN INVESTMENT IN THESE SECURITIES INVOLVES A HIGH DEGREE OF RISK. HOLDERS MUST RELY ON THEIR OWN ANALYSIS OF THE INVESTMENT AND ASSESSMENT OF THE RISKS INVOLVED.

ARCA BIOPHARMA, INC.

WARRANT TO PURCHASE COMMON STOCK

This certifies that, for value received, BioMed Realty, L.P., with its principal office at 17190 Bernardo Center Drive, San Diego, California 92128, or its permitted assigns (the “Holder”), is entitled to subscribe for and purchase at the Exercise Price (as defined below) from ARCA biopharma, Inc., a Delaware corporation, with its principal office at 8001 Arista Place, Suite 200, Broomfield, Colorado 80021 (the “Corporation”), up to a number of Exercise Shares (as defined below), upon the terms and subject to the adjustments as provided herein.

This Warrant is being issued pursuant to the terms of the Lease Termination and Warrant Purchase Agreement, dated September 18, 2009 (the “Lease Termination and Warrant Purchase Agreement”) by and among the Corporation, BMR-201 Industrial Road LLC, a Delaware limited liability company, and the Holder. Capitalized terms used herein but not otherwise defined shall have the meanings given to them in the Lease Termination and Warrant Purchase Agreement.

1. Definitions. As used herein, the following terms shall have the following respective meanings:

(a) “Exercise Period” shall mean the time period commencing with the date of this Warrant and ending on the earlier of (i) the date that is seven (7) years from date of this Warrant, or (ii) the commencement of any liquidation, bankruptcy, insolvency, dissolution or winding-up (or the occurrence of any analogous proceeding) of the Corporation.

(b) “Exercise Price” shall mean $3.82 per share subject to adjustment pursuant to the terms herein, including Section 5 below.

(c) “Exercise Shares” shall mean 130,890 shares of common stock of the Corporation (the “Common Stock”) subject to adjustment pursuant to the terms herein, including Section 5 below.

2. EXERCISE OF WARRANT.

2.1 IN GENERAL. The rights represented by this Warrant may be exercised in whole or in part at any time during the Exercise Period, by delivery of the following to the Corporation at its address set forth above (or at such other address as it may designate by notice in writing to the Holder):

A. an executed Notice of Exercise in the form attached hereto;

B. payment of the Exercise Price either (i) in cash or by check, or (ii) pursuant to Section 2.2 below; and

C. this Warrant.

Upon the exercise of the rights represented by this Warrant, a certificate or certificates for the Exercise Shares so purchased, registered in the name of the Holder or persons affiliated with the Holder, if the Holder so designates, shall be issued and delivered to the Holder as soon as practicable after the rights represented by this Warrant shall have been so exercised.

The person in whose name any certificate or certificates for Exercise Shares are to be issued upon exercise of this Warrant shall be deemed to have become the holder of record of such shares on the date on which this Warrant was surrendered and payment of the Exercise Price was made, irrespective of the date of delivery of such certificate or certificates, except that, if the date of such surrender and payment is a date when the stock transfer books of the Corporation are closed, such person shall be deemed to have become the holder of such shares at the close of business on the next succeeding date on which the stock transfer books are open. In the event that this Warrant is exercised in respect of fewer than all of the Exercise Shares issuable on such exercise at any time prior to the date of expiration of this Warrant, a new certificate evidencing the remaining Warrant will be issued, in a form substantially identical hereto, in the name of the Holder, and delivered to the Holder or to another person that the Holder has designated for delivery as soon as practicable.

This Warrant shall be null and void, and the rights represented hereby shall automatically expire immediately upon the expiration of the Exercise Period.

2.2 Net Exercise. Notwithstanding any provisions herein to the contrary, if the fair market value of one share of the Corporation’s Common Stock is greater than the Exercise Price (at the date of calculation as set forth below), in lieu of exercising this Warrant by payment of cash, the Holder may elect to receive shares equal to the value (as determined below) of this Warrant (or the portion thereof being canceled) by surrender of this Warrant at the principal office of the Corporation together with the properly endorsed Notice of Exercise, in which event the Corporation shall issue to the Holder a number of shares of Common Stock computed using the following formula:

Where:

For purposes of the above calculation, for so long as the Corporation’s Common Stock is traded in a public market, the fair market value of each share of Common Stock shall be the closing price of a share of Common Stock for the business day immediately before the day the Holder delivers its Notice of Exercise to the Corporation. If the Corporation’s Common Stock is not traded in a public market, the fair market value of one share of Common Stock shall be determined by the Corporation’s Board of Directors in good faith.

3. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE CORPORATION. The Corporation hereby represents and covenants to the Holder as of the date hereof, and with respect to Sections 3.3 and 3.5 for so long as the Holder holds the Warrant or any Exercise Shares, as follows:

3.1 Covenants as to Exercise Shares. The Corporation covenants and agrees that all Exercise Shares that may be issued upon the exercise of the rights represented by this Warrant will, upon issuance, be validly issued and outstanding, fully paid and nonassessable, and free from all taxes, liens and charges with respect to the issuance thereof. The Corporation further covenants and agrees that the Corporation shall at all times during the Exercise Period have authorized and reserved, free from preemptive rights, a sufficient number of shares of its Common Stock to provide for the exercise of the rights represented by this Warrant.

3.2 Notices of Record Date. In the event of any taking by the Corporation of a record of the holders of any class of securities for the purpose of determining the holders thereof who are entitled to receive any dividend (other than a cash dividend which is the same as cash dividends paid in previous quarters) or other distribution, the Corporation shall mail to the Holder, at least ten (10) days prior to the date specified herein, a notice specifying the date on which any such record is to be taken for the purpose of such dividend or distribution.

3.3 Health Care / Lodging Facilities. The Corporation does not operate or manage any health care facilities (including a congregate care facility or assisted living facility) or lodging facilities or provide any person, under a franchise, license or otherwise, rights to any brand name under which any lodging facility or health care facility is operated.

3.4 Percentage of Outstanding Stock. As of the date hereof, the Exercise Shares for which this Warrant may be exercised in full represent less than five percent (5.0%) of the voting interest and less than five percent (5.0%) of the value of the outstanding stock of the Corporation.

3.5 Notification. Upon the written request of the Holder, the Corporation shall, within five (5) days confirm in writing to the Holder whether the Exercise Shares for which this Warrant may be exercised in full constitute greater than five percent (5.0%) of the voting interest and/or greater than five percent (5.0%) of the value of the outstanding stock of the Corporation.

4. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE OF HOLDER.

4.1 Acquisition of Warrant for Personal Account. The Holder represents and warrants that it is acquiring the Warrant solely for his, her or its account for investment and not with a view to or for sale or distribution of said Warrant or any part thereof, other than potential transfers between affiliates. The Holder also represents that the entire legal and beneficial interests of the Warrant and Exercise Shares the Holder is acquiring is being acquired for, and will be held for, his, her or its account only.

4.2 Securities Are Not Registered.

(a) The Holder understands that the Warrant and the Exercise Shares have not been registered under the Securities Act of 1933, as amended (the “Act”) on the basis that no distribution or public offering of the stock of the Corporation is to be effected. The Holder realizes that the basis for the exemption may not be present if, notwithstanding his, her or its representations, the Holder has a present intention of acquiring the securities for a fixed or determinable period in the future, selling (in connection with a distribution or otherwise), granting any participation in, or otherwise distributing the securities. The Holder has no such present intention.

(b) The Holder is an “accredited investor” within the meaning of Regulation D promulgated under the Act.

(c) The Holder recognizes that the Warrant and the Exercise Shares must be held indefinitely unless they are subsequently registered under the Act or an exemption from such registration is available.

(d) The Holder is aware of Rule 144 adopted under the Act and the conditions of permitted sales in reliance thereof.

4.3 Disposition of Warrant and Exercise Shares. The Holder further agrees not to make any disposition of all or any part of the Warrant or Exercise Shares in any event unless and until:

(a) The Corporation shall have received a letter secured by the Holder from the Securities and Exchange Commission stating that no action will be recommended to the Commission with respect to the proposed disposition; or

(b) There is then in effect a registration statement under the Act covering such proposed disposition and such disposition is made in accordance with said registration statement; or

(c) The Holder shall have notified the Corporation of the proposed disposition and shall have furnished the Corporation with a statement of the circumstances surrounding the proposed disposition; provided, however, that such statement will not be required if the disposition is permitted under Rule 144 of the Act, except in unusual circumstances.

5. ADJUSTMENT OF EXERCISE PRICE AND EXERCISE SHARES; EFFECT OF ORGANIC CHANGES; CUTBACKS

5.1 Adjustment of Exercise Price and/or Exercise Shares. In the event of changes in the outstanding capital stock of the Corporation by reason of stock dividends, splits, recapitalizations, reclassifications, combinations or exchanges of shares, separations, reorganizations, liquidations, or the like, the number and class of shares available under the Warrant in the aggregate and the Exercise Price shall be correspondingly adjusted to give the Holder of the Warrant, on exercise for the same aggregate Exercise Price, the total number, class, and kind of shares as the Holder would have owned had the Warrant been exercised immediately prior to the event and had the Holder continued to hold such shares until after the event requiring adjustment. The Corporation will provide prompt written notice to the Holder describing the change and the corresponding adjustment of the Exercise Price or number of Exercise Shares subject to this Warrant made pursuant to this section.

5.2 Reorganization, Reclassification, Consolidation, Merger or Sale. If any recapitalization, reclassification or reorganization of the capital stock of the Corporation, or any consolidation or merger of the Corporation with another corporation, or the sale of all or substantially all of its assets or other transaction shall be effected in such a way that holders of the Corporation’s Common Stock shall be entitled to receive stock, securities, or other assets or property, including, without limitation, upon conversion of such Common Stock (an “Organic Change”), then, as a condition of such Organic Change, lawful and adequate provisions shall be made by the Corporation whereby the Holder hereof shall thereafter have the right to purchase and receive (in lieu of the shares of the Common Stock of the Corporation immediately theretofore purchasable and receivable upon the exercise of the rights represented hereby) such shares of stock, securities or other assets or property as may be issued or payable with respect to or in exchange for a number of outstanding shares of such Common Stock equal to the number of shares of such stock immediately theretofore purchasable and receivable upon the exercise of the rights represented hereby. In the event of any Organic Change, appropriate provision shall be made by the Corporation with respect to the rights and interests of the Holder of this Warrant to the end that the provisions hereof (including, without limitation, provisions for adjustments of the Exercise Price and of the number of shares purchasable and receivable upon the exercise of this Warrant) shall thereafter be applicable, in relation to any shares of stock, securities or assets thereafter deliverable upon the exercise hereof and following such Organic Change the Corporation or its successor shall promptly issue to Holder an amendment to this Warrant reflecting such adjustments.

5.3 Certain Events. If any change in the outstanding Common Stock of the Corporation or any other event occurs as to which the other provisions of this Section 5 are not strictly applicable or if strictly applicable would not fairly protect the purchase rights of the Holder of the Warrant in accordance with such provisions, then the Board of Directors of the Corporation shall make, in good faith, an adjustment in the number and class of shares available under the Warrant, the Exercise Price or the application of such provisions, so as to protect such purchase rights as aforesaid. The adjustment shall be such as to give the Holder of the Warrant upon exercise for the same aggregate Exercise Price the total number, class and kind of shares as he would have owned had the Warrant been exercised prior to the event and had he continued to hold such shares until after the event requiring adjustment. The Corporation will provide prompt written notice to the Holder describing the change and the corresponding adjustment of the Exercise Price or number of Exercise Shares subject to this Warrant made pursuant to this section.

6. FRACTIONAL SHARES. No fractional shares shall be issued upon the exercise of this Warrant as a consequence of any adjustment pursuant hereto. All Exercise Shares (including fractions) issuable upon exercise of this Warrant may be aggregated for purposes of determining whether the exercise would result in the issuance of any fractional share. If, after aggregation, the exercise would result in the issuance of a fractional share, the Corporation shall, in lieu of issuance of any fractional share, pay the Holder otherwise entitled to such fraction a sum in cash equal to the product resulting from multiplying the then current fair market value of an Exercise Share by such fraction.

7. NO STOCKHOLDER RIGHTS. This Warrant in and of itself shall not entitle the Holder to any voting rights or other rights as a stockholder of the Corporation.

8. TRANSFER OF WARRANT. Subject to applicable laws and any restrictions on transfer set forth in this Warrant, this Warrant and all rights hereunder are transferable, by the Holder in person or by duly authorized attorney, upon delivery of this Warrant and the form of assignment attached hereto to any transferee designated by Holder.

9. LOST, STOLEN, MUTILATED OR DESTROYED WARRANT. The Corporation covenants to the Holder that, upon receipt of evidence reasonably satisfactory to the Corporation of the loss, theft, destruction or mutilation of this Warrant or any stock certificate and, in the case of any such loss, theft or destruction, upon receipt of an indemnity reasonably satisfactory to the Corporation, or in the case of any such mutilation, upon surrender and cancellation of such Warrant or stock certificate, the Corporation will make and deliver a new Warrant or stock certificate, of like tenor, in lieu of the lost, stolen, destroyed or mutilated Warrant or stock certificate.

10. NOTICES, ETC. Any notice required or permitted under this Warrant shall be given in writing and shall be deemed effectively given: (a) upon personal delivery to the party to be notified, (b) when sent by confirmed facsimile if sent during normal business hours of the recipient; if not, then on the next business day, (c) five (5) business days after having been sent by registered or certified mail, return receipt requested, postage prepaid, or (d) one (1) business day after deposit with a nationally recognized overnight courier, specifying next day delivery, with written verification of receipt. All communications shall be sent to (i) the Corporation at the address set forth above, Attention: Chief Executive Officer and General Counsel; or to (ii) the Holder at his, her or its address set forth above, or at such other address as any such party may designate by ten (10) days advance written notice to the other parties hereto.

11. ACCEPTANCE. Receipt of this Warrant by the Holder shall constitute acceptance of and agreement to all of the terms and conditions contained herein.

12. GOVERNING LAW. This Warrant and all rights, obligations and liabilities hereunder shall be governed and construed under the laws of the State of Colorado in all respects as such laws are applied to agreements among Colorado residents entered into and performed entirely within Colorado, without giving effect to conflict of law principles thereof. The Corporation and Holder agree that any action brought by any party under or in relation to this Warrant, including without limitation to interpret or enforce any provision of this Warrant, shall be brought in, and each agrees to and does hereby submit to the jurisdiction and venue of, any state or federal court located in the County of Denver, Colorado.

[Signature Page Follows]

IN WITNESS WHEREOF, the Corporation has caused this Warrant to be executed by its duly authorized officer as of October 18, 2009.

NOTICE OF EXERCISE

(1)  The undersigned hereby elects to purchase              shares of the Common Stock of ARCA biopharma, Inc. (the “Corporation”) pursuant to the terms of the attached Warrant, and tenders herewith payment of the exercise price in full, together with all applicable transfer taxes, if any.

The undersigned hereby elects to purchase              shares of the Common Stock of the Corporation pursuant to the terms of the net exercise provisions set forth in Section 2.2 of the attached Warrant, and shall tender payment of all applicable transfer taxes, if any.

(2) Please issue a certificate or certificates representing said shares of Common Stock in the name of the undersigned or in such other name as is specified below:

ASSIGNMENT FORM

(To assign the foregoing Warrant, execute this form and supply required

information. Do not use this form to purchase shares.)

FOR VALUE RECEIVED, the foregoing Warrant and all rights evidenced thereby are hereby assigned to

NOTE: The signature to this Assignment Form must correspond with the name as it appears on the face of the Warrant, without alteration or enlargement or any change whatsoever. Officers of corporations and those acting in a fiduciary or other representative capacity should file proper evidence of authority to assign the foregoing Warrant.