Exhibit 8.1

Loeb & Loeb LLP   10100 Santa Monica Blvd.
Suite 2200
Los Angeles, CA 90067


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November 3, 2021

Petra Acquisition, Inc.

5 West 21st Street

New York, NY 10010

Re: Registration Statement of Petra Acquisition, Inc.

Ladies and Gentlemen:

We have acted as United States counsel to Petra Acquisition, Inc. (“Petra”), in connection with the proposed business combination (the “Business Combination”) contemplated by an agreement and plan of merger, dated as of August 29, 2021 (as it may be amended from time to time, the “Merger Agreement”), which provides for a Business Combination between Petra, Petra Acquisition Merger Inc., a Delaware corporation and a wholly-owned subsidiary of Petra (“Merger Sub”), and Revelation Biosciences, Inc., a Delaware corporation (“Revelation”). Pursuant to the Merger Agreement, Merger Sub will merge with and into Revelation with Revelation surviving the merger as a wholly owned subsidiary of Petra.

The Business Combination and certain other related transactions are described in the Registration Statement of Petra on Form S-4 under the Securities Act of 1933, as amended (the “Securities Act”), filed on September 17, 2021 (Registration Number 333-259638) as amended through the date hereof (the “Registration Statement”).

In rendering this opinion, we have reviewed the Registration Statement and have assumed with your approval the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as copies, and the completeness and accuracy of the documents reviewed by us. We have assumed with your approval and have not verified the accuracy of the factual matters and representations set forth in the Registration Statement.

This opinion addresses only certain U.S. federal tax consequences of the Business Combination to certain holders of Petra securities.

Based on the foregoing and subject to the assumptions, limitations and qualifications stated in the Registration Statement and herein, we hereby confirm and adopt as our opinion the statements of United States federal income tax law on the date hereof as set forth in the Registration Statement under the captions “Material U.S. Federal Income Tax Consequences—U.S. Holders—Certain U.S. Federal Income Tax Consequences to U.S. Holders of Petra securities of Exercising Redemption Rights” and “Material U.S. Federal Income Tax Consequences—U.S. Holders—U.S. Federal Income Tax Consequences of Ownership and Disposition of Petra securities.”

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For the United States offices, a limited liability partnership including professional corporations. For Hong Kong office, a limited liability partnership.

Petra Acquisition, Inc. November 3, 2021 Page 2

This opinion is based upon the existing provisions of the Internal Revenue Code of 1986, as amended, Treasury Regulations promulgated thereunder, published revenue rulings and procedures from the United States Internal Revenue Service (“IRS”) and judicial decisions, all as in effect on the date hereof. Any such authority is subject to change, and any change may be retroactive in effect and may affect our opinion as set forth herein. Our opinion is based on the facts, assumptions and representations set forth in the Registration Statement and this opinion. If any of the facts, assumptions or representations is not true, correct or complete, our opinion may not be applicable. We undertake no responsibility to update this opinion or to advise you of any developments or changes as a result of a change in legal authority, fact, representation, assumption or document, or any inaccuracy in any fact, representation or assumption, upon which this opinion is based, or otherwise.

Our opinion is not binding on the IRS or a court. The IRS may disagree with one or more of our conclusions, and a court may sustain the IRS’s position

Very truly yours,

/s/ Loeb & Loeb LLP

Loeb & Loeb LLP