(a) Permitted agreements, relating to bank shares. A national bank may require a borrower holding shares of the bank to execute agreements:

(1) Not to pledge, give away, transfer, or otherwise assign such shares;

(2) To pledge such shares at the request of the bank when necessary to prevent loss; and

(3) To leave such shares in the bank's custody.

(b) Use of capital notes and debentures. A national bank may not make loans secured by a pledge of the bank's own capital notes and debentures. Such notes and debentures must be subordinated to the claims of depositors and other creditors of the issuing bank, and are, therefore, capital instruments within the purview of 12 U.S.C. 83.