In the biosynthesis of proteins, each amino acid passes from the aminoacyl adenylate to become an amino acid ester and finally a 2′ (3′) peptidyl ester of the AMP residue at the end of a tRNA. Consequently, the chemistry of protein synthesis is the chemistry of aminoacyl and peptidyl AMP. Our data has revealed properties of 5′-AMP and its esters which should allow the preferential catalytic synthesis of l-amino acid peptides via a bis(2′, 3′-aminoacyl) ester intermediate. Results in this paper concern one step in the proposed process and show that preexisting Ac-l-Phe monoester reacts about 2.5-times faster to form diester than preexisting Ac-d-Phe monoester. © 1991.