Nucleophilic efficiency of free amino acids in chymotrypsin-catalyzed acyl transfer in ice at -18°C using ethyl esters of N-maleyl-r.-tyrosine and L-tyrosine as the acyl group donors has been studied. Although the amino acids did not act as acyl acceptors in liquid water, the high yields of peptides were obtained in frozen solutions at pH 10.5 (before freezing). The efficiency of amino acids in the formation of the corresponding dipeptides depended on the substrate used, and decreased in the order Ser,Thr,Gln>Lys>Cit>Ala>Gly>Asn>Arg>Glu>V~>Om>Asp (with no peptide formed with His, Leu, Be and Pro) for N-maleyl-L-tyrosine ethyl ester and Ser>Lys>Om>Arg,Cit>Gln>Thr>Asn>Ala>Gly (with no peptide formed with Glu, Val, Asp, His, Leu, Be and Pro) for L-tyrosine ethyl ester. a-Chymotrypsin; Nucleophile specificity; Frozen solution

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