Mechanisms and prevention of trifluoroacetylation in solid-phase peptide synthesis

Abstract

A novel mechanism for trifluoroacetylation in solid-phase peptide synthesis, independent of the coupling step, has been elucidated. It involves the presence of trifluoroacetoxymethyl groups on the resin support, which react with resin-bound amines by an intersite nucleophilic reaction. The trifluoroacetoxymethyl groups are generated from preexisting hydroxymethyl sites during treatment with trifluoroacetic acid in dichloromethane or by acidolysis of the benzyl ester bond between the peptide and the resin. The transfer of trifluoroacetyl from hydroxyl to amine occurs during the subsequent neutralization with tertiary amine. The mechanism was first elucidated by model studies with aminomethyl-resins. Then the expected transfer of trifluoroacetyl groups from trifluoroacetoxymethyl-resin to the alpha-amino group of N(epsilon)-benzyloxycarbonyllysine benzyl ester in solution was demonstrated; k(2), 6 x 10(-4) M(-1). Lysine-resins were used to examine the extent of trifluoroacetylation under the conditions of solid-phase peptide synthesis. After a series of acid/base cycles simulating synthetic conditions but without coupling, the poorly nucleophilic alpha-amino group was approximately 1-2% trifluoroacetylated per cycle when attached to resins already containing hydroxymethyl groups. Standard benzyl ester resins without preexisting hydroxymethyl groups gave comparable levels of trifluoroacetylation after the first few synthetic cycles because of gradual acid cleavage of the ester and accumulation of trifluoroacetoxymethyl sites. Peptide chain termination resulting from trifluoroacetylation by this mechanism could be prevented (<0.02% per cycle) by the use of the aminoacyl-4-(oxymethyl)-phenylacetamidomethyl-resin support, which can be synthesized free from extraneous functionalities and which is stable to trifluoroacetic acid under the conditions of solid-phase peptide synthesis.