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Mechanisms and Prevention of Trifluoroacetylation in Solid-Phase Peptide Synthesis


A novel mechanism for trifluoroacetylation in custom peptide synthesis, independent of the coupling step, has been elucidated. It involves the presence of trifluo roacetoxymethyl groups on the resin support, which react with resin-bound amines by an intersite nucleophilic reaction. The trifluoroacetoyymethyl groups are generated from preexistinghydrorymethyl sites during treatment with trifluorpacetic acidin dichloromethane or by scidolysis of the benzyl ester bond
between the peptide and the resin. The transfer of triluoro acetyl from hydrosyI to amine occurs during the subsequent neutralization with tertiary amine. The mechanism was first elucidated by model studies w ith aminomethyl-resins. Then the expected transfer of trifluoroacetyl groups from trifuoroace toymethyl-resin to the a-amino group of N-benzyloxycar boallysine benzyl ester in solution was demonstrated; k2.6X10- M-1 see-!. Lysine-resins were used to examine the extent of tifluoroacetylation under the conditions of  custom peptide synthesis. Afher a series of acid/base .cycles simulating synthetic conditions but without coupling, the poorly nucleo-philic aamino group was approximately 1-2% trifluoroacety lated per cycle W hen attached to resins already containing hy droxymethy! groups. Slandard benzyl ester resins withoul preesisting hydroxymethyl groups gave comparable levels of frifluoroacetylation after the first few synthetic eyeles because of gradual acid cleavage of the ester and accumulation of trifluoroacetoxymethyl sites Peptide chain termination resuting from trifluoroacetylation by this mechanism could be prevented (<0.02% per eycle) by the use of the aminoaeyl-4orymethy)
phenylacetamidomethy-resin support, which can be synthe sized free from extraneous functionalities and which is stable to tifluoroacetic acid under the conditions of solid-phase pep
tide synthesis.

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