Evaluation of the trifluoromethanosulfonic acid/trifluoroacetic acid/thioanisole cleavage procedure for application in solid-phase peptide synthesis

As an extension of our investigation of peptidyl-resin linkage stability to wards different cleavage procedures used in the solid-phase peptide synthes is (SPPS) technique, the present paper evaluated the trifluoromethanesulfon ic acid (TFMSA)/trifluoroacetic acid (TFA)/thioanisole method, varying the type of resin (benzhydrylamine-resin, BHAR; methylbenzhydrylamine-resin, MB HAR and 4-(oxymethyl)-phenylacetamidomethyl-resin, PAMR) and peptide resin- bound residue (Gly and Phe). The vasoactive angiotensin II (AII, DRVYIHPF) and its /Gly(8)/-AII analogue linked to those resins used routinely in tert -butyloxycarbonyl (Boc)-SPPS chemistry were submitted comparatively to a ti me course study towards TFMSA/TFA cleavage. At VC, /Gly(8)/-AII was complet ely removed from all resins in less than 6 h, but the hydrophobic Plies moi ety-containing All sequence was only partially cleaved (not more than 15%) from BHAR or MBHAR in this period. At 25 degreesC, /Gly(8)/-AII cleavage ti me decreased to less than 2 h irrespective of the solid support. and quanti tative removal of All from PAMR and MBHAR occurred in less than 3 h. Howeve r, about 10-15 h seemed to be necessary for cleavage of All from BHAR, and in this extended cleavage reaction a significant increase in peptide degrad ation rate was observed. Regardless of the cleavage temperature used, the d ecreasing order of acid stability measured for resins was BHAR > MBHAR > PA MR. Collectively, these findings demonstrated the feasibility of applying T FMSA/TFA solution as a substitute for anhydrous HF at the cleavage step in Boc-SPPS methodology. Care should be taken however, as the cleavage efficac y depends on multiple factors including the resin, peptide sequence. the ti me and temperature of reaction.