Comparison of methods for the Fmoc solid-phase synthesis and cleavage of a peptide containing both tryptophan and...

A major side reaction which can occur during the synthesis of Trp-containing peptides is modification of the Trp indole by reactive carbonium ion species released during acidolytic cleavage. [Asn2,Trp4]Dynorphin A-(1-13), a sequence which is very susceptible to Trp modification, was chosen as a model peptide to compare the effectiveness of various methods proposed to minimize Trp modification during Fmoc solid-phase synthesis. The peptide was synthesized with the side chain of Trp unprotected and cleaved by Reagent K [82.5% trifluoroacetic acid (TFA)/5% phenol/5% water/5% thioanisole/2.5% ethanedithiol (EDT)] [King, D.S. et al. (1990) Int. J. Peptide Protein Res. 36, 255-266], Reagent R [90% TFA/5% thioanisole/3% EDT/2% anisole] [Albericio, F. et al. (1990) J. Org. Chem. 55, 3730-3743], TFA containing 20% EDT and 4% water [Riniker, B. & Hartmann, A. (1990) in Peptides: Chemistry, Structure, and Biology (Rivier, J.E. & Marshall, G.R., eds.), pp. 950-952, Escom, Leiden], and TFA containing trialkylsilane, MeOH, and ethylmethyl sulfide [Chan, W.C. & Bycroft, B.W. (1992) in Peptides: Chemistry, Structure, and Biology, Op. cit., pp. 613-614]. Cleavage with Reagent K, Reagent R and TFA containing 20% EDT and 4% water yielded similar results; in addition to the desired peptide, the crude product contained 22-30% of a side product which appeared to result from Trp modification by a Pmc group. Cleavage with the trialkylsilane-containing mixture gave the lowest recovery of the desired peptide and the highest levels of Pmc-containing peptides.(ABSTRACT TRUNCATED AT 250 WORDS)