Identification of coupling conditions proceeding with low C-terminal epimerization during the assembly of fully protected backbone-substituted peptide segments

The coupling of Fmoc-Asp(OBu(t))-L-Phe-OH 1 and H-Lys(Boc)-PepsynKA 2 was used as a model to assess C-terminal epimerization in solid-phase segment condensation. A wide range of coupling reagents and reaction conditions were examined and an optimal combination of good coupling rate along with low phenylalanine epimerization was achieved by using 1-hydroxybenzotriazole-catalysed diisopropylcarbodiimide reaction in dichloromethane. Application of these conditions in the coupling of backbone protected HIV-1(Bru) tat protein related segments(13-mer + resin-bound 16-mer) gave quantitative reaction within 6 h, and a maximum of < 3% epimerization of C-terminal lysine.