Application of tryptic condensation strategy for the synthesis of α-melanocyte stimulating hormone

The use of trypsin in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) paired with N,N-dimethylformamide (DMF) has been proposed for the enzymatic condensation of peptides. The tryptic condensation strategy was applied to the synthesis of the 13-peptide, α-melanocyte stimulating hormone (α-MSH), which contains two susceptible points to trypsin, in a low-water-containing solvent system, 4% H₂O in HFIP/DMF (1/1, v/v). The N-terminal 8-peptide segment (S1) ending with Arg and C-terminal 5-peptide with the side-chain protections at Lys and Trp (S2) were prepared by the stepwise coupling on p-nitrobenzophenone oxime resin and by a conventional method, respectively. Both segments were condensed by the aid of trypsin. The acid component was converted into the partially protected α-MSH at as high as 95% conversion determined by reversed-phase HPLC. When the side-chain of Lys at the 11-position was not protected, α-MSH was obtained only in 35% yield, and was contaminated with products of secondary hydrolysis. Although the Lys¹¹-Pro¹² sequence was very poorly susceptible to trypsin, the side-chain of Lys had to be protected in order to be inert to trypsin under the synthetic conditions. HFIP is demonstrated as a good solvent with DMF to allow the efficient tryptic condensation of peptides. The strategy increased the value of the enzymatic condensation as practical method by avoidance of secondary hydrolysis, high dissolution of peptides and retention of activity of enzyme.