Studies on conformational consequences of i to i + 3 side-chain cyclization in model cyclic tetrapeptides

In an effort to explore the effect of ring size on the biologically active conformation of cyclic analogs of the mating pheromone alpha-factor (WHWLQLKPGQPMY) from Saccharomyces cerevisiae, eight cyclic tetrapeptides corresponding to the KPGQ portion of alpha-factor were synthesized. These N-alpha-acetyl/carboxyl amide terminal cyclic tetrapeptides were prepared on a 4-methylbenzhydrylamine resin using orthogonal Boc, Fmoc, OFm and OtBut protecting groups and HOBt-DIPC accelerated active esters or urethane-protected N-carboxyanhydrides. On-resin cyclization of the side-chain amino and carboxyl groups of the first and fourth residues, respectively, was performed with the BOP reagent to generate lactams containing 14-18 atoms. HF cleavage resulted in two products, the desired cyclic tetrapeptide and a major side product. All peptides were purified to near homogeneity (> 99%) by using reversed-phase HPLC and were characterized by FBMS and 1H NMR. Certain constrained cyclic tetrapeptides appear to be a mixture of isomers at room temperature as evidenced by HPLC and NMR. The major side product has been identified as a cyclo dimer, obtained as a consequence of interchain cyclization on the resin. CD analysis in several solvents gives evidence that some of the cyclic tetrapeptides exist in beta-turn conformations.