Linear oligopeptides. Part 352. Synthesis, characterization and solution conformational analysis of C α -methyl-homo-phenylalanine [(αMe)Hph] containing peptides

For the first time a number of derivatives and terminally blocked model peptides (to the pentapeptide level) of the sterically demanding Cα-methyl-homo-phenylalanine, (αMe)Hph, residue have been synthesized (by solution methods) and fully characterized. The results of a solution conformational analysis, performed by using FTIR and 1H NMR spectroscopies, favour the conclusion that (αMe)Hph is as potent a β-turn and helix promoter as (αMe)Phe (Cα-methylphenylalanine) and (αEt)Phe (Cα-ethylphenylalanine), and more potent than the Phe parent amino acid. In addition, a CD study of Nαpara-bromobenzoylated peptides suggests that the relationship between (αMe)Hph α-carbon chirality and the prevailing screw sense of the turn and helical structures that are formed is opposite to that found for (αMe)Phe and (αEt)Phe peptides, i.e. L-amino acids give right-handed helicities. This relationship is the same as that exhibited by protein amino acids, including Phe.