Effect of phenyl ring position in the C-alpha-methylated alpha-amino acid side chain on peptide preferred conformation

The preferred conformations of C-alpha-methyl phenylglycine, C-alpha-methyl phenylalanine, and C-alpha-methyl homophenylalanine residues, as determined in model peptides (including homo-peptides) by Fourier transform ir absorption, H-1-nmr, CD, and x-ray diffraction techniques, are compared with the aim of investigating the effect of phenyl ring position in the C-alpha-methylated amino acid side chain. This study shows that (a) beta-turn and 3(10)-helical structures are preferentially adopted by peptides rich in these C-alpha-methylated, aromatic alpha-amino acids and (b) turn and helix handedness is critically biased by the position of side-chain branching. (C) John Wiley & Sons, Inc.