Mag: a C-alpha-methylated, side chain unsaturated alpha-amino acid. Introduction into model peptides and conformational preferences

By a chemo-enzymatic approach we synthesized the chiral, C-alpha-methylated alpha-amino acid Mag, characterized by a side-chain C-gamma=C-delta bond. We also prepared a series of model peptides containing Mag in combination with Aib and Ala. All of the peptides were fully characterized and their conformational preference was determined in solution by FT-IR absorption and H-1 NMR investigations. X-Ray diffraction analyses of L-Mag, a derivative and three peptides are also presented. We find that this C-alpha-methylated alpha-amino acid is an excellent beta-turn and 3(10)-helix former. A peptide with two Mag residues one on top of the other after one complete turn of the 3(10)-helix has been synthesized and characterized.