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Using a chemo-enzymatic approach we prepared the highly lipophilic, chiral, Calpha-methylated alpha-amino acid (alphaMe)Aun. Two series of terminally protected model peptides containing either D-(alphaMe)Aun in combination with Aib or L-(alphaMe)Aun in combination with Gly were synthesized using solution methods and fully characterized. A detailed solution conformational analysis, based on FT-IR absorption, 1H NMR and CD techniques, allowed us to determine the preferred conformation of this amino acid and the relationship between chirality at its alpha-carbon atom and screw sense of the helix that is formed. The results obtained strongly support the view that D-(alphaMe)Aun favors the formation of the left-handed 3(10)-helical conformation.

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For the first time a number of terminally protected model peptides (to the pentamer level) of the sterically demanding α-amino acid C α -methyl,C α -diphenylmethylglycine, (αMe)Dip, in combination with either Ala or Gly residues, have been synthesized (by solution methods) and fully characterized. In a parallel synthesis the corresponding peptides based on the related α-amino acid C α -methyl,C α -isopropylglycine, (αMe)Val, have also been prepared. The results of a comparative conformational analysis, performed by using FTIR absorption, 1 H NMR, and X-ray diffraction techniques, favour the conclusion that, in contrast to the potent β-turn and 3 10 -helix promoter (αMe)Val, (αMe)Dip may induce either a folded or a fully extended conformation. These findings indicate that, despite the common C α -methylated and C β -branched features, (αMe)Dip and (αMe)Val are characterized by partially divergent conformational bias. † Experimental procedures for the synthesis of the new derivatives and peptides are available as supplementary data. For direct electronic access see http://www.rsc.org/suppdata/p2/a9/a909856i. See Instructions for Authors available via the RSC web page (http://www.rsc.org/ authors).The NMR spectra of the new derivatives and peptides are available as supplementary data from BLDSC (SUPPL. NO. 57694, pp. 16