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Abstract The crystal-state preferred conformations of two tripeptides, one tetrapeptide, and one pen- tapeptide, each containing a single residue of the chiral, Cα,α-disubstituted glycine Cα-methyl, Cα-benzylglycine [(αMe)Phe], have been determined by X-ray diffraction. The tripeptides are Z-L-(αMe)Phe-(Aib)2-OH dihydrate and Z-Aib-D-(αMe)Phe-Aib-OtBu, the tetrapeptide is Z-(Aib)2-D-(αMe)Phe-Aib-OtBu, and the pentapeptide is pBrBz-(Aib)2-DL-(αMe)Phe-(Aib)2-OtBu. While the two tripeptides are folded in a β-bend conformation, two such conformations are consecutively formed by the tetrapeptide. The pentapeptide adopts a regular 310-helix promoted by three consecutive β-bends. This study confirms the strong propensity of short peptides containing Cα-methylated α-aminoacids to fold into β-bends and 310-helical structures. Since Aib is achiral, the handedness of the observed bends and helices is dictated by the presence of the (αMe)Phe residue. In general, we have found that the relationship between (αMe)Phe chirality and helix handedness is opposite to that exhibited by protein aminoacids. A comparison with the preferred conformation of other extensively investigated Cα-methylated aminoacids is made.
