Structural Versatility of Peptides From C-alpha,alpha-disubstituted Glycines - Synthesis, Characterization, and Solution and Crystal-state Conformational-analysis of Homopeptides From C-alpha-methyl-c-alpha-isopropylglycine, [(alpha-me)val]

Terminally blocked homodi- and homotripeptides from (alphaMe)Val, a C(alpha,alpha)-disubstituted glycine, were prepared by solution methods and fully characterized. The preferred conformation in chloroform solution was assessed by FT-IR and H-1 NMR as a function of concentration and addition of perturbing agents. The molecular and crystal structures of the dipeptide and tripeptide amides, Z-[D-(alphaMe)Val]n-NHiPr (n = 2, 3) were also determined by X-ray diffraction. While the dipeptide amide adopts a type-III' beta-turn conformation stabilized by a 1 <-- 4 C=O...H-N intramolecular H bond, the tripeptide amide is folded in an incipient left-handed 3(10)-helix. These results confirm that (i) the (alphaMe)Val residue is an effective beta-turn and helix promoter and (ii) the relationship between (alphaMe)Val chirality and helix screw sense is the same as that exhibited by protein amino acids. A comparison is made with the conclusions obtained from published work on homopeptides from other C(alpha)-methylated alpha-amino acids.