Linear oligopeptides. 277. 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 (αMe)Val, a C α,α -disubstituted glycine, were prepared by solution methods and fully characterized. The preferred conformation in CHCl 3 solution was assessed by FTIR and 1 H NMR as a function of concentration and addition of perturbing agents. The molecular and crystal structures of the dipeptide and tripeptide amides, Z-(D-(αMe)Val] n -NHiPr (n= 2, 3) were also determined by X-ray diffraction. While the dipeptide amide adopts a type-III' β-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