An extended planar C₅ conformation and a 3₁₀-helical structure of peptide foldamer composed of diverse α-ethylated α,α-disubstituted α-amino acids

Optically active peptide foldamers Tfa-[(S)-(αEt)Leu]-[(S)-(αEt)Nva]-Deg-[(S)-(αEt)Nle]-OEt (10) and Tfa-[(S)-(αEt)Val]-[(S)-(αEt)Leu]- [(S)-(αEt)-Nva]-Deg-[(S)-(αEt)Nle]-OEt (11) composed of diverse α-ethylated α,α-disubstituted α-amino acids were synthesized. The dominant conformation of these peptides in solution was an unusual, fully extended planar conformation, and that in the crystal state was both right-handed (P) and left-handed (M) 3₁₀-helical structures in 10 and a P 3₁₀-helical structure in 11, respectively. The preferred planar C₅ conformation of the peptides prepared from chiral α-ethylated α,α-disubstituted α-amino acids was drastically different from the 3₁₀-helical structure of the peptides prepared from chiral α-methylated α,α-disubstituted α-amino acids.