| Abstract: | The molecular and crystal structures of the Cα-tetrasubstituted, δ-branched α-amino acid Cα-methyl-homophenylalanine, H-d -(αMe)Hph-OH, and three peptides (to the pentamer level), including the homotripeptide, have been determined by X-ray diffraction. The peptides are Z-l -(αMe)Hph-(l -Ala)2-OMe, pBrBz-d -(αMe)Hph]3-OtBu and Ac-(Aib)2-l -(αMe)Hph-(Aib)2-OtBu. All the (αMe)Hph residues prefer φ,ψ torsion angles in the helical region of the conformational map. The two terminally blocked tripeptides adopt a β-bend conformation stabilized by a 1→4 C = O?H-N intramolecular H-bond. The terminally blocked pentapeptide is folded in a regular 310-helix. In general, the relationship between (αMe)Hph α-carbon chirality and helix handedness is the same as that exhibited by protein amino acids. A comparison is also made with the conclusions extracted from published work on peptides from other types of Cα-alkylated aromatic α-amino acids. © Munksgaard 1996. |
