On the majority rule in the helix sense bias of syndiotactic homo-peptides from C-alpha-methyl leucine

The terminally blocked, syndiotactic, linear homo-peptides from the C-alpha-tetrasubstituted alpha-amino acid C-alpha-methylleucine of general formula pBrBz-[(alpha Me)Leu](n)-OR (n=2-5; R=H, OtBu) [(D-L)(x)-(D)(y), with x=1, 2 and y=0, 1](a) have been synthesized by solution methods and fully characterized. In solution, the longest peptides appear to adopt predominantly an intramolecularly H-bonded, right-handed helical structure, as assessed by IR absorption and CD techniques. While the tripeptide ester pBrBz-D-(alpha Me)Leu-L-(alpha Me)Leu-D-(alpha Me)Leu-OtBu does not assume any regular secondary structure in the crystal state, as determined by X-ray diffraction analysis, the tripeptide free acid pBrBz-D-(alpha Me)Leu-L-(alpha Me)Leu-D-(alpha Me)Leu-OH and the pentapeptide ester pBrBz-[D-(alpha Me)Leu-L-(alpha Me)Leu](2)-D-(alpha Me)Leu-OtBu are folded in (incipient) right-handed 3(10)-helices. These latter results indicate that a helix screw sense bias can be produced in homo-peptides even by a limited excess of one enantiomer of the constituent amino acid.