Block sequential polypeptides of L‐alanine and glycine with D, L‐glutamic acid

Sequential polypeptides of L ‐alanine(A) and glycine(G), which were incorporated between two blocks of poly( D , L ‐glutamic acid) (DL), were synthesized by applying Merri‐field's solid‐phase method. On the basis of optical rotatory dispersion criteria, DL(A) 38 ‐DL was found to assume the α‐helix in the whole range of the water‐methanol system; whereas other block sequential polypeptides were found to assume the random‐coiled conformation in water and partly the α‐helix at the high methanol content. The stability of the α‐helix decreased in the order: DL(A) 38 DL, DL(A 2 G) 10 DL, DL(A 2 G) 6 DL, and DL(A 3 G) 7 DL. This phenomenon may be explained in terms of the dependence of hydrophobic bonding between the C 3 H group of the ith L ‐alanine regularly arranged on the surface of the α‐helix and the C 2 H group of the ( i + 3)th residue on whether the residue is alanine or glycine. The role which the methanol plays in stabilizing the α‐helix is also discussed.