Conformational induction in copolypeptides of the form AnBm.

Abstract

AbstractThe polymerization kinetics and optical rotatory properties of AnBm polypeptides have been studied, where A = D,L‐Tyr, D,L‐TyrZ, D,L‐LysZ, L‐AspOBzl, or L‐Asp‐ONBzl, and B = D,L‐GluOR (R = Me or Bzl). In most cases where An and Bm prefer the same helix sense, the polymerization of A N‐carboxyanhydride (initiated by Bm in dioxane) shows first order kinetics and produces a monotonic change in optical rotation, while if opposite helix senses are preferred, the kinetics are multiphasic and the change in rotation reverses direction after the addition of several residues. The rotation change in the latter case is interpreted to mean that the helix in the A block is initially induced to take the nonpreferred sense, as originally suggested by Doty and Lundberg from similar observations on (D‐GluOBzl)n‐(L‐GluOBzl)m. It is found here that the CD spectra for the latter polymer show the sign changes required by this hypothesis. The optical rotation curves and CD spectra for (D,L‐Tyr)n‐(L‐GluOBzl)20 suggest, by analogy, that (L‐Tyr)n prefers the same helix sense as (L‐GluOBzl)n. However, it is found that the opposite conclusion is equally consistent with the data if one considers the effects of possible changes in side‐chain conformation on these data in accordance with the calculated CD spectra of Chen and Woody. The optical rotation curves for (D‐GluOBzl)n‐(L‐GluOBzl)20, (D‐Tyr)n‐(L‐GluOBzl)20, and (L‐Tyr)n‐(L‐GluOBzl)20 are all found to be consistent with a two‐state equilibrium model in which the A block initially takes on an induced conformation and has an increasing tendency to revert to its preferred conformation as n increases. It is concluded that in both D‐Tyr and L‐Tyr the side‐chain and/or the backbone conformation is induced by the neighboring L‐GluOBzl block, and the data do not distinguish which type of change is occurring. These results are discussed in connection with other observations bearing on the helix sense of (L‐Tyr)n.