Macromolecular Stereochemistry of Poly(P-biphenylmethyl l-glutamate): Linkage Between Biphenyl Twist Sense and Polypeptide Conformation and Observation of Microaggregation-Driven, Sudden, Temperature-Dependent Chiral Optical Changes

Michael P. Reidy, Mark M. Green

Research output: Contribution to journalArticle

Abstract

In the expectation that the normally equally populated left- (M) and right-handed (P) twist senses of a biphenyl moiety would, in the side chain of a polypeptide, be perturbed by the helical conformation of the main chain and therefore potentially reveal characteristics of the side-chain conformation, we have carried out the syntheses of the ortho- and para-substituted biphenylmethyl esters of poly(L-glutamic acid) and poly(L-aspartic acid). In the aspartate series the appropriate N-carboxyanhydrides yielded polymer for both the ortho- and para-substituted biphenylmethyl esters. The para isomer was not soluble in solvents appropriate to ultraviolet circular dichroism measurements while the ortho isomer indicated the absence of the a-helical conformation. In the glutamate series although we could not obtain a crystalline iV-carboxyanhydride from the ortho isomer, in the para isomer we were able to obtain a high polymer, which, in a variety of solvents, both adopts the a-helical conformation and displays a strong positive circular dichroism band associated with the biphenyl chromophore. This chiral optical characteristic is almost entirely lost on dichloroacetic acid denaturation of the polypeptide. As a complete surprise, dilute solutions of poly(pbiphenylmethyl L-glutamate) in several helicogenic solvents, with the notable exception of chloroform, exhibit a sudden, large, almost concentration-independent, reversible change in their optical activity properties just below 0 °C. Intrinsic viscosity and light-scattering measurements point to aggregation associated with this change while ultraviolet, infrared, and excimer fluorescence measurements offer no evidence for significant conformational changes of the polypeptide chain or of the relationships among the biphenyl groups.

Original languageEnglish (US)
Pages (from-to)4225-4234
Number of pages10
JournalMacromolecules
Volume23
Issue number19
DOIs
StatePublished - Jan 1 1990

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ASJC Scopus subject areas

  • Organic Chemistry
  • Polymers and Plastics
  • Inorganic Chemistry
  • Materials Chemistry

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