Random coil configurations of polypeptide copolymers

Abstract

The average dimensions of unperturbed, random-coil polypeptide copolymers composed either of l -alanine (or other residues having -CH 2 R′ side chains) and glycine or of d - and l -alanine are treated theoretically, and results of numerical calculations are presented. The effect of amino acid sequence as well as over-all composition is investigated. Introduction of minor proportions of glycine residues into a stereoregular (i.e., all d or all l ) polymer having -CH 2 R′ side chains brings about a disproportionately large decrease in the average chain dimensions. The dimensions are also sensitive to the degree of regularity in the stereoconfigurations of the substituted α-carbons of successive residues. In random-sequence polypeptides, the unperturbed dimensions decrease monotonously with the degree of racemization. In a racemic dl -polypeptide the chain dimensions are predicted to be strongly dependent on the distribution of d - and l -residues. As the sequence distribution is changed to give successively pure stereopolymer, block copolymer, random copolymer, and finally alternating d - and l -copolymer, the chain dimensions are predicted to fall continuously; mean square dimensions calculated for the alternating d - l -copolymer are less than one-tenth of those for the stereoregular (e.g., all l -) polymer. Preliminary experimental results are presented for random d - l -polyglutamates and glycine-glutamic acid copolymers for comparison with the theoretical predictions. The major results of theory are substantiated.