Conformational studies of polymers and copolymers of <scp>L</scp>‐aspartate esters. I. Preparation and solution studies

Abstract

Abstract An alcoholysis method is described for the modification of high molecular weight poly(β‐benzyl L ‐asparatate); by this method the benzyl groups in the polypeptide have been replaced by methyl, ethyl, isopropyl, n ‐propyl, and phenethyl groups to give a series of copolymers of each of the corresponding aspartate esters with benzyl L ‐aspartate. By repeating the reactions, replacement of better than 99% has been achieved in some cases to give in effect the homopolymer. Optical rotatory dispersion studies show that of all the systems studied only poly(β‐methyl L ‐aspartate) has the left‐handed helix sense, the others are right‐handed. It is shown further that the helix sense is not an intrinsic property of the nature of the aspartate side chain. Raising the temperature of chloroform solutions of the right‐handed form of the copolymers of benzyl L ‐aspartate and ethyl L ‐aspartate results in a transition to the left‐handed helix, the temperature of the transition being dependent on the composition of the copolymer. Also poly(β‐ n ‐propyl L ‐aspartate) undergoes a transition from the right‐ to the left‐handed helix form at 59°C. These results suggest a general pattern of behavior of poly(aspartate esters) and that with suitable conditions of solvent and temperature they may be in either the right‐ or left‐handed helical form.