INTERMOLECULAR ASSOCIATION AND SUPRAMOLECULAR ORGANIZATION IN DILUTE-SOLUTION .2. LIGHT-SCATTERING AND OPTICAL-ACTIVITY OF POLY(P-BIPHENYLYLMETHYL L-GLUTAMATE)

Static and dynamic light scattering measurements are reported on dilut e solutions of poly-(p-biphenylylmethyl L-glutamate), PBPMG, in the he licogenic solvent THF, over a temperature range encompassing an observ ed reversible transition in the chiral optical properties of these sol utions. The results confirm the expected helical conformation for this polypeptide, as well as complete dissolution of the polymer at temper atures above the transition temperature. Association is observed at lo wer temperatures, with the chains in an ordered parallel array within the aggregates. The association occurs with increasingly poor solvent conditions as the temperature is decreased, with the degree of associa tion in the aggregate increasing concurrently with the change in the o ptical rotation and circular dichroism (CD) spectrum. It is concluded that the intermolecular association of the chains in the parallel arra y alters the chiral electronic environment of the biphenyl group, with out a conformation change in that chromophore, resulting in the observ ed transition in the optical rotation and the CD spectrum. Unlike the behavior reported for certain other aggregated systems, the optical tr ansition observed-with solutions of PBPMG does not involve the initial formation of a disordered aggregate, which orders itself on reduced s olvent quality or temperature, but rather a spontaneous association, s uch as expected on crossing a phase boundary, with a concurrent optica l transition.