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 dilute solutions of poly(p-biphenylylmethyl L-glutamate), PBPMG, in the helicogenic solvent THF, over a temperature range encompassing an observed reversible transition in the chiral optical properties of these solutions. The results confirm the expected helical conformation for this polypeptide, as well as complete dissolution of the polymer at temperatures above the transition temperature. Association is observed at lower 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 association in the aggregate increasing concurrently with the change in the optical rotation and circular dichroism (CD) spectrum. It is concluded that the intermolecular association of the chains in the parallel array alters the chiral electronic environment of the biphenyl group, without a conformation change in that chromophore, resulting in the observed transition in the optical rotation and the CD spectrum. Unlike the behavior reported for certain other aggregated systems, the optical transition observed with solutions of PBPMG does not involve the initial formation of a disordered aggregate, which orders itself on reduced solvent quality or temperature, but rather a spontaneous association, such as expected on crossing a phase boundary, with a concurrent optical transition.