Differentiation between the α‐helical and coillike conformations of poly(L‐glutamic acid) in aqueous solution through binding of pinacyanol

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Abstract
Pinacyanol in the presence of an excess of poly(L‐glutamic acid) [polymer/dye ratio (P/D) > 10] exhibits different absorption spectra in the visible region when bound to the slightly charged polypeptide in the α‐helical conformation or to the nearly completely dissociated polypeptide in the coillike conformation. These spectra reveal aggregation of the dye bound to the macromolecular chain in both conformations, although in the coillike one different kinds of aggregates may be present. Dye binding is accompanied by the appearance of CD bands in the visible region which are also different for the α‐helical and the coillike conformations. The aggregates formed in the presence of the latter change slowly in time and seem to induce some conformational changes in the polypeptide chain. Furthermore, they have been found to be, at least partially, stable with respect to a subsequent reversal to the α‐helical conformation. All results could be qualitatively interpreted assuming that in the coillike conformation, ordered regions exist along the chain as proposed by Krimm and Tiffany.