SYNOPSISThe coil-helix transition has been studied for alkali metal salts of poly (L-glutamic acid) ( P L G ) , i.e., PLGLi, -Na, -K, and -Cs, in aqueous organic solvent systems. Dependence of the transition on the solvent composition has been qualitatively discussed in terms of the solvent dielectric constant D , Gutmann's acceptor number AN, and water activity G . The helix formation induced by addition of alkali chlorides has also been studied. The sharpness of the transition has been interpreted as a measure of reduction of electrostatic energy of helical PLG through contact ion-pair formation between a counterion and carboxyl anion.I NTRO DUCT10 N Conformational transitions of charged polypeptides such as poly (L-glutamic acid) ( P L G ) and poly ( Llysine) ( P L L ) in aqueous solutions have been extensively studied as model systems of protein folding and unf01ding.l-l~ It has been well known that addition of organic solvents with low dielectric constants to the polypeptide systems enhances stability of the a-helix conformation. As possible causes for the stabilization, unionized side-chain-side-chain interaction as well as hydrogen bonding between peptides have been suggested, although contribution of counterion binding, especially for fully charged polypeptides, has attracted much less attention. We have studied the coil-helix transition of alkali salts of PLG in aqueous alcohols. 16 We have found that smaller counterions ( N a + and Lif ) stabilize the ahelix conformation rather than larger ones (K' and Cs' ) and that the specificity is observed in the presence of a small amount of alkali, which was added to suppress the protonation of the charged groups by the protic solvents. Although the coil-helix transition of PLG salts in aqueous organic solvents have also been studied by Daoust et al.I7 and others,[8][9][10][11][12][13][14][15][16][17][18][19][20] the transition behaviors observed in these studies seem to reflect not only counterion specificity but also the contribution of protonation by protic solvents.In the present study, we have performed CD measurements on alkali salts of PLG in aqueous alcohols (MeOH, EtOH, 2-PrOH, t-BuOH) and aqueous dioxane, which contain small amount of alkali hydroxides. We have investigated dependencies of the helix content on the counterion species, on the solvent composition, and on salt concentration. We aim at elucidation of the mechanism of these dependencies. In the following, we consider, in advance, possible factors that influence the coil-helix transition of charged polypeptides in aqueous organic solvents.It is apparent that many factors concert, compete, and counterbalance for the helix formation in such mixed solvent systems. These are, however, classified into two groups: one influences the transition through the intramolecular hydrogen bond of polypeptides, i.e., modulation of strength of the hydrogen bond and solvation of the peptide groups. The other controls the transition through the electrostatic interaction between polymer charges. Addition of organi...
SYNOPSISThe coil-helix transitions of poly (L-glutamic acid) in aqueous alcohol solutions have been investigated for mixed counterion systems. It has been found that coexistence of two kinds of counterion species, i.e., two alkali metal counterions, alkali and alkaline earth metal, and two alkaline earth metals, specifically stabilizes or destabilizes the helix conformation depending upon the combination of the counterion species. The most striking enhancement of the helix content was observed for the combination of Lit and K + counterions. It has been suggested that the helix stabilization is attributed to the reduction of the free energy in the contact ion pair formation between the polymer charges and the counterions in the mixed counterion systems.
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