Specific helix stabilization of poly(L‐glutamic acid) in mixed counterion systems

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Abstract
The 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 Li+ 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. © 1993 John Wiley & Sons, Inc.