Abstract
Effects of divalent counterions (Mg 2+, Ca 2+, Sr 2+, Ba 2+) on the helix formation of poly( l-glutamic acid) are investigated in aqueous alcohol (MeOH, EtOH, 2PrOH, tBuOH) solutions. To observe helix formation inherent to the divalent counterions, the chlorides were added to a poly( l-glutamate) having an inert monovalent counterion, tetrabutylammonium cation. Marked counterion- and alcohol-specificities were observed for the dependence of helix content, θ, on the alcohol content. The θ values for most Mg 2+ and Ca 2+ systems decreased with increasing alcohol content in a higher alcohol region, while those for Sr 2+ and Ba 2+ systems were almost constant. A specific helix stabilization was observed for mixed divalent counterion systems. These results are self-consistently interpreted by taking into account contact ion-pair formation between the counterions and the polymer charges on the coil conformation.
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