HELIX FORMATION OF POLY (L-GLUTAMIC ACID) IN THE PRESENCE OF ALKALINE-EARTH METAL-CATIONS IN AQUEOUS ALCOHOL-SOLUTIONS

Effects of divalent counterions (Mg2+, Ca2+, Sr2+, Ba2+) on the helix formation of poly(L-glutamic acid) are investigated in aqueous alcohol (MeOH, EtOH, 2PrOH, tBuOH) solutions. To observe helix formation inhe rent 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, theta, on the alcohol content. The t heta values for most Mg2+ and Ca2+ systems decreased with increasing a lcohol content in a higher alcohol region, while those for Sr2+ and Ba 2+ systems were almost constant. A specific helix stabilization was ob served for mixed divalent counterion systems. These results are self-c onsistently interpreted by taking into account contact ion-pair format ion between the counterions and the polymer charges on the coil confor mation.