Method of determining the relative stability of different conformational states of biological macromolecules

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 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, theta, on the alcohol content. The theta values for most Mg2+ and Ca2+ systems decreased with increasing alcohol content in a higher alcohol region, while those for Sr2+ and Ba2+ 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.