Double conformational transition of alkali metal poly(L-glutamate)s in aqueous ethanol: counterion mixing effect revisited

Abstract

It was first found that alkali metal poly(L-glutamate)s show the coil-globule transition and the coil-helix transition sequentially in aqueous ethanol with increasing the solvent concentration. The counterion specificity for the former transition, i.e., Na(+)>K(+), Rb(+)>Li(+), Cs(+), proved to be somewhat different from that for the latter; Na(+)>Li(+)>K(+)>Rb(+)>Cs(+). Counterion mixing effects were also observed for both transitions; the most effective combinations to induce the transitions were Li(+)/Cs(+) and Li(+)/K(+), respectively. Solution viscometry, circular dichroism and alkali metal NMR line width measurements for the single-counterion systems revealed that the contact ion-pair formation hardly occurs for Li(+) even at the collapsed globule state and in the helix conformation. Unexpectedly, however, the specific binding of Li(+) was induced just by mixing with K(+) or Cs(+), when the helix content concomitantly increased. Mechanism for the counterion-specific "double transition" as well as the counterion mixing effect is discussed referring to the size-fitting model that has been proposed for the coil-globule transition of alkali metal poly(acrylate)s.