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

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.

Highlights► Alkali metal poly(l-glutamate) shows “globule-helix” transition in aqueous ethanol. ► Mixing of smaller and larger counterions induces α-helix formation. ► Contact-ion pair formation is not necessary for α-helix formation.