title: 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.
authors.affiliation: Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Tokyo, 152-8550, Japan.
authors.forename: Masayasu
authors.initials: M
authors.lastname: Hasuike
authors.forename: Shigeki
authors.initials: S
authors.lastname: Kuroki
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Satoh
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D000431:Ethanol / Q000737:chemistry*; D008672:Metals, Alkali / Q000737:chemistry*; D011099:Polyglutamic Acid / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012997:Solvents / Q000737:chemistry; D014867:Water / Q000737:chemistry*
chemical_list: D000180:Acrylic Resins; D008672:Metals, Alkali; D012997:Solvents; D014867:Water; D011099:Polyglutamic Acid; D000431:Ethanol; C006903:carbopol 940