Abstract
The conformational transition of poly(L‐agrignine) by binding with various mono‐, di‐, and polyvalent anions, especially with SO, was studied by CD measurements. The intramolecular random coil‐to‐α‐helix conformational transition and the subsequent transition to the β‐turn‐like structure was caused by binding with SO. The binding data obtained from equilibrium dialysis experiments showed that the α‐helical conformation of poly(L‐arginine) is stabilized at a 1:3 stoichiometric ratio of bound SO to arginine residue; at higher free SO concentrations, the α‐helix converts to the β‐turn‐like structure accompanied by a decrease in amount of bound SO. The same conformaitonal transition of poly(L‐arginine) also occurred in the solutions of other divalent anions (SO, CO, and HPO) and polyvalent anions (P~2~O, P~3~O). Among the monovalent anions examined, CIO and dodecyl sulfate were effective in including α‐helical conformation, while the other monovalent anions (OH^−^, Cl^−^, F^−^, H~2~PO, HCO and CIO) failed to induce poly(L‐arginine) to assume the α‐helical conformation. Thus, we noticed that, except for dodecyl sufate, the terahedral structure is common to the α‐helix‐forming anions. A well‐defined model to the α‐helical poly(L‐arginine)/anion complex was proposed, in which both the binding stoichiometry of anions to the arginine residue and the tetrahedral structure of anions were taken into consideration. Based on these results, it was concluded that the tetrahedral‐type anions stabilize the α‐helical conformation of poly(L‐arginine) by crosslinking between two guanidinium groups of nearby side chains on the same α‐helix through the ringed structures stabilized by hydrogen bonds as well as by electrostatic interaction. Throughout the study it was noticed that the structural behavior of poly(L‐arginine) toward anions is distinct from that of poly(L‐lysine).