Ion-binding and conformation of poly-L-methionine S-methylsulfonium salts in added salt systems.

Abstract

AbstractIn order to study the type of ion binding and the conformation of several salts of poly‐L‐methionine S‐methylsulfonium hydroxide, the viscosity, conductance, counterion activity, and optical rotatory dispersion of the polysalts were measured in systems with a small amount of added salt. It was shown that the ion binding of chloride and bromide salts was of a diffuse binding type due only to the electrostatic potential of the polyion, and that both polysalts underwent no conformational transition by the addition of a simple salt, NaCl for chloride salt and NaBr for bromide salt, and retained a random coil conformation. Iodide and thiocyanate salts showed a conformational change, probably from the random coil into the β form, with increasing concentrations of NaI and NaSCN, respectively. On the other hand, perchlorate salt existed in the α‐helix conformation in part even in pure aqueous solution, and the fraction of α‐helix increased on the addition of NaClO4. On considering several possible situations, it is suggested that there is a specific and strong interaction between the polyion and the small counterion in iodide, thiocyanate, and perchlorate salts.