The effects of oxidative reaction on ionic permeabilities through polypeptide membranes containing –SH groups in the side chains

The effects of the oxidative reaction on the ionic permeabilities through polypeptide membranes containing thiol groups in the side chains were investigated. Membranes were derived from poly(glutamyl‐cysteine) (PGC) containing 12% (in mol %) of SH groups in the side chains, and from poly( L ‐cysteine) (PlCys) containing 26% of thiol groups in the side chains, respectively. On the basis of the polarographic analysis, it can be estimated that more than 80% of SH groups in the membranes could be oxidized by using 10 mM solution of iodine. The membrane potentials across the PGC membrane were increased by the iodine oxidation, indicating that SH groups of PGC were converted to highly dissociable groups, i.e., sulfonic acid derivatives. The decrease in the ionic permeabilities through the PGC membrane by the oxidation, therefore, was attributed to the decrease in ionic solubility into the membrane owing to the increase in fixed charge density of the membrane (Donnan exclusion effect). On the other hand, no influence of the oxidation on the membrane potentials of the PCys membrane suggests the possibility of oxidative S‐S cross linking in the membrane. The oxidation‐induced permeability decrease of the PCys membrane could be explained in terms of the decrease in ionic diffusibility associated with the formation of the side chain cross linkings between SH groups in the membrane.