Salt-induced pH changes in spinach chloroplast suspension. Changes in surface potential and surface pH of thylakoid membranes

A pH decrease in chloroplast suspension in media of low salt concentration was observed when a salt was added at pH values higher than 4.4, while at lower pH values a pH increase was observed. The salt-induced pH changes depended on the valence and concentration of cations of added salts at neutral pH values (higher than 4.4) and on those of anions at acidic pH values (lower than 4.4). The order of effectiveness was trivalent > divalent > monovalent. The pH value change by salt addition was affected by the presence of ionic detergents depending on the sign of their charges. These characteristics agreed with those expected from the Gouy-Chapman theory on diffuse electrical double layers. The results were interpreted in terms of the changes in surface potential, surface pH and the ionization of surface groups which result in the release (or binding) of H⁺ to (or from) the outer medium.The analysis of the data of KCl-induced pH change suggests that the change in the surface charge density of thylakoid membranes depends mainly on the ionization of carboxyl groups, which is determined by the surface pH. When the carboxyl groups are fully dissociated, the surface charge density reaches ?1.0 ± 0.1 · 10^?3 elementary charge/square Å.Dependence of the estimated surface potential on the bulk pH was similar to that of electrophoretic mobility of thylakoid membrane vesicles.