Salt dependence of the electrical potential at the photosynthetic membrane in steady-state light and its structural consequence

Abstract

(1) The change in potential difference across the functional membrane of photosynthesis has been measured as a function of KCI concentration of the suspending medium under saturating, continuous illumination. The measurements have been carried out through the potential-indicating electrochromic absorption changes. Between 1 and 100 mM KCI, a decrease from 80 to about 0 mV was found. The salt dependence can be described by a Donnan model which is based on buffer groups located within the inner space of the system. A Gouy-Chapman model based on buffer groups located at the plane of the inner membrane surface does not fit the data of the measurements. The strong salt dependence may explain apparent discrepancies between the values of the light-induced potential changes reported in the literature. (2) Measurements of the membrane conductance of the ionophore, nonactin-K +, as a function of the internal proton concentration have been carried out. Also, in this case, the results can be explained by a Donnan model. The consequence of both results, i.e., distribution of the buffer groups of thylakoids within the inner space, is possibly realized by membrane-bound proteins which project up to about 20 nm into the inner thylakoid space, as well as through unbound proteins (see Fig. 7). Inter alia, the carboxyl groups of aspartic and glutamic acids of these proteins may be the effective buffer groups. Plastocyanin may also contribute to this effect in a minor way.