Freezing of isolated thylakoid membranes in complex media: I. The effect of potassium and sodium chloride,nitrate, and sulfate

Thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were subjected to a freeze-thaw cycle in the presence of a buffered medium containing sorbitol as a cryoprotectant and various combinations of potassium and sodium chloride, nitrate, and sulfate. Above a certain total salt concentration, an increase in the concentration of a single electrolyte, or of potassium plus sodium salts with identical anions, always led to a decrease in photophosphorylation activity. A similar effect was obtained with combinations of nitrate plus chloride with identical cations and of KNO₃ plus NaCl. By contrast, in the presence of suitable combinations of NaNO₃ plus KCl, NaNO₃ plus sulfates, and chlorides plus sulfates, inactivation of photophosphorylation was diminished, sometimes dramatically, at initial molarities of nitrate or chloride which alone caused partial or complete membrane damage. When NaNO₃, KCl, and potassium or sodium sulfate were simultaneously present during freezing, thylakoids were affected very little over a wide range of concentration. Diminution or prevention of inactivation of photophosphorylation by suitable combinations of two or more cryotoxic inorganic salts can be explained by postulating that the different solutes act on different sites and that each reduces the concentration of the others by colligative action, together with specific effects of the various electrolytes on individual membrane sites.