The effect of chloride on the thermal inactivation of oxygen evolution

The effect of Cl depletion on the sensitivity of the oxygen-evolving complex of Photosystem II (PS II) to heat treatment was examined by a parallel study of the Hill activity (H2O→2,6-dichlorophenolindophenol), Cl(-) binding (by (35)Cl-NMR) and Mn release (by EPR). The extent of thermal inactivation in spinach thylakoids was found to depend on the degree of Cl(-) depletion in the sample. In partially Cl(-)-depleted thylakoids, mild heating (38°C, 3 min) was found to eliminate inflections in plots of both Hill activity versus [Cl(-)] (at low light intensity) and excess (35)Cl-NMR linewidth versus [Cl(-)] (in the dark). In PS II membranes, the same treatment reduced the differences between the linewidth maxima and minima, particularly in the region of 0.3 mM and 7.0 mM Cl(-), as compared to unheated membranes. These results indicate that mild heating affects the Cl(-)-binding domains within the oxygen-evolving complex, OEC, EPR measurements of the temperature dependence of Mn release from heated thylakoids show that Mn release begins to correlate with the loss of Hill activity only at higher temperatures, where the OEC is already substantially inactivated. We conclude from these studies that the Cl(-)-binding domains of the OEC constitute a principal site of damage by heat treatment.