The effects of cations on the structure and photochemistry of the photosystem II core complex

We have studied the effects of cations and detergents on the structure (molecular weight) and photochemistry of Triton X-100 Photosystem II subchloroplast particles (TSF-IIa). The effect of Mg²⁺ ions on activity depended on the Triton X-100 content of the preparation. If the residual Triton X-100 was not removed prior to assay, MgCl₂ increased the rate of electron transport, acting at a site on the reducing side of Photosystem II. Lowering the pH also increased the rate of electron transport. If the Triton X-100 was removed from the particles, both MgCl₂ and NaCl caused a decrease in the rate of electron transport. Addition of Triton X-100 caused a reversible decrease in the number of active Photosystem II reaction centers. Both cations and Triton X-100 had a profound effect on the molecular weight of the Photosystem II particles as determined by gel filtration. At 20 °C, addition of 0.05% Triton X-100 decreased the molecular weight from a high value (≥800,000) to 250,000. At 4 °C, addition of 1 mm MgCl₂ or 100 mm NaCl increased the molecular weight of the complex. In the absence of these salts 67% of the protein eluted with a molecular weight of 460,000 (the rest was >800,000-in the void volume). In the presence of these salts all of the material had a molecular weight of ≥800,000. A similar effect was observed when the pH was lowered from 8 to 6. Further work is needed to determine whether there is a correlation between the changes in molecular weight and activity.