Molecular Weight and the Dodecyl Sulphate Binding of a Thylakoid Membrane Polypeptide Involved in a Reaction on the Oxygen-Evolving Side of Photosystem II

The molecular weight of a thylakoid membrane polypeptide with the apparent molecular weight 11 000 was determined by measurement of the sedimentation velocity, the diffusion and the ef­ fective partial specific volume. The molecular weight was found to be 6300 and that of the polypeptide-dodecyl sulphate micelle was found to be 11 200. The frictional ratio was 1.35. In ad­ dition, we determined the binding of dodecyl sulphate onto the polypeptide by equilibrium dialysis. We found that 1 g polypeptide binds 0.77 g sodium dodecyl sulphate which corresponds to 17 molecules dodecyl sulphate bound per polypeptide chain. In the absence of dodecyl sulphate the polypeptide aggregates. The molecular weights of the aggregates are in 0.01 M sodium phosphate buffer pH 7.2 150 000 and in a 1 :1 mixture of 0.01 M phosphate buffer and 96% ethanol 365 000. The frictional ratios were 1.07 and 1.16 respectively which points at a spherical shape. The experimental conditions for the determination of the dodecyl sulphate binding were critically scrutinised.