Two-dimensional (2D) crystals of photosystem II (PS II) treated with various concentrations of the zero-length crosslinker 1-ethyl-3-(3-dimethylaminopropyl)carbodi-imide (EDC) were analysed by electron microscopy in conjunction with crystallographic image processing. The preparations were characterized by SDS/PAGE and oxygen-evolution measurements, and the effectiveness of cross-linking was monitored by measuring the level of protection afforded against high concentrations of NaCl and CaCl2, which normally remove extrinsic proteins from PS II. We found that low concentrations of EDC (0.25%) increase the order of 2D crystals of PS II. Treatments with EDC concentrations higher than 0.5% did not improve the order of 2D crystals but induced gross structural changes, which were correlated with a decrease in oxygen evolution activity. Structural changes due to cross-linking did not affect packing or symmetry of the 2D crystals, further supporting the conclusion that PS II has a monomeric nature in vivo.

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