Crystallization and electron paramagnetic resonance characterization of the complex of photosystem I with its natural electron acceptor ferredoxin

The formation of a transient complex between photosystem I and ferredoxin is involved in the process of ferredoxin photoreduction in oxygenic photosynthetic organisms. Reduced ferredoxin is an essential redox intermediate involved in many assimilatory processes and is necessary for the reduction of NADP⁺ to NADPH. Single crystals from a complex of photosystem I with ferredoxin were grown using PEG 400 and CaCl₂ as precipitation agents. The crystals diffract x-rays to a resolution of 7-8 Å. The space group was determined to be orthorhombic with the unit cell dimensions a = 194 Å, b = 208 Å, and c = 354 Å. The crystals contain photosystem I and ferredoxin in a 1:1 ratio. Electron paramagnetic resonance (EPR) measurements on these crystals are reported, where EPR signals of the three [4Fe-4S] clusters F_A, F_B, F_X, and the [2Fe-2S] cluster of ferredoxin were detected. From the EPR spectra observed at three particular orientations of the crystal in the magnetic field, the full orientation pattern of the F_A^- g-tensor was simulated. This simulation is consistent with the presence of 12 magnetically inequivalent F_A^- clusters per unit cell with the C₃ axis of the PSI trimers oriented at (23°, 72°, 77°) to the unit cell axes.