Cytochrome c₆ from the green alga Monoraphidium braunii. Crystallization and preminary diffraction studies

Cytochrome c₆, a plastocyanin functionally interchangeable electron carrier between the chlorophyll molecule P700 of photosystem I and cytochrome f from cytochrome b₆f complex, has been isolated from the green alga Monoraphidium braunii and crystallized by the vapour-diffusion technique in sodium citrate. Crystals belong to space group R3, with cell dimensions a = b = 51.93 (5) and c = 80.5 (1) Å (hexagonal axes), with one molecule per asymmetric unit. They diffract beyond 1.9 Å under a Cu Kα rotating-anode source, with an anomalous signal that allows the positioning of the heme Fe atom in the unit cell.