Crystallization and a 5 A X-ray diffraction study of Aphanothece sacrum ferredoxin.

A chloroplast-type ferredoxin containing two non-heme iron and two labile sulfur atoms per molecule was prepared from Aphanothece sacrum. Crystals were obtained by dialysis against 75% saturated a-monium sulfate solution, and belong to the tetragonal system with cell dimensions a = b = 92.2 A and c = 47.6 A, containing four molecules in an asymmetric unit. The electron density map at 5 A resolution was calculated by using the best phase angles determined by the single isomorphous replacement method coupled with the anomalous dispersion effect. An anomalous dispersion difference Fourier map for the native crystal clearly showed four humps corresponding to the iron atoms in an asymmetric unit. The electron densis surface.