Auracyanin B structure in space group P6₅

The structure of auracyanin B, a `blue' copper protein produced by Chloroflexus aurantiacus, has previously been solved and refined in the hexagonal space group P6₄22 with a single molecule in the asymmetric unit. The protein has now been crystallized in space group P6₅, with unit-cell parameters a = b = 115.9, c = 108.2 Å. In the new crystal form, the asymmetric unit contains four protein molecules. The structure has been solved by molecular replacement and refined at 1.9 Å resolution. The final residuals are R  =  19.2% and R_free  =  21.9%. In relation to the earlier crystal structure, the doubling of the unit-cell volume and the lower symmetry are explained by small rotations of the molecules with respect to one another.