Abstract
Few examples of pseudomerohedrally twinned macromolecular crystals have been described in the literature. This unusual phenomenon arises when a fortuitous unit-cell geometry makes it possible for twinning to occur in a space group that ordinarily does not allow twinning. Here, the crystallization, structure determination and refinement of the cocaine hydrolytic antibody 15A10 at 2.35 A resolution are described. The crystal belongs to space group P2(1), with two molecules in the asymmetric unit and unit-cell parameters a = 37.5, b = 108.4, c = 111.3 A and beta fortuitously near 90 degrees; the refined twinning fraction is alpha = 0.43. Interestingly, the non-crystallographic symmetry (NCS) and twin operators are nearly parallel, which appears to be a relatively frequent situation in protein crystals twinned by merohedry or pseudomerohedry.