Pseudo-merohedral twinning in monoclinic crystals of wild-type human brain neuroglobin.

The purification, crystallization and successful structure determination by molecular replacement of wild-type human brain neuroglobin at 1.8 A resolution is reported. The apparent space group was orthorhombic C222(1), but the real space group was monoclinic P2(1), which resulted from twinning. Indeed, the unit-cell parameters, a = 31.2, b = 139.1, c = 31.2 A, beta = 102 degrees , display a fortuitously close to c and twinning by the operator l, -k, h occurs. Twinning was not evident from the initial analysis of intensity distribution, but pseudo-merohedral twinning was revealed by the Padilla and Yeates test based on local intensity differences. A twinning fraction of 0.5 was determined in SHELXL, indicating a perfect hemihedrally twinned crystal. To date, this type of twinning has been reported in more than ten structures, which makes it quite a common case in proteins.