DETWINNING OF HEMIHEDRALLY TWINNED CRYSTALS BY THE LEAST-SQUARES METHOD AND ITS APPLICATION TO A CRYSTAL OF HYDROXYLAMINE OXIOREDUCTASE FROM NITROSOMANAS-EUROPAEA

With the development of methods to crystallize proteins to sizes suita ble for X-ray analysis, structures of many proteins have been determin ed to elucidate the relationships between their structures and functio ns. However, crystals of biological macromolecules have been frequentl y found to be twinned by adherence of two crystal lattices. Some cryst als show splitting of diffraction spots owing to the different tilts o f the two lattices, Others pretend to be single crystals with no split spots, and their symmetries of intensity distribution vary with every data set. These latter have been called hemihedral, and in them the u nique axes of the two crystals are exactly reversely parallel with eac h other. The observed intensities were found to be detwinned by the le ast-squares method after Britton's algorithm [Britton (1972). Acta Cry st. A28, 296-297], when several sets of intensity data were observed. Fortran programs have been successfully used to determine the scale fa ctors and degrees of twinning from the data of hydroxylamine oxidoredu ctase. The analysis of data collected at various positions of one crys tal of this enzyme suggested a heterogeneous distribution of twinning in the crystal.