Detection and characterization of merohedral twinning in two protein crystals: bacteriorhodopsin and p67^phox

The estimation of the twinning ratio in crystals of two proteins, bacteriorhodopsin and a truncated form of p67^phox, is described. Various approaches are combined to detect and determine accurately the twinning ratio. For each protein, three data sets from crystals exhibiting twinning ratios ranging from 0 to almost 50% are analysed. Self-rotation functions and R_sym values considering space groups of higher symmetry are indicative of twinning. Precise values of the twinning ratios are derived from Yeates' statistical approaches and Britton plots. The twinning ratios are also obtained by analysing the second-, third- and fourth-order moments of the intensity distribution. The twinning ratios obtained from the various approaches are compared. Second-, third- and fourth-order moments of the intensity distribution are useful indicators for evaluating the twinning precisely in order to correct the intensities or to screen rapidly for non-twinned crystals.