The direct rotation function: rotational Patterson correlation search applied to molecular replacement

Most rotation functions try to achieve maximal correlation between two Patterson functions by systematically rotating one and computing the overlap with the other. In contrast, the direct rotation function rotates a search model relative to the crystal unit cell and evaluates the linear correlation coefficient (Patterson correlation, PC) between squared normalized structure-factor amplitudes of the observed and calculated diffraction data. Structure factors are calculated from the rotated search model in a P1 unit cell identical to that of the target crystal. PC makes use of all self-Patterson vectors of the search model. A comparison of the direct rotation function, a real-space rotation function, and a fast rotation function suggests that the direct rotation function provides a considerable enhancement of the signal-to-noise ratio compared to other two. Combined with PC refinement, the direct rotation function was successful in solving multidomain macromolecular crystal structures.