Improvement of SAS triple invariant estimates for macromolecular direct-methods phasing

Single-wavelength anomalous dispersion (SAS) data can in principle be phased by direct methods since a priori estimates of the three-phase structure invariants can be computed from these data. The mean phase error of the most reliable triple estimates for a small protein, however, is typically no better than 60°, and does not bode well for applications to larger structures. A procedure is described that can substantially lower the error in these estimates and introduce a larger number of useful triple invariants into the phasing process. The mean phase error of the most reliable triples for a 2.5 Å resolution data set from a Pt derivative of a 115-residue protein was reduced from 55 to 25° by this method. It was also possible to identify a significant number of the poorest triple estimates, those with mean phase errors approaching 90°, such that they could be reliably down-weighted or excluded from the phasing process.