Direct-method SAD phasing of proteins enhanced by the use of intrinsic bimodal phase distributions in the subsequent phase-improvement process

Abstract

A modified SAD (single-wavelength anomalous diffraction) phasing algorithm has been introduced in the latest version of the program OASIS. In addition to direct-method phases and figures of merit, Hendrickson-Lattman coefficients that correspond to the original unresolved bimodal phase distributions are also output and used in subsequent phase-improvement procedures in combination with the improved phases. This provides the possibility of rebreaking the SAD phase ambiguity using the ever-improving phases resulting from the phase-improvement process. Tests using experimental SAD data from six known proteins showed that in all cases the new treatment produced significant improved results.