Resolution of phase ambiguity in macromolecular crystallography

Abstract

The chapter presents a procedure that is applicable to single isomorphous replacement (SIR) data without anomalous scattering information as well as to pure single-wavelength anomalous scattering (SAS) data alone, and it does not require the presence of noncrystallographic symmetry. Test results shows that it is very effective; not only can it remove the phase ambiguity but it will also refine the phases at the same time. In actual application to SIR data, the method has even produced results that appear to be superior to those obtained from the multiple isomorphous replacement method. One of the situations in which the phase ambiguity problem occurs is when the isomorphous replacement method is applied to a noncentrosymmetric structure using only one isomorphous data set. The method demonstrates a remarkable effectiveness for overcoming the phase ambiguity problem that has the major obstacle in the use of SIR or SAS data alone for macromolecular studies. The computer programs used in the studies has been assembled into a program package.