Effect of errors, redundancy, and solvent content in the molecular replacement procedure for the structure determination of biological macromolecules.

The power of molecular replacement as a tool for analyzing macromolecular structures such as viruses has been demonstrated by an increasing number of successful determinations. We examine here the effects of noncrystallographic redundancy, N; the fraction of solvent volume (1- U/V); error in structure amplitude measurements, R; the fraction, f, of the unique data that were measured; error in the description of the noncrystallographic symmetry; and definition of the molecular envelope. The formula P = (Nf)1/2/R(U/V) has been derived and represents the inherent phasing power (P) for a given problem. The ability of "solvent flattening" procedures to determine phases was analyzed and found to be analogous to the effect of noncrystallographic redundancy. However, in the limiting case, the effect of solvent flattening approaches the power of Sayre's equations.