Determination and correlation of heavy atom positions in the derivatives used in the X-ray analysis of ribonuclease A at 2.5 Å resolution

Abstract

The analysis of four heavy atom derivatives of ribonuclease A is described leading to isomorphous phasing at 2.5 A resolution. Differences in the characteristics of the derivatives are discussed with respect to the determination of their heavy atom parameters and the correlation of sites between derivatives to produce a self consistent set. The criteria used to ensure consistency are described. In the final phase analysis using refined heavy atom parameters the least squaresR factors of the derivatives were .124, .143, .101, and .067 respectively, and the mean figure of merit for the 3444 observed reflexions was 0.66.