The application of direct methods to the analysis of heavy-atom derivatives

Abstract

Heavy atoms in isomorphous derivatives of concanavalin-A and formylmethionine transfer RNA have been found by applying the tangent formula in three dimensions to EΔ's, the normalized moduli of the differences between the scaled amplitudes of the derivative and parent structure factors. A full three-dimensional set of starting phases was computed from trial heavy-atom constellations derived from symbolic-addition analysis of the centrosymmetric axial projections. This strategy compares favorably with the analysis of two- and three-dimensional difference Patterson syntheses usually employed to find heavy atoms, and is flexible enough to incorporate other sources of structural information when available. It is preferable when circumstances, such as high-symmetry space groups, data of poor quality or low resolution, and multiple heavy-atom sites per asymmetric unit make the Patterson syntheses difficult to interpret. Problems peculiar to the application of direct methods to the solution of isomorphous heavy-atom derivatives are discussed.