The crystal structure of myoglobin: Phase determination to a resolution of 2 Å by the method of isomorphous replacement

Abstract

The method of multiple isomorphous replacement was used to solve the structure of the protein myoglobin to a resolution of 2 A. EDSAC II was programmed to calc. the phase angles, and the statistical treatment of errors proposed by Blow and Crick (CA 54, 1965a) has been used throughout, leading not to the most probable electron-d. map but to the one with the least mean square error over the entire unit cell. Problems inherent in adapting the process for a digital computer are discussed, and examples are given of probability curves for typical phase detns. A method of refining heavy-atom parameters in the course of phase detn. is presented. Comparisons of the most probable and least-error electron-d. maps are presented in the region of the haem group, and it is shown that the latter map gives slightly better results. In an appendix a means of trial-and-error least-squares refinement of heavy-atom parameters using data from a centrosymmetric projection is discussed. [on SciFinder(R)]