A GENERAL PHASING ALGORITHM FOR MULTIPLE MAD AND MIR DATA

Jordi Bella and Michael G. Rossmann

Department of Biological Sciences
Purdue University, West Lafayette, IN 47907, USA

A general algorithm has been developed for combining phasing information from Multiple-wavelength Anomalous Dispersion (MAD) data collected on multiple X-ray absorption edges, corresponding to different types of anomalous scatterers on a given crystal, or multiple MAD data from different derivative crystals. The algorithm also is appropriate for combining MAD data with multiple isomorphous replacement (MIR) data from different derivative crystals. This algorithm is based in an alternative geometrical construction of the Harker diagram, originally developed by Rossmann in 1961 for the phase determination of the crystal structure of horse oxy-hemoglobin (1). A heavy-atom phasing and refinement program originally written by Rossmann in 1967 has been adapted to determine protein phases by using all MAD and MIR derivatives simultaneously. In this technique the "native" data is considered equivalent to a "null-derivative" that contains neither an anomalous nor an isomorphous component. Several numerical tests will be presented for different experimental scenarios: a double MAD experiment on (diffraction data at two absorption edges), combination of two MAD experiments from different derivatives, and combination of MAD data with MIR data from multiple crystals. There is a direct relationship between this method and the Karle equations (2) used in MAD phasing, and it will be shown how these can be reformulated as a particular case of the algorithm.

Cullis, A.F., Muirhead, H., Perutz, M.F., Rossmann, M.G., North, A.C.T., Proc. Roy. Soc. A265, 15-38 (1961).

Karle J. Int. J. Quantum Chem.: Quantum Biol. Symp. 7, 357-367 (1980).