Multiwavelength Anomalous Diffraction in Macromolecular Crystallography

Abstract

Multiwavelength anomalous diffraction (MAD) is the fastest growing method of structure determination in macromolecular crystallography. At least twenty-five new structures solved with MAD were published in the past year. Many factors contribute to the growth of MAD, and its future is extremely bright. The experience gained over the past several years is now being generalized to make MAD more accessible. This paper aims to present a practical overview of MAD. I first review the observational equation for MAD and describe the basis of the phasing signal and how it is estimated for specific problems. This is followed by a discussion of the design of a MAD experiment, schemes for data analysis and phasing, and considerations in solving the anomalous-scatterer partial structure. Finally, there is a discussion of selenomethionine as a phasing vehicle. More comprehensive reviews of MAD have been published by W. A. Hendrickson, who pioneered its development and application in macromolecular crystallography [1,2]. Several short papers covering various aspects of MAD are included in the proceedings of the 1997 CCP4 Study Weekend [Recent Advances in Phasing, K. Wilson, G. Davies, and S. Bailey, eds., SERC Daresbury Laboratory, Warrington, U.K, 1997, in press].