Multiwavelength anomalous diffraction phasing

Membrane-integrated proteins were always hard to express in cell-based systems in sufficient quantity for structural analysis. In cell-free systems, they can be produced on a milligrams per milliliter scale, which, combined with labeling with stable isotopes, is also very amenable forNMR spectroscopy [157-161]. Possible applications of in vitro expression systems also include incorporation of selenomethionine (Se-Met) into proteins for multiwavelength anomalous diffraction phasing of protein crystal structures [162], Se-Met-containing proteins are usually toxic for cellular systems [163]. Consequently, rational design of more efficient biocatalysts is facilitated by quick access to structural information about the enzyme. 

Phase information can also be obtained by Multiwavelength Anomalous Diffraction experiments... 

The intensity differences obtained in the diffraction pattern by illuminating such a crystal by x-rays of different wavelengths can be used in a way similar to the method of multiple isomorphous replacement to obtain the phases of the diffracted beams. This method of phase determination which is called Multiwavelength Anomalous Diffraction, MAD, and which was pioneered by Wayne Hendrickson at Columbia University, US, is now increasingly used by protein cystallographers. 

X-ray data are recorded either on image plates or by electronic detectors The rules for diffraction are given by Bragg s law Phase determination is the major crystallographic problem Phase information can also be obtained by Multiwavelength Anomalous Diffraction experiments... 

X-ray crystallography is a widely used teclmique for the three-dimensional structure analysis of RNA molecules. The extraction of information from X-ray diffraction patterns from RNA crystals requires proper phase determinations using methods such as multiwavelength anomalous dispersion in which placement of specific heavy atoms is required. Seleiuum-modi-fled ribonucleotides have been used for these purposes by Du et al. (36), because the selenium atom does not seem... 

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