Crystallography anomalous dispersion methods

The most demanding element of macromolecular crystallography (except, perhaps, for dealing with macromolecules that resist crystallization) is the so-called phase problem, that of determining the phase angle ahkl for each reflection. In the remainder of this chapter, I will discuss some of the common methods for overcoming this obstacle. These include the heavy-atom method (also called isomorphous replacement), anomalous scattering (also called anomalous dispersion), and molecular replacement. Each of these techniques yield only estimates of phases, which must be improved before an interpretable electron-density map can be obtained. In addition, these techniques usually yield estimates for a limited number of the phases, so phase determination must be extended to include as many reflections as possible. In Chapter 7,1 will discuss methods of phase improvement and phase extension, which ultimately result in accurate phases and an interpretable electron-density map. 

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... 

It is well known that the absolute configuration of chiral compounds was first determined by the Bijvoet using X-ray crystallography [1]. In X-ray crystallography, since the anomalous dispersion effect of heavy atoms can be measured accurately under proper conditions, the absolute stereostructure obtained is unambiguous and reliable. In addition, the molecule can be projected as a three-dimensional structure, and, therefore, the method has been employed extensively. However,... 

In the case of x-ray diffraction, important advances include the use of robotics to ensure successful growth of crystals, the development of synchrotron sources for macromolecular crystallography, the use of direct methods and anomalous dispersion in phasing, and the development of automated methods of map htting. 

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