The major problem in X-ray crystallography is to determine the phase angles of the X-ray reflections. In protein crystallography this problem is solved by the application of either isomorphous replacement or molecular replacement or multiple wavelength anomalous dispersion. In small molecule crystallography a completely different solution is applied. There, direct methods are the standard techniques for determining the phase angles of the structure factors. They use the principle that phase information is included in the intensities and this principle depends on the basic assumptions that the electron density is always positive [F(000) included in the Fourier summation] and the crystal consists of discrete atoms that are sometimes even considered to be equal. Phase relations based on probability theory have been formulated and these phase relations are applied to suitably chosen clusters of reflections. Although these direct methods work perfectly well for small molecule crystals, it has thus far not been easy to extend them successfully to protein crystals (Karle, 1989). However, the field has attracted great attention and the first successful results have been published in Acta Crystallogra • D49 Part 1, January 1993.
KeywordsPhase Angle Maximum Entropy Electron Density Distribution Protein Crystal Maximum Entropy Method
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