International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F, ch. 13.3, pp. 276-277
Section 13.3.4. Phased translation function^{a}Department of Biological Sciences, Columbia University, New York, NY 10027, USA |
If an atomic model needs to be placed in an electron-density map that has been obtained through other methods (e.g. the multiple-isomorphous-replacement method or partial model phases), the phased translation function can be used (Bentley & Houdusse, 1992; Colman et al., 1976; Read & Schierbeek, 1988; Tong, 1993). It is essentially based on the correlation between observed and calculated electron-density values throughout the unit cell: Therefore, the phased translation function can also be evaluated by the FFT technique. As with the Patterson-correlation translation function, the phased translation function can be placed on an `absolute' scale by introducing appropriate normalizing factors or by converting the results to R factors or correlation coefficients. It should be noted that the prior phase could be in the wrong hand, so both and may need to be tried in the phased translation function.
The stationary molecules contribute a constant to the phased translation function and are not shown in equation (13.3.4.1). However, the phase information from the stationary molecules can be applied to the observed structure-factor amplitudes, and the phased translation function, rather than the Patterson-correlation translation function, can be used in the search for additional molecules (Bentley & Houdusse, 1992; Driessen et al., 1991; Read & Schierbeek, 1988). This could prove especially useful in locating the last few molecules in cases where there are several molecules in the asymmetric unit.
References
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