Abstract

Density modification techniques try to improve the phases of poorly resolved electron density maps given by isomorphous replacement by correcting the systematic errors of the maps according to known physical properties. The phases computed from the corrected maps are combined with the observed moduli through a suitable weighting scheme. A new refinement strategy is proposed which considers the observed moduli and the moduli of the Fourier coefficients of the 'best' map as isomorphous pairs, the Fourier transform of the known systematic errors being a 'heavy-atom contribution'. The lack of closure is assumed to have a Gaussian probability law and gives the basis of the weighting scheme as in a true single isomorphous replacement. The application of the technique to the multiple isomorphous replacement map of E. coli methionyl-tRNA-synthetase yields dramatic improvements.

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