W0461

Accurate studies of a high resolution Fourier synthesis require the full set of structure factors to be used when calculating the synthesis. Structure factors with unknown phase or even with unknown amplitude may be restored with a reasonable accuracy through density modification methods. For the case of Aldose Reductase, measured at 0.9 A resolution, a special type of solvent flattening was tested for restoring about 20 000 (10% of the full set) structure factors in the 0.9Å resolution zone [1]. This flattening is based on the connectivity analysis of the Fourier synthesis map and is applied to small ‘drops’ only. The outlines of the procedure used are general for iterative density modification methods. Each cycle of the procedure consists of the following steps: the synthesis is calculated with the current set of structure factors (the weighted MAD-phased synthesis is used at the first step of the procedure); the modification of the synthesis is performed; new values of structure factors are calculated from the modified synthesis; a new set of values of structure factors is obtained combining calculated phases with the observed magnitudes (if these latter are known), and taking both the calculated magnitude and phase for reflections with unknown magnitudes. The map modification is based on the observation that the small drops in the maps represent usually noise, while real structural features are represented by more extended regions. The modification discussed consists in reducing of density corresponding to sharp narrow peaks, while the larger ‘blobs’ of a density in the solvent region are left unchanged. Such procedure combines the features of traditional density modification methods with the connectivity based phasing [2].