Low-resolution phase extension and refinement by maximum entropy

The problem of phase refinement and extension at very low resolution (30-25 Å) is treated with an algorithm that combines a maximum-entropy approach, a binary modelling of the electron density, refinement of the proposed map against the observed amplitudes and solvent flattening outside a molecular envelope. The algorithm is applied to data for the complex of aspartyl-tRNA and aspartyl-tRNA synthetase in three different cases: (1) X-ray amplitudes and phases calculated from a partial model; (2) mixed observed and calculated X-ray amplitudes and phases from a partial model; and (3) observed neutron amplitudes and phases from a very approximate model. The change of correlation with the correct map at 30 Å resolution is used as a measure of correctness. Upon application of the algorithm, this correlation changes from 59 to 97% in case 1, from 59 to 77% in case 2 and from 72 to 90% in case 3. In all cases, the method is successful in correcting large phase errors, deleting noise regions and producing the correct low-resolution molecular image.