Phasing at resolution higher than the experimental resolution

Limited experimental resolution is a unavoidable feature in macromolecular crystallography: it may hinder or make difficult the determination of the crystal structure. A novel procedure is presented which from an approximate electron-density map extrapolates the moduli and phases of non-measured reflections beyond and behind the experimental resolution limit. Applications to a set of test structures show that the extrapolation can be successfully accomplished. As a consequence, the phase estimates of the observed reflections are subsequently improved and the interpretability of the corresponding electron-density map increases. The use of the extrapolated values for the non-measured reflections provides additional information for the map, which shows a resolution higher than the experimental resolution.