Ab initio phasing at resolution higher than experimental resolution.

Owing to the limited experimental resolution of data in macromolecular crystallography, ab initio phasing is successful only when atomic or quasi-atomic resolution data are available. It is shown that extrapolating the moduli and phases of non-measured reflections beyond and behind the experimental resolution limit makes the ab initio phasing process more efficient and leads to crystal structure solution even in cases in which the standard SIR2004 program does not succeed. Moreover, use of the extrapolated values improves the quality of the final electron-density maps and makes the recognition of the correct structure among several trial structures easier.