Crystal structure solution of small-to-medium-sized molecules at non-atomic resolution

SIR2011: a new package for crystal structure determination and refinement

SIR2011, the successor ofSIR2004, is the latest program of theSIRsuite. It can solveab initiocrystal structures of small- and medium-size molecules, as well as protein structures, using X-ray or electron diffraction data. With respect to the predecessor the program has several new abilities:e.g.a new phasing method (VLD) has been implemented, it is able to exploit prior knowledge of the molecular geometryviasimulated annealing techniques, it can use molecular replacement methods for solving proteins, it includes new tools like free lunch and new approaches for electron diffraction data, and it visualizes three-dimensional electron density maps. The graphical interface has been further improved and allows the straightforward use of the program even in difficult cases.

Relaxation of the resolution requirements for direct-methods phasing

Shake-and-bake phasing methods have permitted the ab initio solution of crystal structures containing more than 1000 independent non-H light atoms (C, N, O). The success of these procedures is critically dependent upon having diffraction data measured to at least 1.2 Å resolution. A new target function R(2)(φ(h)) is introduced into the shake-and-bake procedure along with a real difference map strategy whereby this resolution limit can be appreciably lowered toward 1.5 Å. These improvements, when applied to moderately high resolution data, may now allow one the possibility to solve structures that are twice as large as could have been solved previously.