Ab initio structure determination of a pharmaceutical compound, prednisolone succinate, from synchrotron powder data by combination of a genetic algorithm and the maximum entropy method

The structure of a pharmaceutical compound, prednisolone succinate, has been determined by a newly developed analytical method for ab initio structure determination and refinements from X-ray powder data. It consists of a genetic algorithm (GA) for the structure determination process and the maximum entropy method (MEM), including its variations, for the structure refinement process. The crystal structure of prednisolone succinate has two molecules in the asymmetric unit and the total number of degrees of freedom is 25. In the structure refinement process, the various kinds of MEM analyses, such as difference MEM and omit MEM, in addition to ordinary MEM, play crucial roles in obtaining a better structural model. The data used in the present study were collected at BL02B2, SPring-8, which allowed the use of a wide d-spacing range, d > 1.0 angstrom. The reliability factors, R-wp and R-I, of the final structure analysis by Rietveld refinement are less than 0.013 and 0.048, respectively. The structure determined in the present study gives no CHECKCIF alert relating to the fundamental structure, such as interatomic distances and bond angles, even in chemical bonds involving H atoms.