Powder diffraction based structural studies of pharmaceuticals

Powder X-ray study of racemic (2RS,3RS)-5-amino-3-[4-(3-methoxyphenyl)piperazin-1-yl]-1,2,3,4-tetrahydronaphthalen-2-ol

The structure of the title benzovesamicol analogue, C(21)H(27)N(3)O(2), an important compound for the diagnosis of Alzheimer's disease, has been determined by X-ray powder diffraction. The title compound was firstly synthesized and characterized by spectroscopic methods (FT-IR, and (13)C and (1)H NMR). The compound is a racemic mixture of enantiomers which crystallizes in the monoclinic system in a centrosymmetric space group (P2(1)/c). Crystallography, in particular powder X-ray diffraction, was pivotal in revealing that the enantio-resolution did not succeed. The piperazine ring is in a chair conformation, while the cyclohexene ring assumes a half-chair conformation. The crystal packing is dominated by intermolecular O-H···N hydrogen bonding which links molecules along the c direction.

Counteracting stagnation in genetic algorithm calculations by implementation of a micro genetic algorithm strategy

A new strategy for implementing the concept of a "micro genetic algorithm" within a standard genetic algorithm (GA) procedure is proposed. The strategy operates by applying criteria to test for the occurrence of stagnation within the population of a standard GA calculation, and triggering the micro-GA procedure whenever stagnation is detected. The micro-GA is implemented in terms of the parallel evolution of a number of small sub-populations (comprising predominantly new randomly generated structures together with a few of the best structures from the stagnated population), and the sub-population of highest quality following the micro-GA procedure is used in the construction of the next population of the standard GA calculation. The micro-GA procedure is applied in the context of a GA for carrying out direct-space structure solution from powder X-ray diffraction data, and the results demonstrate that this strategy is an effective means of promoting structural diversity within a stagnated population, leading to significantly improved evolutionary progress. This strategy may prove to be more generally applicable as an approach for alleviating problems due to stagnation in GA calculations in other fields of application.

Enhanced Efficiency of Direct-Space Structure Solution from Powder X-ray Diffraction Data in the Case of Conformationally Flexible Molecules

A strategy is reported for assessing the feasibility of molecular conformations within direct-space structure-solution calculations of organic molecular crystal structures from powder X-ray diffraction data, focusing in particular on the genetic algorithm technique for structure solution in which fitness is defined as a function of the whole-profile figure-of-merit Rwp. The strategy employs a readily computed distance-based function to assess the feasibility of the molecular conformation in each trial structure generated in the genetic algorithm calculation, and structures considered to have low-feasibility conformations are penalized within the evolutionary process. The strategy is shown to lead to significant improvements in the success rate of structure-solution calculations in the case of flexible molecules with a significant number of conformational degrees of freedom.