Direct Methods and Powder Data: State of the Art and Perspectives

Abstract

Solving crystal structures by applying direct methods to single-crystal data is a relatively easy task for structures with up to 100 atoms in the asymmetric unit. Their successful application to powder data is still a challenge unless the size of the structure is moderate. The rate of success depends on several factors like the efficiency of the full-pattern-decomposition programs, the peak overlapping, the presence of preferred orientation in the powder specimen, the nature of the background, the type of radiation used for collecting experimental data etc. The main factors are analysed in order to provide a clear description of the specific problems that have to be forced when direct phasing from powder data is attempted.

Characterization of the Products of the Heme Detoxification Pathway in Malarial Late Trophozoites by X-ray Diffraction

“…For this crystal system there are only two possible space groups: either the two hemes are related by inversion symmetry in space group P-1 or they are crystallographically inequivalent and the space group is P1. In principle, Wilson statistics can be used to determine the presence of an inversion center from a diffraction pattern, but the method is notoriously inaccurate for powder data (13). In the present case the mean of ͉E ⅐ E Ϫ1 ͉ is 0.892 for 266 data out to 28°in Fig.…”

Characterization and structural determination of a new sodium lanthanum phosphate thiosulfate, Na2La[PO4][S2O3]

“…The background was approximated by a Chebyshev polynomial. Reflection intensities were extracted from the diffraction pattern using the full-pattern-decomposition (FPD) program [31]; 248 F 2 values extracted from the data were used as input for the direct methods routine of the DIRDIF96 program [32]. The reliability factors after FPD procedure for Na 2 La[PO 4 ][S 2 O 3 ] were: χ 2 = 1.59%, R wp = 6.39%, and R p = 4.90%.…”

Application of simulated annealing approach for structure solution of molecular crystals from X-ray laboratory powder data

“…For crystal structure determination from powder diffraction data one can use a number of methods that have been discussed in details in recent reviews (Giacovazzo, 1996;Harris & Tremayne, 1996;Le Bail, 1998;Louër, 1999). The most known of them, implemented in available programs, are the direct methods (Jansen, Peschar, Schenk, 1992a;1992b;Altomare, Cascarano, Giacovazzo, Guagliardi, Burla, Polidori, Camalli, 1994), the maximum entropy and likelihood-ranking method (Bricogne & Gilmore, 1990), the direct grid search (Masciocchi, Bianchi, Cairati, Mezza, Pulati, Sironi, 1994;Chernyshev & Schenk, 1998) and the global optimization procedures: genetic algorithm (Kariuki, Serrano-Gonzalez, Johnston, Harris, 1997) and Monte Carlo based methods (Harris, Tremayne, Lightfoot, Bruce, 1994;Andreev, Lightfoot, Bruce, 1997).…”