Methods for Investigating Crystallographic Structure

Rudiments of Crystallography I 3.4 Methods for Investigating Crystallographic Structure 39... 

Three main tendencies have been underlined in recent studies of structure and action mechanism ofbacterial photosynthetic reaction centers. The crystallographic structure of the reaction centers from Rps. viridis and Rb. spheroids was initially determined to be 2.8 and 3 A resolutions (Michel and Deisenhofer et al., 1985 Allen et al., 1986). Resolution and refinement of these structures have been subsequently extended to 2.2, 2.3 and 2.6 A. (Rees et al., 1989 Stowell et al., 1997, Fyfe and Johns, 2000 Rutherford and Faller, 2001). Investigations of the electronic structure of donor and acceptor centers in the ground and exited states by modern physical methods with a combination ofpico-and femtosecond kinetic techniques have become more precise and elaborate. Extensive experimental and theoretical investigations on the role of orbital overlap and protein dynamics in the processes of electron and proton transfer have been done. All the above-mentioned research directions are accompanied by extensive use of methods of sit-directed mutagenesis and substitution of native pigments for artificial compounds of different redox potential. 

Work on direct methods was continued independently by Jerome Karle and Herbert Hauptman, ° Joseph Gillis, William H. Zachariasen, David Sayre, William Cochran, and Isabella Karle. Phase relationships were clearly found for the crystal structure of deca-borane (which is centrosymmetric), but were not so clear for some other structures. Gillis showed, however, that often certain inequalities were nearly satisfied, and this observation led to subsequent investigations of the probability relationships among the structure factors. Karle and Hauptman went on to show how the use of inequalities can restrict the range of phase angles for non centrosymmetric structures.. All of these studies led to the direct methods now used routinely in small-molecule crystallographic laboratories. 

In the best-designed studies of polymorphic or solvate systems, the purpose of the vibrational spectroscopy investigation should be to gather information from the observed pattern of vibrational frequencies and to use these data to understand the structural aspects that yield crystallographic differences. Once suitable spectral features are identified from this work, they can be used to develop easily performed methods for the quantitative analysis of one polymorph (or solvate) in the presence of the other. Unfortunately, all too many workers are merely... 

The earliest investigations of the crystal structure of cerium (Hull, 1921) indicated that cerium had both a face centered cubic (fee) and a hexagonal close-packed (hep with cl a = 1.62) structure at room temperature and one atmosphere. Subsequent studies revealed only the fee phase with a ==5.15 A, which today is identified as y-Ce (see table 4.2 for additional crystallographic data). About 25 years ago the a-Ce phase was identified by X-ray methods at both high pressure and room temperature (Lawson and Tang, 1949) and at low temperature and atmospheric pressure (Schuck and Sturdivant, 1950) to have a fee structure but with a = 4.83 A. The a phase, with a volume about 17% less than that of y-Ce, is also known as the collapsed fee phase. This large volume difference is also observed in dilatometric and volumetric studies of the a y transformation. 

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