Judging crystal quality

When these drastic measures are required to yield good crystals, the crys-tallographer is faced with the question of whether the resulting fragment is worthy of the arduous effort to determine its structure. This question is similar to the basic issue of whether a protein has the same structure in crystal and in solution, and the question must be answered in the same way. Specifically, it may be possible to demonstrate that the fragment maintains at least part of the biological function of the intact molecule, and further, that this function is retained after crystallization. 

The acid test of a crystal s suitability for structure determination is, of course, its capacity to give sharp diffraction patterns with clear reflections at large angles from the X-ray beam. A brief inspection of crystals under a low-power light microscope can also provide some insight into quality and can help the crystallographer pick out the most promising crystals. 

Another useful physical property of the crystal is its density, which can be used to determine several useful microscopic properties, including the protein molecular weight, the proteinlwater ratio in the crystal, and the number of protein molecules in each asymmetric unit (defined later). Molecular weights from crystal density are more accurate than those from electrophoresis or most other methods (except mass spectrometry) and are not affected by dissociation or aggregation of protein molecules. The proteinlwater ratio is used to clarify electron-density maps prior to interpretation (Chapter 7). If the unit cell is symmetric (Chapter 4), it can be subdivided into two or more equivalent parts called asymmetric units (the simplest unit cell contains, or in fact is, one asymmetric unit). For interpreting electron-density maps, it is helpful to know the number of protein molecules per asymmetric unit. 

Crystal density is measured in a graduated cylinder by suspending the crystal in a density gradient made by mixing water-saturated organic solvents such as xylene and carbon tetrachloride. The crystal will settle through the liquid until its density matches that of the liquid mixture and then remain suspended there. Drops of salt solutions of known density are used to calibrate the gradient. 

The product of the crystal density and the unit-cell volume (determined from crystallographic analysis, Chapter 4) gives the total mass within the unit cell. This quantity, expressed in daltons, is the sum of all atomic masses in one unit cell. If the protein molecular mass and the number of protein molecules per unit cell are known, then the remainder of the cell can be assumed to be water, thus establishing the proteinlwater ratio. 

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For trypanosomal TIM we experimented with three different cocktails of 32 compounds (Table 4). Molecules were chosen in such a way that they would be compatible, soluble, cheap, and as varied as possible. Each compound was present at a concentration of 1 m M The final cocktail solutions were clear and devoid of precipitate. Since this was a pilot experiment both subcocktails were checked at each stage of the dichotomic strategy. Only the soak with cocktail 1 revealed electron density that could not be accounted for by water molecules, hereafter called peak X. The soaks with cocktails 2 and 3 led to featureless difference Fourier maps. The quality of the data and refinement can be inspected from Table 5, while Figure 9 illustrates the dichotomic search to identify peak X. An oxidized molecule of DTT, identified in the high-resolution structure of the native TIM crystals [24], served as an internal reference to judge the quality of the data and the noise level in the final difference Fourier maps. 

Quality control systems usually used for judging the quality of oils and fats or oil blends used in margarine production could evaluate color, color stabihty, flavor, flavor stabihty, free fatty acid, peroxide value, active oxygen method (AOM) stabihty, iodine value, shp melting point, fatty acid composition, refractive index, crystallization rate, and sohd fat/temperature relationship (solid fat index) (5, 91, 112, 113). 

Figure 16 shows the minority carrier lifetime derived from the time resolved photoluminescence decay curve. The minority carrier lifetime increases from 1.66 ns (as-grown) to 4.66 ns after the plasma treatment, and gradually decreases with increasing annealing temperature. It is difficult to judge the crystal quality only by the... 

Once preliminary crystallizing conditions have been found, they usually have to be optimized to grow large, diffraction quality crystals. There are several methods that are commonly used for improving the conditions. Diffraction quality crystals can only be judged by diffraction experiments, and the requirements will vary depending on what is required. 

Checking the data quality is strongly recommended inspection of the Wilson plot and data reduction statistics is very useful in judging the extent of the resolution to which the data can realistically be used. Pathologically bad data, for example those from a split crystal, twinned data, systematically incomplete data, low resolution overloads will always make model building and refinement hard if not impossible. 

The color of the crystals is pale yellow to almost white. A pink shade shows the presence of minor impurities. The quality of the product is conveniently judged from the ultraviolet spectrum. The bands at wave numbers 35,000 and 44,100 cm.-1 with molar extinction coefficients of 73 and 333, respectively, are typical of the chloropentaammine. Spectra of pink crystals show less resolved bands. These products are, however, sufficiently pure to serve as starting materials for further reactions. The molar conductivity in aqueous solution is 253 cm.2 ohm-1 mole-1. The crystals are soluble in water but insoluble in HC1 or in common organic solvents. 

Sample Selection and Mounting. The sample should now be safe for a few minutes, and one can proceed to selection and mounting of a crystal. Selection of a suitable crystal is similar to procedures commonly used for chemically stable samples. A sowing needle held with a pin vice, and a sharp razor blade are useful tools. The binocular microscope should have a polarizing attachment to assist in judging quality of crystallinity. 

The quality of a crystal structure cannot be better than the quality of the experimental data on which it is based. The experimental data can be judged by the following criteria. 

The issue of how to reliably and usefully characterize the quality of a single-crystal sample of Ba2YCu307 is not fully resolved. Resistivity and susceptibility are relatively crude indicators. The suitability of any crystal for a given measurement and subsequent interpretation must be judged with attention to possible sample limitations. 

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