Multiple crystals

S. Mourachov. Cellular automata simulation of the phenomenon of multiple crystallization. Comput Mater Sci 7 384, 1997. 

The probing of the active site of an enzyme by using multiple crystal structures containing different small molecules was originally described using different organic solvents as the probe molecules [21]. This technique showed how the information derived from the small molecules binding in... 

Despite the improvements outlined above, the modest yield through the resolution process (around 30% yield of 97% ds salt) and the need for multiple crystallizations... 

To test this hypothesis beyond CO adsorption on Pt(l 11), Weaver et al. compared CO and NO stretching frequencies on multiple crystal facets of Pt, Rh, Pd, and Ir in UHV and electrochemical environments.58 With the exception of NO and CO on Pt(l 11), in which both unsolvated and D20 solvated environments were examined, only unsolvated UHV environments were considered. In this comparison, the same... 

The preparation of immiscible polymer blends is another way to disperse a bulk polymer into fine droplets. It has been reported for several polymers that when they are dispersed in immiscible matrices into droplets with average sizes of around 1 pm, they usually exhibit multiple crystallization exotherms in a differential scanning calorimetry (DSC) cooling scan from the melt (at a specific rate, e.g., 10 Cmin ). Frensch et al. [67] coined the term fractionated crystallization to indicate the difference exhibited by the bulk polymer, which crystallizes into a single exotherm, in comparison with one dispersed in a large number of droplets, whose crystallization is fractionated temperature-wise during cooling from the melt. 

Linear trans-quinacridones exhibit multiple crystal modifications. This phenomenon is reflected in the different reflection angles which are observed in the X-ray diffraction spectra of the pigment powders. This effect may be exemplified by describing the polymorphous phases of unsubstituted quinacridone. 

KcsA crystals suitable for X-ray crystallographic analysis using synchrotron radiation were obtained and the data collected and analyzed for multiple crystals and six different data sets as described in the 1998 Science publication (reference 15). The final KcsA pore structure, including amino acid residues 23 to 119 of the K+ channel, refined to 3.2 A. The X-ray data were deposited in the Protein Data Bank with the accession number 1BL8. 

In the general multiple-crystal beam conditioner case, there is no universal formula for broadening. Rather, the duMond diagram is constructed for the beam conditioner and the shape of the passed band in and is determined. The specimen crystal is then represented on the duMond diagram and scanned... 

Fewster obtains this from the increase of satellite widths as a function of satelhte order. The trae integral breadths or FWHM values are first found with a double or multiple crystal diffractometer this is simple measurement, and with a powder... 

The use of several crystals, one behind the other, in a multiple crystal holder with automatic change and data updating in the event of imminent failure of a quartz CrystalSix. 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

As with ceramics and metals, polymer crystals can have multiple crystal forms. Polyethylene has a metastable monoclinic form and a orthohexagonal high pressure form. A list of some of the more common polymers and their corresponding crystal strnctnres is given in Table 1.24. Finally, X-ray diffraction can be used to determine the amorphous to crystalline ratio in semicrystalUne polymers in much the same way that Eq. (1.61) can be used. Figure 1.66 shows a schematic illustration of the X-ray diffraction patterns for semicrystalline and amorphous polyethylene. The estimation of crystalline content is based upon a ratio of the peak areas in the two samples. 

Ammonium nitrate (AN) was considered early as an environment-friendly alternative to AP but its multiple crystal phase-transitions at low temperatures and its poor performance precluded its use. The nitramine-based propellants are also likely to emerge as potential eco-friendly propellants as the combustion products are non-toxic and non-smoky, although the present day nitramine propellants do not match the high performance and high burn rates of AP-based composite propellants [62, 63]. At the same time, high pressure exponent and unstable combustion prevent their application in large rockets due to safety considerations [20]. The inclusion of Al powder and other additives increases the burn rate and also eliminates the combustion instability. 

During data collection from crystals exposed to fragment(s), collect a data set that is complete in the low-resolution shells and has high redundancy. Also beneficial will be the highest resolution data possible, so examination of multiple crystals to select the one with suitable qualities is crucial (see Note 12). 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

X-ray crystal stmcture of hydroperoxo-ferric complex in CPO was determined as a result of cryogenic photoreduction of oxy-ferrous precursor during data collection in the synchrotron beam.133 Three data sets with low, medium, and high dose have been prepared from the data collected on multiple crystals with the goal to resolve the structures with well-defined redox state. Because of the fast reduction, the low-dose data set already had sufficient contribution of Compound 0, and the stmcture of Compound III could not be determined. The medium-dose data set was resolved to 1.75 A resolution and revealed a well-defined 0-0 ligand with 100% occupancy. Assignment of this stmcture to hydroperoxo- ferric versus peroxo-ferric was done based on the QM/MM calculation. 

Most crystallization processes produce particles whose sizes cover a range of varying breadth. If the particles consist of single crystals, the resulting distribution is a crystal size distribution (CSD) on the other hand, if the particles consist of agglomerates or other combination of multiple crystals, the distribution is a particle size distribution. In either case, the distribution is expressed in terms of either population (number) or mass. The popu-... 

As long as a simultaneous docking of water molecules and ligands is an unsolved problem, it remains common practice to consider essential water molecules as a fixed part of the binding site. Preplaced water molecules may either correspond to recurrently observed waters found in multiple crystal structures of the tar-... 

With multiple crystal structures and amino acid sequences of a large number of iLBPs, the invariant and highly conserved amino acid sites can be analyzed to determine if they are needed for correct folding, for function, or in some instances for both. However, even in a small protein of known structure, individual roles of conserved residues can be difficult to determine. This is easiest if the comparisons are limited to a subfamily in which the level of invariant positions is relatively high. 

This is not a method commonly used for coordination compounds, which do not often melt without decomposition. Where applicable, it can be used by cooling from a high-temperature melt to room temperature, or by cooling a room-temperature liquid to a lower temperature. The latter is a specialized technique, usually carried out in situ on a diffractometer, with monitoring of the crystal growth by optical and X-ray methods the sample is contained in a sealed capillary tube, and selective heating may be applied by an infra-red laser to develop a single crystal. Twins and multiple crystals often result from these methods. 

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