Polymorphism identification method

DNA testing is rapidly becoming the predominant technique for human identification. The restriction fragment-length polymorphism (RFLP) method requires large amounts of DNA (20-100 ng) and is extremely time-consuming and labor-intensive. 

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

The prediction of crystal structures by ab initio quantum mechanical methods, and the identification of stable polymorphic forms and the conditions under which they will crystallize, is of great interest to the pharmaceutical industry. Some progress has been made towards this goal in recent years [17, 18] with a degree of success for small and conformationally simple pharmaceuticals. The methods are still a number of years away from routine use in the day to day research and development environment. 

A NIR identification library shonld encompass all the raw materials used by the manufacturer in order to be able to identify all possible snbstances and avoid or minimize errors. The method to be nsed should allow the unequivocal identification of each componnd present in the library and the exclnsion of those not present. It should also be able to distinguish between very similar compounds nsed in different applications (e.g. products with different particle sizes, polymorphs, products in different grades or from different suppliers). 

Probabilistic methods can be applied in dose-response assessment when there is an understanding of the important parameters and their relationships, such as identification of the key determinants of human variation (e.g., metabolic polymorphisms, hormone levels, and cell replication rates), observation of the distributions of these variables, and valid models for combining these variables. With appropriate data and expert judgment, formal approaches to probabilistic risk assessment can be applied to provide insight into the overall extent and dominant sources of human variation and uncertainty. 

Mapping of eukaryotic chromosomes has involved additional methods which are discussed in Sections E and G,l. These incluse radiation hybrid mapping,104 use of meiotic recombination, identification of restriction fragment length polymorphisms (RFLPs described in Section E,7), and use of expressed sequence tags (ESTs), short DNA sequences deduced from mRNA molecules transcribed from the DNA.105 106... 

It is now recognised that a substantial number of proteins, especially enzymes, are polymorphic in that they exist in the cell as multiple molecular forms differing in certain of their physico-chemical properties. Each form of a polymorphic enzyme is called an isoenzyme or isozyme. Electrophoretic techniques provide convenient methods whereby this protein heterogeneity can be investigated and the approach has been widely exploited to characterise parasites. In short, aqueous parasite extracts are electro-phoresed, or focused isoelectrically, and separated proteins are stained generally (usually with Coomassie Blue) or more specifically with a histochemical (enzyme) stain (the zymogram technique). Further details of individual procedures and the use of the approach in parasite identification are to be found in a number of recent reviews (104,258,413,536,615,856). 

Single-crystal diffraction methods (whether based on X-ray or neutron sources) are those that carry the most complete information on the intimate nature of the crystals and therefore provide the most valuable tools for identification, characterization and comparison of polymorphs and pseudo-polymorphs. It is difficult to deny that one of the reasons for the outpouring of new results in the field of crystal engineering is the quantum leap represented by the commercial availability of single-crystal diffractometers equipped with area detectors. These devices have not only reduced the time of data collection by an order of magnitude with... 

Physical stability, the propensity of compounds to adopt different polymorphs depending on methods of isolation is also now beginning to be tackled by the computational chemist. Ideally, molecules which can only crystallise as a single polymorph would be preferred, although preparation of a compound in the desired (most stable) polymorph, can usually be mastered by the experimental chemist. Nonetheless, identification of the 50% of organic molecules likely to crystallise in different polymorphs would be useful, especially if this can be predicted from the molecular structure of the compounds even before they are synthesised. 

As noted in the previous section, the phenomenon of polymorphism is not new to chemistry. Nineteenth century chemists were very much aware of the properties of solids, and in the decades preceding the development of spectroscopic and X-ray crystallographic methods, the characterization of solids was a crucial aspect of the identification of materials. Chemists grew crystals carefully in order to obtain characteristic morphologies and then determined physical properties such as colour, interfacial angle, indices of refraction, melting point, and even taste (e.g. Schorlemmer 1874 Senechal 1990 Kahr and McBride 1992). Being critically observant was essential, for there was little other information to rely on. 

Another approach was taken by Winchell (1943, 1987), who prepared a compilation of all organic compounds whose optical properties are sufficiently well known to permit identification by optical methods . The compilation is arranged in the same fashion as the fourth edition of Beilstein s Handbuch der organischen Chemie (Beilstein 1978), and at the time of its first publication was meant to include all organic compounds whose indices of refraction had been measured. Since indices of refraction differ among them, polymorphs could be easily recognized by different optical properties. The book does contain references to primary sources and drawings of crystals, as illustrated in a typical entry Fig. 1.2. 

X-ray crystallographic methods, which reflect differences in crystal structure, in most cases can be definitive in the identification and characterization of polymorphs, and whenever possible should be included in the analytical methods utilized to define a polymorphic system. 

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