Polymorphism classification

After a temptative structure-based classification of different kinds of polymorphism, a description of possible crystallization and interconversion conditions is presented. The influence on the polymorphic behavior of comonomeric units and of a second polymeric component in miscible blends is described for some polymer systems. It is also shown that other characterization techniques, besides diffraction techniques, can be useful in the study of polymorphism in polymers. Finally, some effects of polymorphism on the properties of polymeric materials are discussed. 

In the second section a classification of the different kinds of polymorphism in polymers is made on the basis of idealized structural models and upon consideration of limiting models of the order-disorder phenomena which may occur at the molecular level. The determination of structural models and degree of order can be made appropriately through diffraction experiments. Polymorphism in polymers is, here, discussed only with reference to cases and models, for which long-range positional order is preserved at least in one dimension. 

In humans, erythrocytes contain an esterase that displays genetic polymorphism [86], This esterase has been called esterase D (ES-D), a name without connection to the above-presented A-, B-, and C-classification. Three carboxylesterases named HU1, HU2, and HU3 have been found in human liver microsomes. Other tissues where esterases have been found include brain, plasma, stomach, small intestine, and colon [79]. 

Sihcon dioxide exists in several structural forms polymorphic crystalline sihca, synthetic quartz crystals, amorphous silica, and vitreous silica. This classification is not complete as there are other forms of silica synthesized for speciahzed apphcations. Various forms of sihca are mentioned briefly below. 

Gordon et al. (48) recently performed a pathway-based pharmacogenomic study on rectal cancer treated with chemoradiation in which they evaluated 21 polymorphisms in 18 genes involved in the critical pathways of cancer progression (drug metabolism, tumor microenvironment, cell cycle control, and DNA repair). They applied the CART analysis and found that a classification tree with four genes (lL-8, lCAM-1, TGF-, and... 

The following section shows specific examples of association of dicoordinated mononuclear gold units which have been classified according to the nuclearity of the aggregate. Despite this general classification, the different polymorphic forms of a specific compound are discussed together, as well as the structures of complexes with small ligand variations. 

Small changes in substituents may lead to different aurophilic aggregation patterns. Thus, different structures have been described for very similar species and polymorphs of different nuclearities have even been found for one complex. Thus, a classification based only on the nuclearity of the aggregates is not only difficult, but may also hide other interesting comparisons. For these reasons some chain structures have been mentioned above. The following tries to complete those commentaries. 

NIR spectroscopy was utilized by Aldridge and coworkers86 to determine, in a rapid manner, the polymorphic quality of a solid drug substance. Two computational methods, Mahalonobis distance and soft independent modeling of class analogy (SIMCA) residual variance, were used to distinguish between acceptable and unacceptable samples. The authors not only determined that the Mahalonobis distance classification yielded the best results, they addressed one of the key implementation issues regarding NIR as a PAT tool. 

Two isoenzymes, peptidase A and glutathione reductase, have been reported to have polymorphic forms in negro populations and little or no variants in Caucasians (13, 14, 15). Thus, should the rare variant be demonstrated in a stain, there would be a high probability concerning the ethnic origin of the blood. In addition, research has been initiated into the use of Gm and Inv typing to assist in the anthropological classification of bloodstains (2, 17). For example, the combination of Gm factors 1,... 

This serves as an example of the extremes one can go if one tmly wants to better understand the process and the form of the compound in the product. This approach is less important for detecting polymorphic form conversion in the solid state, since polymorphic solubility typically varies by less than a factor of two and tablets at very low strength would generally be classified as a type one by the biopharmaceutical classification system. In addition, polymorphs rarely have pronounced differences in chemical stability. On the other hand, if a compound is somewhat unstable and significant amount of amorphous material is potentially present, this approach can be used to determine if it is responsible for drug instability in the tablet formulation. In any event, this reference serves as a very good example of the strength of a powerful approach that has not been widely applied in our industry. 

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