Polymorphism definition

Proton transfer along a hydrogen bond poses an interesting question about polymorph definition. In fact, proton mobility along a hydrogen bond (say from... 

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

Essential to the definition of Pharmacogenetics is the term genetic polymorphism. It is extrapolated that there are at least three million genetic polymorphisms in the human genome. Historically, a genetic polymorphism was defined as a genetic variation with a population frequency of 1% and above, but the larger inter-ethnic variation of population frequencies makes a strict definition based on such frequencies impractical. The most common molecular type of polymorphism is the... 

There are other soUd states which sometimes confuse the measurement and definition of solubiUty. The dmg may crystaUize as a hydrate, i.e. under inclusion of water molecules. If the hydrate form is more stable than the pure form it may be difficult to measure the intrinsic solubility of the drug at all. Often drugs tend to precipitate in an amorphous form, often under the inclusion of impurities. As with metastable polymorphs, such amorphous precipitates may lead to erroneously high solubility measurements. CommerciaUy, drugs are often crystallized in salt form, e.g. as the hydrochloride salt, a cation with a chloride anion. In these co-crystallized salts, a much lower solubility than the intrinsic solubility will typi-... 

Meyer UA. Genotype or phenotype the definition of a pharmacogenetic polymorphism. Pharmacogenetics 1991 1 66-67. 

In Chapter 1 we introduced the term phase. A phase is a state that has a particular composition and also definite, characteristic physical and chemical properties. We may have several different phases that are identical in composition but different in physical properties. A phase can be in the solid, liquid or gas state. In addition, there may exist more than one distinct crystalline phase. This is termed polymorphism, and each crystalline phase represents a distinct polymorph of the substance. 

There is a diversity of opinion regarding definitions and benefits of pharmacogenetics and pharmacogenomics.1 3 For example, pharmacogenetics is often considered to be the study of inter-individual variations in DNA sequence related to drug absorption and disposition (pharmacokinetics, PK) or drug action (pharmacodynamics, PD). Polymorphic variation in the genes that encode the functions of transporters,... 

A ML can be simply defined as a one-molecule thick 2D film, but the molecular surface density has to be defined for each molecule-substrate system because it depends on the shape, size and relative orientation of the molecules. To clarify this point let us consider the examples of PTCDA and Ceo on the Ag(l 11) surface. The surface density of the substrate is 1.4 x 10 atoms cm , which is usually defined as 1 ML as a reference limit. The surface density of the (102) plane of PTCDA, the cleavage plane, is 8.4 x 10 and 8.3 x 10 cm (molecules cm ) for the monocliiuc a and polymorphs, respectively. Therefore, full coverage corresponds to 0.02 ML according to this definition. On the other hand, the surface density of a full hexagonal layer of closed-packed Ceo molecules corresponding to the (111) plane in the fcc-Ceo crystal is 1.2 x 10 " cm . Thus, Ceo would fully cover the Ag(l 11) surface at a coverage of 0.09 ML. However, other authors define 1 ML as... 

Although the information available from synthetic studies strongly indicates a P-T control of the chlorite polymorph, natural minerals appear to exhibit both polymorph, 7 and 14 8, at low temperature. Most diagenetic chlorites correspond to a 7 8 polymorph. However, there are occurrences especially in deep ocean sediments of a 14 8 phase. The contradiction cannot be resolved with the information available at present. It is probably reasonable to assume that the 7 8 polymorph stable for all chlorite compositions (i.e., various Fe +, Mg, A1 ratios) and that the 14 8 forms are metastable at low temperatures. However, this is certainly not definitive. 

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