Polymorphism selective pressure

Furthermore, it has been observed that placental PGP is of great importance in limiting the fetal penetration of various potentially harmful or therapeutic compounds [64]. High PGP levels may have a role in the protection of fetuses from harmful tropical plant metabolites (from which many drugs are derived). An originally balanced polymorphism may have been preferentially selected by improved prenatal growth and development as well as improved postnatal health. This selective pressure may maintain the C allele in populations of African descent. 

Observed differences between strains of rats and mice, as described below, may be the result of gene polymorphisms. In cases involving insecticide selection pressure, resistant populations may arise as a result of direct mutations of insecticide-metabolizing enzymes and/or insecticide target sites that are passed on to succeeding generations. 

Ulrich CM, Carlson CS, Sibert J, Poole EM, Yu JH, Wang LH, Sparks R, Potter JD, Bigler J (2005) Thromboxane synthase (TBXASl) polymorphisms in African-American and Caucasian populations evidence for selective pressure. Hum Mutat 26 394-395... 

Beside mid-IR, near-IR spectroscopy has been used to quantitate polymorphs at the bulk and dosage product level. For SC-25469 [34], two polymorphic forms were discovered (a and /3), and the /3-form was selected for use in the solid dosage form. Since the /3-form can be transformed to the a-form under pressure by enantiotropy, quantitation of the /3-form in the solid dosage formulation was necessary. Standard mixtures of both forms in the formulation matrix were prepared, and spectra were measured in the near-IR via diffuse reflectance. Utilizing a standard, near-IR multiple linear regression, statistical approach, the a- and /3-forms could be predicted to within 1% of theoretical. This extension of the diffuse reflectance IR technique shows that quantitation of polymorphic forms at the bulk and/or dosage product level can be performed. 

The structures of three forms of Rh203 have been determined, the normal (corundum) form, a high-temperature orthorhombic form, and a high-pressure polymorph made at 1200°C under 65 kbar. In this high-pressure form there are pairs of face-sharing octahedra (as in corundum) but a different selection of shared edges the structure may be regarded as built from slices of the corundum structure. 

Catalysts based on transition metal molybdates, typically bismuth, cobalt and nickel molybdates [2-6], have received recent attention. Of the transition metal molybdates, those based on nickel, and in particular the stoichiometric NiMo04, have attracted the greatest interest. NiMo04 presents two polymorphic phases at atmospheric pressure a low temperature a phase, and a high temperature P phase [2,7]. Both phases are monoclinic with space group dim. These phases differ primarily in the coordination of molybdenum which is distorted octahedral in the a phase and distorted tetrahedral in the P phase. The P phase has been shown to be almost twice more selective in propene formation than the a phase for comparable conversion at the same temp>erature [2]. A similar effect has been noted for oxidative dehydrogenation of butane, with the P phase being approximately three times more selective in butene formation than the a phase [8]. The reason for the difference in selectivities is unknown, but the properties of the phases are known to be dependent on the precursors from which they are derived. Typically, nickel molybdates are prepared by calcination of precipitated precursors. 

Field D, Wills C. Abundant microsatellite polymorphism in Saccharomyces cerevisiae, and the different distributions of microsatellites in eight prokaryotes and S. cerevisiae, result from strong mutation pressures and a variety of selective forces. Proc Natl Acad Sci USA 95 1647-1652, 1998. 

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