Fractional molecule

Filtration through cross-linked dextrans (e.g. Sephadex, Pharmacia, Uppsala, Sweden) makes it possible to fractionate molecules, including proteins, on a commercial scale. It is possible to separate the casein and whey proteins by gel filtration but the process is uneconomical on an industrial scale. 

B. Blue Dextran 2000 was eluted during gel filtration in a volume of 36.4 mL from a 2.0 X 40 cm (diameter X length) column of Sephadex G-50, which fractionates molecules in the molecular mass range 1 500 to 30 000. 

This is probably the most popular use of gel chromatography. Because of the ability of a gel to fractionate molecules on the basis of size, gel filtration complements other purification techniques that separate molecules on the basis of polarity and charge. 

Gel electrophoresis is a powerful and versatile method to resolve mixtures of different nucleic acid molecules and allows the fractionated molecules (i) to be viewed directly, (ii) to be recovered in pure form or (iii) to be characterized directly by hybridization. Hybridization of the probes to fractionated DNA ( Southern technique ) (Southern, 1975) or fractionated RNA ( Northern technique ) (Al-wine et al., 1979) can be achieved after the transfer of the resolved molecules to a membrane, but in some cases also directly in the gel using oligonucleotide probes ( unblot ) (Purrello and Balazs, 1983 Tsao et al., 1983). The steps in these protocols are summarized in Table 9.1. Simultaneous extraction of DNA and RNA (Section 3.4.3) (Chan et al., 1988) may be advantageous when the mass of tissue available is small. 

Part of the petroleum refinement process involves adjusting the percentage of each hydrocarbon fraction to match commercial demand. For example, the demand for gasoline is higher than that for kerosene. As a result, chemical reactions convert the larger kerosene-fraction molecules into molecules... 

Generally, it is preferable to avoid using fi actional coefficients because they could be interpreted as implying the existence of fractional molecules. But we can easily eliminate any fractions by multiplying the entire equation by a factor that removes them. In this case multiplying by 2 will do the trick, converting the 1/2 to 1. This brings us to a final form of the balanced equation ... 

Mark-Houwink relationships are also important for the application of the universal calibration procedure in GPC/ SEC of polymer molecules, where the calibration curve is expressed in terms of the size of fractionated molecules against retention volumes. The intrinsic viscosity of a polymer (expressed in cm /g) is, in practice, a measure of the... 

For coal with C H N 0 S as constituents, the individual combustible fuel fraction molecules will undergo combustion described by the... 

Elementary reactions cannot involve fractional molecules. We may choose to write a balanced stoichiometric equation so that it contains fractional molecules, e.g.,... 

One other factor that must be considered in analyzing reactions is the principle of microscopic reversibility. This principle states that an elementary reaction must follow the same path in both the forward and reverse directions. The practical imphcation of microscopic reversibility is that a reaction must pass the simplicity tests and the fractional molecule test in both directions. A reaction cannot be elementary in one direction and not in the other. Consider the reaction... 

Fractional molecules Molecularity (number of molecules colliding) Total of bonds broken and formed Not permitted One or two (unimolecular or bimolecular) Elementary termolecular processes are rare, and must be regarded with suspicion One or two, preferably. Three is acceptable. Some elementary reactions may involve as many as four. However, when the total is three or higher, look for simpler pathways, i.e., a sequence of simpler reactions rather than a single, complex reaction... 

While this is properly balanced, it is preferable to have whole numbers instead of fractions for stoichiometric coefficients, because it is not possible to have fractional molecules. Let s multiply all coefficients by 2 ... 

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