Competitive phenomenon

Accordingly, this chapter deals mainly with some of the very recent advances in knowledge of iron transport systems in enteric bacteria. I review critically and place in historic perspective current understanding of the competition phenomenon between phages, colicins, and sidero-phores for outer membrane receptors, the resistance to certain colicins imparted by iron, and, finally, the mechanism of siderophore-mediated iron uptake. 

The extreme potency of siderophores in counteracting the B-group colcins suggests an important biological role for such molecules in a mechanism which resembles, but which is fundamentally different in nature from, the outer membrane competition phenomenon. 

The following example illustrates the above competition phenomenon, in which CH3 can be largely adsorbed on the Si site and SiCl3 on the C site. 

The retention model in adsorption chromatography developed by Snyder and Soczewinski is based on the assumption that there is flat adsorption in a monomolecular layer on a homogeneous adsorption surface. The adsorption is understood as a competition phenomenon between the molecules of the solute and the solvent on the adsorbent surface, so that the retention of a sample molecule requires the displacement of one or more previously adsorbed polar solvent molecules. Later, the model was corrected for adsorption on a heterogeneous surface of adsorbent. To first approximation, the solute-solvent interactions in the mobile and stationary phases are assumed to compensate each other and possible liquid-liquid partition effects are neglected. In this case, the retention in a mixed binary mobile phase comprising a nonpolar solvent, A (usually an aliphatic hydrocarbon), and a polar solvent, B, can be described by eqn [1] ... 

Adsorption from solution is sometimes used to assess the adsorptive capacity and hence the surface area of adsorbents. For instance, iodine (1,2), dyes (3,4), organic mixtures (5) or surfactants (6) have been used, but this never lead to a method as wide and generally accepted as those based on gas adsorption, like the BET, BJH, 0 j or Dubinin-Radushkevitch methods (7). The two main reasons are certainly that adsorption from solution is a competitive phenomenon between solvent and solute which is more difficult to interpret than the adsorption of a single gas and also that solute molecules are generally subject to more specific interactions with the surface than the gas molecules conventlonaly used (N, Ar. ..). 

Koopal and co-workers [186] have extended this thermodynamic analysis to investigate the competitive wetting of a solid by two relatively immiscible liquids. They illustrate the tendency of silica to be preferentially wet by water over octane, a phenomenon of importance in oil reservoirs. 

For very fast reactions, the competition between geminate recombmation of a pair of initially fomied reactants and its escape from the connnon solvent cage is an important phenomenon in condensed-phase kinetics that has received considerable attention botli theoretically and experimentally. An extremely well studied example is the... 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

It is well known that electrospray ionization (El) suffers from suppression effects when polar/ionic compounds other than the analyte(s) of interest, such as those originating from the sample matrix, are present, with this phenomenon being attributed to competitive ionization of all of the appropriate species present [33]. Matrix effects can, therefore, be considerable and these have two distinct implications for quantitative procedures, as follows ... 

In addition, the data indicated that the strong inhibition = 4/iM) of the exchange reaction by the unphosphorylated sugar, a phenomenon reported for many E-IIs [73,104-106] was due to competition between fructose and fructose-l-P for the binding site on the enzyme. The important implication of this result was that it would indicate that unphosphorylated II possessed a high affinity for the sugar, which, subsequently, was confirmed by binding studies. Furthermore, it was concluded that... 

In many ecosystems, plants tend to pattern themselves as pure stands or as individuals spaced in rather specific densities or configurations. Many desert species show obvious zones of inhibition around which few, if any, alien species are able to invade. These patterns often cannot be adequately explained by competition alone, and are probably caused by a combination of factors including allelopathy. The phenomenon happens with herbaceous plants as well as woody shrubs and trees. 

A final source of evidence for the formation of inclusion complexes in solution has been derived from kinetic measurements. Rate accelerations imposed by the cycloamyloses are competitively inhibited by the addition of small amounts of inert reagents such as cyclohexanol (VanEtten et al., 1967a). Competitive inhibition, a phenomenon frequently observed in enzymatic catalyses, requires a discrete site for which the substrate and the inhibitor can compete. The only discrete site associated with the cycloamyloses is their cavity. 

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