Coulombic effects reactions

The signs + and are for repulsion and attraction, respectively.) L is called Onsager radius [73] at which the Coulomb interaction energy equals the thermal energy kBT. Finally, for the Coulomb attraction (see more details in Chapter 4), the effective reaction radius is [67]... 

Enzymes are a class of macromolecules with the ability both to bind small molecules and to effect reaction. Stabilizing forces such as hydrophobic effects only slightly dominate destabilizing forces such as Coulombic forces of equal polarity thus the Gibbs free enthalpy of formation of proteins, AGformation, is only weakly negative. 

The variation in the kinetics observed for this reaction are related to charge accumulation on the oxygen-evolving enzyme, as recently clearly demonstrated by Brettel et al [162], who measured 820 nm absorption changes as a function of flash number. When S(, or Sj was present, P-680 was reduced largely with a t /2 of 50 ns, while, when S2 or S3 was present, P-680 reduction slowed down (t,/2 240 ns). This was attributed to a coulombic effect of the positive charge present in the S2 and S3 states (Ref 162, and see Section 3.5). When the 02-evolving enzyme is destroyed by, for example, treatment with Tris, the donation of electrons to P-680 is slowed down dramatically. This reaction is sensitive to the pH (fi/2 = 4-6 /us pH 7.0, 14 /JLS pH 5.0) [170]. 

Considering first, however, polar solvents where only the coulomb and screened coulomb potentials are signficant by comparison with thermal energies, it should be emphasized that theory is quite successful in explaining the experimental results. The Debye [68] expression for the rate of reaction has been discussed in Chap. 3, Sect. 1. With an Onsager distance 0.7—1.0 nm, the effective reaction radius is 0.9—1.2 nm or... 

A potential problem for CA modeling is that the databases for metal ion adsorption have generally been developed using models that correct mass action equations for coulombic effects, but coulombic effects on adsorption are poorly understood in soils and sediments. For example, in the diffuse double layer (DDL) model of Dzombak and Morel (1990), a typical molecular scale reaction that describes the adsorption of divalent metal ions by Fe oxides is ... 

The electrostatic effect of an ionic substituent on the reactivity of a functional group subject to attack by another ion persists over a much greater length of intervening chain than is found for the influence of an unionized substituent (compare column B of Table V). This follows of course from Coulomb s law. From the standpoint of polymer reactions, it is important to bear in mind that when charged... 

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