Small thermodynamic driving force

Steenken et al. have concluded that in double-stranded DNA direct hydrogen atom abstraction from 2 -deoxyribose by G(-H) radical is very unlikely due to steric hindrance effects and a small thermodynamic driving force [94]. The EPR studies performed in neutral aqueous solutions at room temperature have shown that, in the absence of specific reactive molecules, the lifetime of the G(-H) radical in double-stranded DNA is as long as -5 s [80]. Therefore, the fates of G(-H) radicals are mostly determined by the presence of other reactive species and radicals. Thus, the G(-H) radical can be a key precursor of diverse guanine lesions in DNA. In the next section we begin from a discussion of the site-selective generation of the G(-H) radical in DNA, and then continue with a discussion of the reaction pathways of this guanine radical. 

If a system is not far from global equilibrium, linear phenomenological equations represent the transport and rate processes involving small thermodynamic driving forces. Consider a simple transport process of heat conduction. The rate of entropy production is... 

Avoid excessively large or small thermodynamic driving forces in processes. 

The more favorable partitioning of [1+ ] to form [l]-OH than to form [2] must be due, at least in part, to the 4.0 kcal mol-1 larger thermodynamic driving force for the former reaction (Kadd = 900 for conversion of [2] to [l]-OH, Table 1). However, thermodynamics alone cannot account for the relative values of ks and kp for reactions of [1+] that are limited by the rate of chemical bond formation, which may be as large as 600. A ratio of kjkp = 600 would correspond to a 3.8 kcal mol-1 difference in the activation barriers for ks and kp, which is almost as large as the 4.0 kcal mol 1 difference in the stability of [1]-OH and [2]. However, only a small fraction of this difference should be expressed at the relatively early transition states for the reactions of [1+], because these reactions are strongly favored thermodynamically. These results are consistent with the conclusion that nucleophile addition to [1+] is an inherently easier reaction than deprotonation of this carbocation, and therefore that nucleophile addition has a smaller Marcus intrinsic barrier. However, they do not allow for a rigorous estimate of the relative intrinsic barriers As — Ap for these reactions. 

In the normal region, thermodynamic driving forces are small. The electron-transfer process is thermally activated, with its rate increasing as the driving force increases. 

The cofactors of both xanthine and aldehyde oxidases belong to the LMoVI(S)(0) subfamily (see Section IV). However, inactive dioxo forms, LMovi(0)2, of both xanthine and aldehyde oxidase are known. These dioxo forms do not catalyze oxidation of the respective substrates of these enzymes. The Mov/Molv redox potential for the inactive bis(oxido) form of xanthine oxidase differs from the oxido-sulfido form by -30 mV (bovine xanthine oxidase) and -lOOmV (chicken liver xanthine oxidase) [91]. Although the difference is small, given the xanthine/uric acid reduction potential (-360 mV), it is possible that the Mov/MoIV couple (-433 mV) of the chicken-liver xanthine oxidase bis(ox-ido) form impedes the effective oxidation of xanthine for redox reasons alone. However, the bis(oxido) form of bovine xanthine oxidase (with a reduction potential of -386 mV) should be able to oxidize xanthine, since the redox potential, and hence the thermodynamic driving force, is sufficient for activity [91,92,99]. As substrate oxidation does not occur, the chemical differences between the bis(oxido) and oxido-sulfido (Movl) forms must be critical to the dramatic difference in activity (see Section VI.E.l). 

The neutral 1,4- and 1,2-quinone methides react as Michael acceptors. However, the reactivity of these quinone methides is substantially different from that of simple Michael acceptors. The 1,6-addition of protonated nucleophiles NuH to simple Michael acceptors results in a small decrease in the stabilization of product by the two conjugated 7T-orbitals, compared to the more extended three conjugated 7T-orbitals of reactant. However, the favorable ketonization of the initial enol product (Scheme 1) confers a substantial thermodynamic driving force to nucleophile addition. By comparison, the 1,6-addition of NuH to a 1,4-quinone methide results in a large increase in the -stabilization energy due to the formation of a fully aromatic ring (Scheme 2A). This aromatic stabilization is present to a smaller extent at the reactant quinone methide, where it is represented as the contributing zwitterionic valence bond structure for the 4-0 -substituted benzyl carbocation (Scheme 1). The ketonization of the product phenol (Scheme 2B) is unfavorable by ca. 19 kcal/mol.1,2... 

Phase dissolution in polymer blends. The reverse process of phase separation is phase dissolution. Without loss of general validity, one may assume again that blends display LCST behavior. The primary objective is to study the kinetics of isothermal phase dissolution of phase-separated structures after a rapid temperature-jump from the two-phase region into the one-phase region below the lower critical solution temperature. Hence, phase-separated structures are dissolved by a continuous descent of the thermodynamic driving force responsible for the phase separation. The theory of phase separation may also be used to discuss the dynamics of phase dissolution. However, unlike the case of phase separation, the linearized theory now describes the late stage of phase dissolution where concentration gradients are sufficiently small. In the context of the Cahn theory, it follows for the decay rate R(q) of Eq. (29) [74]... 

Themiodynamic fluxes J are generally functions of thermodynamic forces X that induce the fluxes. Near the thermodynamic equihbrium, both the thermodynamic driving forces and fluxes of the processes are rather small. In such situations, the values of thermodynamic forces X and the conju gate fluxes J are in a simple linear relationship... 

From equation (11) it follows directly that in a series of reactions, if the work terms and nonadiabaticity parameters are the same or similar, the rates will correlate with the thermodynamic driving force. For small differences, the relationship is linear ... 

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