Bound free-radical hypothesis

The Bound Free-Radical Hypothesis How Does Coenzyme Bi2 Work ... 

Scheme 3. Schematic representation of the bound free-radical hypothesis for Bi2-catalyzed reactions.

Support for the bound free-radical hypothesis (i.e., Scheme 3) comes from a variety of sources and has been summarized in several comprehensive reviews [5, 6, 26-30]. Early evidence was obtained from electron spin resonance (ESR) [31] and ultraviolet (UV) [32] spectroscopic results. Additionally, labeling experiments provided evidence of hydrogen transfer between the substrate of a variety of Bi2-dependent enzymes and the coenzyme [33, 34]. Although objections have been raised to the hydrogen-transfer step [29] on the basis that the abstractions are proposed to take place at relatively unactivated positions, calculations of the thermodynamics of the hydrogen-transfer steps support this mechanistic proposal [21, 35]. 

Implicit in our approach is, of course, acceptance of the bound free-radical hypothesis. As we have seen in the previous sections, this appears to be the most likely mode of action for coenzyme B12. The widespread acceptance [1] of this pathway demonstrates that it has, so far, been able to stand the test of time. Our attention is now turned to theoretical techniques that can provide accurate estimates of thermochemical data. 

This transformation is part of a microbial metabolic pathway in which nicotinate is broken down into ammonia, CO2, acetate and pyruvate [16, 68]. Accepting the bound free-radical hypothesis implies that the cmcial radical rearrangement step can be represented by reaction 4, in which a 2-methyleneglutarate-derived radical (1) is transformed into an (/ )-3-methylitaconate-related radical (2) [6] ... 

This reaction represents the first step in the fermentation of glutamate to acetate and butyrate in many clostridia [4, 80]. Once again, accepting the bound free-radical hypothesis leads to the following radical rearrangement ... 

Our treatment of chain reactions has been confined to relatively simple situations where the number of participating species and their possible reactions have been sharply bounded. Most free-radical reactions of industrial importance involve many more species. The set of possible reactions is unbounded in polymerizations, and it is perhaps bounded but very large in processes such as naptha cracking and combustion. Perhaps the elementary reactions can be postulated, but the rate constants are generally unknown. The quasi-steady hypothesis provides a functional form for the rate equations that can be used to fit experimental data. 

On the other hand, numerous observations favour the opinion that the solvent enters into a chemical combination with nitrocellulose to form solvates. Some of those solvates are stable only at low temperatures. For instance, cellulose dinitrate does not dissolve in methyl alcohol at room temperature, though on cooling it does so. The cellulose nitric ester precipitates again when the solution is heated. Similar behaviour is observed with ethyl alcohol a lower temperature causes nitrocellulose to swell or even to dissolve more readily. The solvent seems likely to be bound to free hydroxyl radicals (Highfield [36]). The hypothesis explains why nitrocellulose is soluble in a mixture of ether and alcohol, though neither of these solvents, when used separately, is capable of dissolving it. It is assumed that first an alcohol solvate of nitrocellulose is formed which then dissolves in the ether. 

They fractionated the melanoidins by gel filtration to determine which molecular weight melanoidins were responsible for this loss. They found that aU of the fractions reacted with the sulfur-containing volatiles. Attanpts to free the melanoidin-bound volatiles through the addition of other free thiols were unsuccessful, suggesting that the sulfur compounds were not simply involved in disnlfide interchange as observed for proteins. Further work strongly supports the hypothesis that the thiols are covalently bound to pyrazinium ions which are oxidation products of l,4,-bis-(5-amino-5-carboxy-l-pentyl) pyrazinium radical ions. 

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