Acids as electron acceptors

The microbial degradation of contaminants under anaerobic conditions using humic acids as electron acceptors has been demonstrated. These included the oxidations (a) chloroethene and 1,2-dichloroethene to CO2 that was confirmed using C-labeled substrates (Bradley et al. 1998) and (b) toluene to CO2 with AQDS or humic acid as electron acceptors (Cervantes et al. 2001). The transformation of l,3,5-trinitro-l,3,5-triazine was accomplished using Geobacter metallireducens and humic material with AQDS as electron shuttle (Kwon and Finneran 2006). 

Bradley PM, FH Chapelle, DR Lovley (1998) Humic acids as electron acceptors for anaerobic microbial oxidation of vinyl chloride and dichloroethene. Appl Environ Microbiol 68 3102-3103. 

Eubacterium acidaminophilum not only reductively deaminates glycine to ammonia and acetate but also expresses enzymes capable of reductively deaminating sarcosine and betaine when cells are cultured in the presence of formate (Hormann and Andreesen 1989). One would expect that the enzymes catalyzing the latter two reactions might be similar to GR. In fact, the substrate-specific protein B for sarcosine reductase was purified and found to be similar to GR protein B from C. sticklandii (Meyer et al. 1995). Apparently, this organism has evolved means to use different amino acids as electron acceptors and preferentially expresses each in response to conditions in the environment. 

The presence of P-CD favors the formation of a charge-transfer (CT) complex between 2-methoxynaphthalene and o-dicyanobenzene, as demonstrated by the variations in the absorption spectrum. The excitation of the charge-transfer complex caused a structureless emission with maximum at 480 nm. Since methoxynaphthalene and dicyanobenzene do not form CT complexes in water, it was concluded that the complexed methoxynaphthalene and the complexed dicyanobenzene aggregate to form a 2 1 1 complex (CD-methoxynaphthalene-dicyanobenzene) [130]. The y-CD promoted formation of charge-transfer complexes between a- and / -naphthylacetic acid (as donors), picric acid, and four isomeric dinitrobenzoic acids (as electron acceptors) was reported in references 147 and 148. 

The Lewis system defines acids as electron acceptors and bases as electron donors. An acid-base reaction involves the transfer of an electron pair from the base to the acid. Although the Lewis bases are largely the same as the Bronsted bases, the range of acids differs markedly. For example, in the Bronsted system the acids donate a proton, but in the Lewis sense the proton is... 

An alternative drivirg force could involve a donor - acceptor interaction. The electron-poor pyridine ring that is coordinated to the copper cation can act as electron acceptor with respect to the aromatic ring of the -amino acid. The fact that donating substituents on the amino acid increase the efficiency... 

Mammalian sulfite oxidase is the last enzyme in the pathway for degradation of sulfur-containing amino acids. Sulfite oxidase (SO) catalyzes the oxidation of sulfite (SO ) to sulfate (S04 ), using the heme-containing protein, cytochrome c, as electron acceptor ... 

A1C13, or S02 in an inert solvent cause colour changes in indicators similar to those produced by hydrochloric acid, and these changes are reversed by bases so that titrations can be carried out. Compounds of the type of BF3 are usually described as Lewis acids or electron acceptors. The Lewis bases (e.g. ammonia, pyridine) are virtually identical with the Bransted-Lowry bases. The great disadvantage of the Lewis definition of acids is that, unlike proton-transfer reactions, it is incapable of general quantitative treatment. 

Cations at the surface possess Lewis acidity, i.e. they behave as electron acceptors. The oxygen ions behave as proton acceptors and are thus Bronsted bases. This has consequences for adsorption, as we will see. According to Bronsted s concept of basicity, species capable of accepting a proton are called a base, while a Bronsted acid is a proton donor. In Lewis concept, every species that can accept an electron is an acid, while electron donors, such as molecules possessing electron lone pairs, are bases. Hence a Lewis base is in practice equivalent to a Bronsted base. However, the concepts of acidity are markedly different. 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

The reductase in Geobacter sulfurreducens is located in the outer membrane and a soluble Fe(III) reductase has been characterized from cells grown anaerobically with acetate as electron donor and Fe(III) citrate or fumarate as electron acceptor (Kaufmann and Lovley 2001). The enzyme contained Fe, acid-labile S, and FAD. An extracellular c-type cytochrome is distributed in the membranes, the periplasm, and the medium, and functions as a reductase for electron transfer to insoluble iron hydroxides, sulfur, or manganese dioxide (Seeliger et al. 1998). 

Pnre cnltnres of organisms that can oxidize propionate either in the presence of a methanogen or nsing snlfate as electron acceptor have been obtained. These include both Syntrophobacter wolinii and Syntrophobacter pfenigii (Wallrabenstein et al. 1995). The interaction of two organisms, therefore, is clearly not obligatory for the ability to degrade these carboxylic acids under anaerobic conditions. 

The genus Thiobacillus, especially the species T. denitrificans catalyzed the oxidation reactions of hydrogen sulfide yielding soluble hydrosulfide compounds, elemental sulfur, and sulfuric acid. Carbonyl sulfide and carbon disulfide are converted to hydrogen sulfide by hydrolysis. Additionally, they are oxidized to SOx and sulfates via microbial action. The reported oxidation reactions of thiosulfate using nitrate as electron acceptor are ... 

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

The electron-transfer paradigm for chemical reactivity in Scheme 1 (equation 8) provides a unifying mechanistic basis for various bimolecular reactions via the identification of nucleophiles as electron donors and electrophiles as electron acceptors according to Chart 1. Such a reclassification of either a nucleophile/ electrophile, an anion/cation, a base/acid, or a reductant/oxidant pair under a single donor/acceptor rubric offers a number of advantages previously unavailable, foremost of which is the quantitative prediction of reaction rates by invoking the FERET in equation (104). 

Cationic polymerisations of vinyl derivatives proceeds by a polar or heterolytic cleavage of the double bond by the action of initiators which act as electron acceptors. Initiation is carried out by proton donation by acids or co-catalysts... 

Organic molecules spontaneously form corresponding cation-radicals on inclusion within activated zeolites (Yoon and Kochi 1988, Yoon 1993, Pitchumani et al. 1997). Zeolites are crystalline alu-mosilicate minerals that are widely used as sorbents, ion exchangers, catalysts, and catalyst supports. As zeolites act as electron acceptors due to the presence of Lewis- or Broensted-acid sites, confined organic compounds occur to be electron donors. Frequently, the interaction of electron donor with electron acceptor centers spontaneously generates cation-radicals and traps the ejected electrons. 

Anaerobic conditions often develop in hydrocarbon-contaminated subsurface sites due to rapid aerobic biodegradation rates and limited supply of oxygen. In the absence of O, oxidized forms or natural organic materials, such as humic substances, are used by microorganisms as electron acceptors. Because many sites polluted by petroleum hydrocarbons are depleted of oxygen, alternative degradation pathways under anaerobic conditions tend to develop. Cervantes et al. (2001) tested the possibility of microbially mediated mineralization of toluene by quinones and humus as terminal electron acceptors. Anaerobic microbial oxidation of toluene to CO, coupled to humus respiration, was demonstrated by use of enriched anaerobic sediments (e.g., from the Amsterdam petroleum harbor). Natural humic acids and... 

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