Humic free radicals

The study on the reaction of several ring-substituted anilines with HS in aqueous solutions showed that the free radical intermediates were converted to stable products by self-coupling or cross-coupling with other radical species (i. e., indigenous humic free radicals becoming incorporated into the HS macromolecule). 

The purposes of this paper are then (a) to refer on some additional results obtained, in order to confirm the previously suggested [9, 10 ] involvement of humic free radicals in the cross coupling reactions of humic substances with xenobiotic phenoxy radicals and (b) to discuss comparatively the different behaviours of humic acids in these reactions, as a function of their different origin and chemical properties. 

This organism is able to oxidize acetate to CO2 under anaerobic conditions in the presence of Fe(III). A study of the intermediate role of humic and fulvic acids used ESR to detect and quantify free radicals produced from oxidized humic acids by cells of G. metallireducens in the presence of acetate. There was a substantial increase in the radical concentration after incubation with the cells, and it was plausibly suggested that these were semiquinones produced from quinone entities in the humic and fulvic structures (Scott et al. 1998). 

Choudhry GG (1984) In Humic substances-photophysical and free radical aspects and interactions with environmental chemicals. Gordon and Breach Science Publishers, New York, 215 pp... 

In 1960 Rex (22) first reported the use of electron paramagnetic resonance spectrometry as a method for demonstrating the presence of stable organic free radicals in humic acid. We felt that this technique might provide useful information about the structure of humic acid which was not readily available by other physical methods, particularly if relations between EPR spectra and chemically modified humic acids could be demonstrated. Our preliminary studies (26) confirmed this presumption. 

One of the most striking properties of humic acid is its change in free radical content upon conversion to the solid sodium salt. This change is reversible upon reacidification of the salt, the spin content returns to its original level (28). In general, the line widths increased about 50% on conversion to the salt. Table III illustrates this effect for many different humic acids. These results seem entirely consistent with the known properties of the salts of quinhydrone as shown in Figure 6 (28). 

Table III. Free Radical Content of Humic Acids and Salts...

Stable radicals were formed during diagenesis of the organic sediment, and they have persisted ever since. The detailed mechanism of enzymatic reactions is not well understood, but it is known that stable free radicals can be produced. Steelink (18) has found that humic acids from soils, peats, and lignites have moderately high concentrations of free radicals, and since the... 

The aliphatic alicyclic hydrocarbon precursor is not well recognized as the major aliphatic component in dissolved humic substances, although it was previously postulated to occur (II). This precursor might arise from terpenoid hydrocarbon lipids, but the data presented in this chapter favor polyunsaturated lipid precursors that are oxidatively coupled and cyclized by free-radical mechanisms (20). Degradative studies have not identified this aliphatic component in recognizable fragments. The quantitative, structural-model approach presented here combines the results of 13C NMR, NMR,... 

Scavengers include overdosed hydrogen peroxide, humic substances, carbonate ions, bicarbonate ions, and oxidation by-products. Free-radical scavengers are compounds that consume chemical species that have at least one unpaired electron. 

Electron spin resonance (ESR) spectroscopy, as reviewed by Senesi and Steelink (1989), has given an awareness of the abundances of free radicals in humic fractions and of the possibilities for interactions that can arise from these. 

A considerable amount of work has been done on comparing free radicals in humic substances and melanoidins by ESR,539 as well as by 13C-CP/MAS NMR spectra and <513C and <515N values. These studies have not been able to define the role played by the melanoidins, but do not exclude their participation either. 

ESR parameters Humic acid Chlorophenoxy compounds Free radical concentration (spins 10 ) Line width (gauss) Spectroscopic splitting factor (g — values) ... 

The application of ESR spectrometry and in particular the determination of free radical concentrations in the interaction products between HAs and chlorophenoxyalkanoic compounds, has experimentally confirmed the occurrence of already suggested homolytic cross-coupling reactions, leading to the incorporation of xenobiotic chlorophenoxy units into the humic polymers through covalent bonds. 

Another technique which is widely used for functional group analysis of humic materials is carbon-13 nuclear magnetic resonance specroscopy ( C-NMR). The C-NMR solid-state spectrum of an aquatic humic acid is shown in Figure 5. The band assignments for the types of carbon that can be detected by NMR are listed in Table III. Again, bands are broadened due to the presence of free radicals in the structure. More information can be obtained with C-NMR regarding the carbon skeleton of the humic... 

The phenoloxidase enzymes (peroxidases, tyrosinases, and laccases) are known to occur extracellularly in soils (16,17), and have been considered to play a role in catalyzing the coupling of aromatic amines to humic substances (3). Peroxidases have been studied most extensively. They use hydrogen peroxide to promote the one electron oxidation of aromatic amines and of phenolic moieties within humic substances to form free radicals. (12,18). 

Thus, peroxidases may potentially effect direct free radical coupling reactions between aniline and humic substances or create additional substrate sites within the fiilvic or humic acid molecules for nucleophilic addition by aniline. A model for the latter pathway can be found in the work with guaiacol and 4-chloroaniline by Simmons et al. (19). Peroxidase catalyzed the coupling of guaiacol, itself not a substrate for nucleophilic addition, into the extended quinonoid dimer which subsequently underwent nucleophilic attack by the chloroaniline ... 

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