Contribution reactions with sulfites

We have found that sulfite and bisulfite undergo one-electron oxidation by many free radicals to produce S03. Rate constants determined for selected radicals are given in Table 1. Measurement of the rate constant over a wide range of pH has, in some cases, allowed the separate determination of rate constants for the oxidation of sulfite and bisulfite. The very strong oxidant OH reacts very rapidly and oxidizes bisulfite faster than sulfite, possibly due to a contribution from hydrogen atom abstraction. For radicals which are relatively weaker oxidants, the reaction with sulfite is the faster. For example, with Br2 the ratio of rate constants for sulfite to bisulfite is about 4 for the even weaker... 

This reaction has many implications for foodstuffs. For example, aroma components possessing a carbonyl group become involatile and do not contribute anymore to the overall flavor. Other nucleophilic reactions include the cleavage of S-S bonds in proteins and addition to C=C bonds of a,(l-unsaturated carbonyl compounds. Control of nonenzymatic browning is based on this latter reaction (McWeeny et al., 1974). A key intermediate of the Maillard reaction, i.e., 3,4-deoxyhexulos-3-ene, is efficiently blocked by a fast reaction with sulfite, leading to formation of 3,4-dideoxy-4-sulfohexosulose, which is much less reactive and in which sulfite is irreversibly bound. 

The three rate constants for Eq. (98) correspond to the acid-catalyzed, the acid-independent and the hydrolytic paths of the dimer-monomer equilibrium, respectively, and were evaluated independently (107). The results clearly demonstrate that the complexity of the kinetic processes is due to the interplay of the hydrolytic and the complex-formation steps and is not a consequence of electron transfer reactions. In fact, the first-order decomposition of the FeS03 complex is the only redox step which contributes to the overall kinetic profiles, because subsequent reactions with the sulfite ion radical and other intermediates are considerably faster. The presence of dioxygen did not affect the kinetic traces when a large excess of the metal ion is present, confirming that either the formation of the SO5 radical (Eq. (91)) is suppressed by reaction (101), or the reactions of Fe(II) with SO and HSO5 are preferred over those of HSO3 as was predicted by Warneck and Ziajka (86). Recently, first-order formation of iron(II) was confirmed in this system (108), which supports the first possibility cited, though the other alternative can also be feasible under certain circumstances. 

For many years phenol was made on a large industrial scale from the substitution reaction of benzene sulfonic acid with sodium hydroxide. This produced sodium sulfite as a by-product. Production and disposal of this material, contaminated with aromatic compounds, on a large scale contributed to the poor economics of the process, which has now been replaced by the much more atom economic cumene route (see Chapter 2, Schemes 2.2 and 2.3). 

These processes could contribute a damaging amount of SO, to the atmosphere if precautions were not taken to remove it. Limestone and sand, which are added to the mixture, form a molten slag that removes many of the impurities as well as the S02. For example, calcium oxide (a basic oxide) from the limestone reacts with the SOz (an acidic oxide) to produce calcium sulfite in a Lewis acid-base reaction ... 

Addition Reaction. The double bond of dehydroalanine and e-methyl dehydroalanine formed by the e-elimination reaction (Equation 6) is very reactive with nucleophiles in the solution. These may be added nucleophiles such as sulfite (44). sulfide (42), cysteine and other sulfhydryl compounds (20,47), amines such as a-N-acetyl lysine (47 ) or ammonia (48). Or the nucleophiles may be contributed by the side chains of amino acid residues, such as lysine, cysteine, histidine or tryptophan, in the protein undergoing reaction in alkaline solution. Some of these reactions are shown in Figure 1. Friedman (38) has postulated a number of additional compounds, including stereo-isomers for those shown in Figure 1, as well as those compounds formed from the reaction of B-methyldehydroalanine (from 6 elimination of threonine). He has also suggested a systematic nomenclature for these new amino acid derivatives (38). As pointed out by Friedman the stereochemistry can be complicated because of the number of asymmetric carbon atoms (two to three depending on derivative) possible. 

O-NO2) [2], Moreover, ozone and sulfur dioxide have been found to positively interfere with this reaction whereas carbon dioxide negatively. Munemorl et al. [3] discovered that the ozone and sulfur dioxide interference is removed by addition of sodium sulfite (Na2S03) to the luminol solution. Whereas the negative contribution from carbon dioxide can be rectified by adjusting the alkalinity of the solution. 

The initial contribution to this volume provides a detailed overview of how spectroscopy and computations have been used in concert to probe the canonical members of each pyranopterin Mo enzyme family, as well as the pyranopterin dithiolene ligand itself. The discussion focuses on how a combination of enzyme geometric structure, spectroscopy and biochemical data have been used to arrive at an understanding of electronic structure contributions to reactivity in all of the major pyranopterin Mo enzyme families. A unique aspect of this discussion is that spectroscopic studies on relevant small molecule model compounds have been melded with analogous studies on the enzyme systems to arrive at a sophisticated description of active site electronic structure. As the field moves forward, it will become increasingly important to understand the structure, function and reaction mechanisms for the numerous non-canonical [ie. beyond sulfite oxidase, xanthine oxidase, DMSO reductase) pyranopterin Mo enzymes. 

The wide use of sulfites in the food industry and the lack of critical information about their effects on foods prompted the editors to seek contributions in this field. Gehman and Osman and Joslyn and Braver-man have come forward with two excellent articles relating to both the fundamental and practical aspects of using sulfites in foods. The former authors have confined themselves to the all-important subject of sugar-sulfite reactions in foods while the latter discuss the many uses for, and effects of sulfites. 

When the yeast starter is added, it is important to avoid antagonism with other strains naturally present in the must. Antagonistic reactions may reduce the fermentation rate and contribute to causing stuck fermentation (Section 3.8.1). For inoculation with DAY to be successful, the yeast starter must be more abundaut aud more active than the indigenous yeasts, which must be inhibited by proper hygiene, sufficiently low temperatures, and appropriate use of sulfite. 

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