Reductones, structure

An apparent anomaly is encountered when a reductone structure, —C=C—C=0, is oxidized. The first equivalent of oxidant produces a... 

Superoxide radicals are another factor in oxidative damage. They can be determined with nitrobluetetrazolium (NBT), which then forms the colourless formazan. When melanoidins scavenge the superoxide radicals, the colour of the NBT persists.490,491 The activity of a glucose-glycine melanoidin on superoxide radicals is equivalent to the effect of 16 units of superoxide dismutase. The effect of the HMM and LMM fractions of this melanoidin is almost the same. The reaction rate constant of the melanoidin was markedly higher than that of ascorbic acid. If this were due to the reductone structures embedded in the melanodin, it is difficult to explain why the reducing power of the melanoidins is only 0.7 that of ascorbic acid.490... 

The antioxidative mechanism of MRPs in vitro and in vivo is currently unclear. However, it is estimated that the reductone structures, the electron donor property (Kato,... 

The chemical principle behind the antioxidant properties of Maillard reaction products is currently not well understood. It is assumed that these properties show both low molecular weight products and high molecular weight melanoidins. As the structure of melanoidins has not been clarified satisfactorily, it is difficult to explain the chemical nature of their antioxidant activity. The active structures are probably reductones and aminoreductones, bound in melanoidin molecules that reduce the products of autoxidation. One of a few identified reductone structures in real food melanoidins is 2,4-dihydroxy-2,5-dimethyl-l-(5-acetamino-5-methoxycarbonylpentyl)-3-oxo-2H-pyrrole bound by a peptide bond. This so-called pronyl-L-lysine (pyrrolinone... 

In addition to reductones, structures capable of binding metal ions that act as oxidation catalysts also have significant antioxidant activities. Although amino acids can bind metals to coordination compounds (see Section 6.2.2.1), metal chelates (M +) with Amadori compounds (4-257) have considerably higher stability and therefore higher antioxidant activity. The increased stability is due to additional metal ion binding by C-2 and C-3 hydroxyl... 

Raman spectra of pyrid-4-one, 353 2-thiophenealdehyde, 13 Raman spectroscopy, in study of tautomerism, 338 Reductones, structure of, 363 Refractive index, as method to study tautomerism, 337 Riedel synthesis, 300 Rutaecarpine, 301, 304... 

Figure 3 Possible mechanism for the generation of hydrogen peroxide by Maillard products with amino reductone structure...

The appearance of free iodine during the periodate oxidation of compounds having an active hydrogen atom (27) or an ene-diol structure (1,39) has frequently been observed, and this implies that further reduction of iodate, formed from periodate during the main reaction, takes place. It has, in fact, been shown that, in acid solution, iodate is fairly readily reduced by such compounds as triose reductone (27), dihydfoxy-fumaric (39), and tartronic (32) acids. 

The reaction of iodate with triose reductone is not only a function of the concentration of the reagents, it is also dependent on the pH of the solution. In solutions of triose reductone more dilute than 10"3M, iodine is set free from iodate, if the pH of the solution is lower than about 3 (55). Dihydroxyfumaric and L-ascorbic acids (26), which also have free ene-diol structures, behave similarly. 

According to Sato and Herring (39), ascorbic acid can function as an antioxidant by interfering with the free radical mechanism due to the presence of the ene-diol portion of the molecule. However, St. Angelo et al (5) reported that reductone-type compounds, such as maltol, kojic acid, 3-hydroxyflavone, etc., were effective antioxidants and suggested that the alpha ketol structure may also play an important role concerning free radical mechanisms. 

Knowledge about the chemical structure of the antioxidative MRP is very limited. Only a few attempts have been made to characterize them. Evans, et al. (12) demonstrated that pure reductones produced by the reaction between hexoses and secondary amines were effective in inhibiting oxidation of vegetable oils. The importance of reductones formed from amino acids and reducing sugars is, however, still obscure. Eichner (6) suggested that reductone-like compounds, 1,2-enaminols, formed from Amadori rearrangement products could be responsible for the antioxidative effect of MRP. The mechanism was claimed to involve inactivation of lipid hydroperoxides. 

Intramolecular redox reactions via enediol chemistry are, of course, well known. Ascorbic acid and the multitude of known related structures which readily form enediols have even been coined reductones by von Euler and Eistert. ... 

Yamazaki, Mason and Piette [63-65] have investigated the mechanism of action of peroxidases using flow ESR apparatus. The peroxidase used (from Japanese turnips) catalyses the oxidation of a number of substrates such as indoleacetic acid, dihydroxyfumarate and triose reductone by hydrogen peroxide. They were able to demonstrate directly the presence of free radical intermediates, a number of which could be identified from their hyperfine structure, and to show a correlation between ESR signal intensity and the kinetics expected for the reaction. This was strong evidence for a mechanism concerning one-electron transfer steps. The steady state concentration of free radicals was proportional to the square root of the enzyme concentration and the main decay route of the radicals was via dismutation. 

Its formation from rhamnose heated with piperidine acetate in ethanol, under the same conditions that produced amino-hexose-reductones from glucose and other hexoses, was described as early as 1963 by Hodge et al., who confirmed the structure by IR and NMR data and proposed a formation pathway. The formation from Amadori intermediates was been reviewed by Vernin (1981). Numerous model systems have confirmed that it is one of the main Maillard-reaction products. For instance we will mention the formation from L-rhamnose and ethylamine (Kato et al., 1972) and from pentose/glycine or alanine, whose mechanism was proposed by Blank and Fay (1996) and Blank et al. (1998), from the intermediate Amadori compound, /V-(l-deoxy-D-pentos-l-yl)glycine. Furaneol is also formed by recombination of... 

These amino reductones described above are usually very unstable reaction intermediate compounds and, therefore, isolation and elucidation of their precise chemical structures by ordinary experimental techniques are rather difficult. However, owing to the recent remarkable progress in computational chemistry, various types of molecular orbital methods are now applicable to obtain needed information about their precise structures and chemical reactivities. For instance, the optimized structure of L-ascqrbic acid, an important acid-reductone in food and biological systems, was obtained by both semi-empirical and ab initio molecular orbital methods (Abe et aL, 1987, 1992). Semi-empirical molecular orbital calculations were also used to elucidate the autoxidation mechanism of L-ascorbic acid (Kurata et aL, 1996a,b). 

As a result of the presence of an enediol structure element in the a-position to the oxo function in the open-chain structures of acetylformoin, this compound belongs to the group of substances called reductones. Substances of this type, e. g., also vitamin C (ascorbic acid), are weakly acidic (Formula 4.71), reductive (Formula 4.72) and exhibit antioxidative properties. The latter are attributed to the possible formation of resonance-stabilized radicals (Formula 4.73) and also to the disproportionation of two radicals with... 

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