Amino-reductones

By Amadori Reaction. — 1-Amino-1-deoxy-D-fnictose derivatives (Amadori compounds) have been prepared by reaction of D-glucose vith a series of aliphatic amino acids (e.g. glycine, P-alanine, y-aminobutyric acid, 5-aminovaleric acid, e-aminocaproic acid and AT-formyl-L-lysine), and characterized by H- and C-n.m.r. and FAB-mass spectrometry and pH-potentiometric titration. The P-pyranose form dominates in aqueous solution. The basicity of the amino-group in these compounds is decreased (by -1.5 of the IKa value) relative to the parent amino acid. The formation of l-)V-butylamino-l-deoxy-4-0-(a-D-glucopyranosyl)-D-fructose by Amadori reaction of maltose and butylamine, and its conversion to the amino-reductone 23 have been reported. ... 

There are a number of chemical reactions which can participate in the formation of various types of amino-reductones including enaminols and enediamines. These amino-reductones are known to play important roles in the process-induced chemical changes in foods, especially in browning and in cooked flavor formation, and thus the over-all quality... 

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). 

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

The most practical method for preventing WOF in meat products is to add antioxidants prepared from natural precursors such as sugars and amino adds by heating them to produce constituents that not only act as antioxidants but serve to enhance meaty flavor as well. The resulting Maillard products have been known to have antioxidant activity in lipid systems (6-8). It is assumed that the antioxidative property of the Maillard reaction is assodated with the formation of low molecular weight reductones and high molecular weight melanoidins (6, 7, 9-13). 

Omura, H. Inoue, Y. Eto, M. Tsen, Y.-K. Shinohara, K. Reaction products of triose reductone with some amino acids. Kyushu Daigaku Nogakubu Gakugei Zasshi 1974, 29, 61-70. 

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. 

Hodge7 has advanced several possible routes for the conversion of the enol form of the 1-amino-l-deoxy-2-ketose into melanoidin, and the evidence to support these mechanisms is considerable. Thus, the enol may be converted into the Schiff base of a furaldehyde, or to a reductone by loss of water. It may also be broken down into smaller fragments (for example, hydroxy-2-propanone or pyruvaldehyde), which react further with amino compounds. The enol may also react with an a-amino acid and be converted to an aldehyde by a Strecker degradation. The compounds thus formed from... 

In the advanced stage, subsequently the ARP opens, via 1,2- or 1,3-enollsation, depending on the pH of the medium, leaving dicarbonyl-compounds, the so called reductones (10) after elimination of the glycine residue. These highly functionalized compounds are very reactive towards nucleophiles, like the amino acids, or may give rise to the formation of condensation products. 

As shown above, glycine is formed again in the course of the Maillard reaction, during the conversion of the ARP into the reductones. Eventually it may react with these reductones or other electrophiles. Therefore the rate of the decrease of the concentration of the amino acid is not similar to the rate of the reaction with the sugar (Figure 1). The rate of consumption of monosaccharide, however, is a good measure of the reaction rate. 

This is a reaction of a-amino acids, in which they are oxidised to the corresponding aldehyde, giving off carbon dioxide, and ammonia is transferred to other components of the system, very little being liberated as such. The reaction is initiated by compounds, such as a-dicarbonyl compounds and their vinylogues, or compounds which can give rise to them readily, such as reductones by dehydrogenation or imino analogues by hydrolysis. The reaction may therefore be represented as follows ... 

Sugars, polysaccharides, polyhydroxycarboxylic acids, reductones, a-dicarbonyl compounds, and quinones will undergo browning in the absence of amino compounds. 

The chemical degradation of carbohydrates, particularly under acidic conditions, produces reductones, furan derivatives, pyruvaldehyde, and so on, which can condense, either among themselves or with amino compounds (Maillard reactions), to produce dark-colored, amorphous products, similar to humic substances. Pyruvaldehyde, which has been held to be an intermediate in Maillard reactions, has been identified in many soils. Such con-... 

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