Ascorbic acid from hexose sugars

Alkazones are very reactive compounds they readily cyclize and aromatize. This renders alkazones of hexoses and higher sugars difficult to isolate from the dark products they form. Other highly reactive species of bis(hy-drazones) are the derivatives of mesoxaldehyde and of ascorbic acid, which... 

Atomic partial charges were subjected to analysis of the detection sensitivity of a variety of compounds measured by the same system. The detection limits and ape values are shown in Table 11.5. The ape values of saccharides were calculated as the aldehyde form, but these sugars exist as hexoses and the concentration of the aldehyde form was not known under these experimental conditions. Therefore, these values were eliminated from the correlation coefficient calculations. The sensitivity of creatinine was very low, therefore creatinine was also eliminated from the calculations. The correlation coefficients were 0.704 (n = 7] for Group 1, and 0.960 [n = 8) for Group 2. In the above calculation, uric and ascorbic acid have two carbonyl groups and, therefore, two ape values were combined. 

Ascorbic acid was identified in 1932 as the antiscorbutic vitamin. Chemically L-ascorbic add is related to the hexose sugars from which it can be synthesized by most animal species. The chief exceptions are primates and guinea-pigs. L-Ascorbic add is very soluble in water and, although when dry it is stable in light and air, it is readily oxidized in solution, particularly in the presence of metals such as copper. On oxidation it is converted first to dehydroascorbic acid but although the ascorbic acid-dehydroascorbic add conversion is reversible, subsequent stages of oxidation are irreversible and the oxidation products have no vitamin activity. 

There are a number of papers in the literature, the results of which suggest that the vitamin is synthesized from a hexose sugar in both plants and animals. Studies on this subject have been concerned with supplying possible precursors and ascertaining the effect on the ascorbic acid content of the tissues. With plant tissue, seeds have been selected by many workers as the best material, because they are devoid of ascorbic acid but produce it during germination, even in the absence of light. The synthesis can therefore be studied under conditions in which photosynthesis cannot occur, and thus must take place at the expense of a carbon source derived from the reserves of the seed. 

Of the other commonly occurring hexose sugars, glucose and fructose, together with sucrose, appear to be equivalent as sources from which ascorbic acid may be formed (Butkewitsch, 1938 ltd and Mizumo, 1948 Mapson et al., 1949). 

The early evidence suggesting that ascorbic acid may be formed from a hexose sugar in animals is very unconvincing. Widenbauer and Koschor-rech (1937) claimed that the ascorbic acid content of slices of intestinal... 

The most convincing evidence that ascorbic acid can be synthesized from a hexose sugar in animals comes from the work of Jackel and his collaborators (1950). When D-glucose labeled uniformly in all positions with was supplied to rats which had been fed chloretone to stimulate the synthesis of the vitamin, L-ascorbic acid was excreted which was also labeled uniformly. This clearly indicates that n-glucose can serve as an ultimate precursor but does not exclude the possibility that the glucose may have to be converted to other sugars before being transformed. 

Foods derived from plant materials contain pentose as well as hexose sugars, which are usually formed under weakly acidic conditions. It is well known that pentose contributes more to browning by the Maillard reaction than hexose does, because the oxo-or reducible form of sugars is higher in pentose than in hexose. Hydroxymethylfurfural (HMF) is one of the major decomposed products of such hexoses as glucose and fructose under acidic conditions, while furfural is the corresponding one of pentose and is also formed by the decomposition of ascorbic acid. 

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