Riboflavin isomers

Wehmeyer et al. (1969) published results on the content of B vitamins (thiamine, riboflavin, and nicotinic acid), vitamin C, and p-carotene and foimd that the morama bean is a good source of both B vitamins and vitamin C, but a poor source of p-carotene. Holse et al. (2010) investigated the content of the eight vitamin E isomers and found that the vitamin E composition in morama beans is dominated by y-tocopherol with 59-234 ng/g, followed by a- and p-tocopherols with 14- 8 gg/g and 1.1-3.3 ng/g, respectively. Eurthermore, traces of 8-tocopherol as well as p- and y-tocotrienols were present in some samples. The remaining two tocotrienols (a- and 8-) were not present in the beans. The presence of a-, p-, and y-tocopherols in the morama bean was also foimd by Mitei et al. (2009) who examined morama oil and by Dubois et al. (1995) who examined two samples of T.fassoglense. 

The procedure to phosphorylate riboflavin derivatives on a preparative scale has recently been improved . These preparations, and also commercial FMN, contain a considerable amount of riboflavin phosphate isomers, which are difficult to separate by column chromatography. This problem is emphasized in the chemical synthesis of FAD where the yield is rather low (20-25 %). In this context, it is surprising that a modification of the synthesis of FAD from FMN published by Cramer and Neuhoeffer has not been noticed by workers in the flavin field. According to Cramer and Neuhoeffer, the yield of the chemical synthesis of FAD is drastically improved ( 70 % pure FAD). The procedure was successfully applied in the author s own laboratory (yield 60-70%). It is expected that the improved procedure of the FAD synthesis will stimulate the active-site directed studies on flavoproteins because the problem of separating FMN or FAD from their synthetic by-products has already been solved by use of FMN- or FAD-specific affinity column... 

Absolute electron affinities can be obtained by classification of biological molecules to establish different values of AAG. This is illustrated for riboflavin, vitamin K, vitamin A, polyazines, and hydroxyprimidines. The Ea of these compounds are also predicted by substitution and replacement rules. Those for the dia-zines range from 0.2 eV to 0.4 eV. The values for 1,2,4 triazine and 1,2,4,5 tetrazine are 0.9 eV and 1.7 eV. The replacement of an additional CH by N increases the Ea by 0.6 eV. Therefore, the predicted values for pentazine and hexazine are 2.2 eV and 2.9 eV. The CURES-EC method gives better approximations to these Ea and can differentiate between isomers. 

An important advance in the synthesis of riboflavin was made by Tishler and associates, by the discovery that iV-(i -D-ribityl)-2-arylazo-4,5-dimeth-ylaniline (but not the -6-arylazo isomer) would react directly with barbituric acid in a weak acid medium. Large amounts of unusually pure riboflavin can be synthesized by this procedure. 

Table 3.28 shows that the composition of hydroperoxide isomers derived from an unsaturated acid by autoxidation ( 02) differs from that obtained in the reaction with 02- The isomers can be separated by analysis of hydroperoxides using high performance liquid chromatography and, thus, one can distinguish Type I from Type II photooxidation. Such studies have revealed that sensitizers, such as chlorophylls a and b, pheophytins a and b and riboflavin, present in food, promote the Type II oxidation of oleic and linoleic acids. 

Riboflavin, mostly as FMN and FAD and less frequently as the free vitamin, is found in aU foods, and its distribution is similar to the distribution of thiamine. In milk (and also in eggs), however, the prevailing form is as riboflavin (about 82%). Riboflavin in milk is in part bound to Uj-casein and fS-casein, about 14% of riboflavin is in the form of FAD and 4% in the form of FMN. Smaller quantities of some other flavins, such as 10-(2-hydroxyethyl)flavin, 7a-hydroxyriboflavin (7-hydroxymethylriboflavin) and its 8a-isomer (8-hydroxymethylriboflavin) are also found in mflk. A higher vitamin content is found particularly in meat and offal, cheeses and some sea fish (Table 5.8). 

The analogue was reduced at 0.3% the rate of riboflavin. However, the reaction was not absolutely stereospecific but the oxidoreductase showed a preference for the R-isomer of [4- H]NADH with attack on the re-face of 5-deazariboflavin. It turns out that a 5-deaza-FAM derivative (called factor F420) has now been found as a natural cofactor in the anaerobic bacteria that produce CH4 from CO2 (289) and makes the study of analogous model systems even more significant. 

Cohen and Kolthoff [11], and Muller [4], The prewave of riboflavin differs in form from that of methylene blue the slope of the former is steeper. The i, t curves for riboflavin at the prewave potentials present an extremely complex picture (Fig. 1). A semi-quinone is mainly formed under these conditions. Brdicka [2] explains the delay in the increase of the current at these potentials by the fact that the semiquinone and the leuco form of riboflavin are surface-inactive in statu nascendi but are converted at a definite rate into surface-active isomers, which then behave autocatalytical-ly towards the conversion of the surface-inactive isomer. 

---