Formation of α-hydroxycarbonyl and α-dicarbonyl compounds during degradation of monosaccharides

Abstract

The formation of α-hydroxycarbonyl and α-dicarbonyl compounds from monosaccharides (glucose, fructose, arabinose, glyceraldehyde, and 1,3-dihydroxyacetone) was studied in three different model systems comprising an aqueous and alkaline solution of potassium peroxodisulfate (K<sub>2</sub>S<sub>2</sub>O<sub>8</sub>), and a solution of sodium hydroxide, respectively. In total, six α-hydroxycarbonyl (in the form of O-ethyloximes) and six α-dicarbonyl compounds (as quinoxaline derivatives) were identified by GC/MS and quantified. Acetol, glycolaldehyde, 1,3-dihydroxyacetone, methylglyoxal, and glyoxal were the most abundant low molecular weight carbonyls. Within the model systems studied, the yield of α-hydroxycarbonyl and α-dicarbonyl compounds was 0.32−4.90% (n/n) and 0.35−9.81% (n/n), respectively. The yield of α-dicarbonyls was higher than that of α-hydroxycarbonyls only in aqueous solution of K<sub>2</sub>S<sub>2</sub>O<sub>8</sub> and in the other two model systems an inverse ratio of these two carbonyl types was found. For the first time, ethylglyoxal was identified as a sugar degradation product and several mechanisms explaining its formation were proposed. The achieved data indicated that low molecular weight α-hydroxycarbonyl and α-dicarbonyl compounds are predominantly formed by a direct retro-aldol reaction and α- and β-dicarbonyl cleavage. It was evident that some compounds were produced from the sugar fragmentation products. Thus, isomerisation, reduction of dicarbonyls by formaldehyde (cross-Cannizzaro reaction), and mutual disproportionation are possible reaction pathways participating in the formation of α-hydroxycarbonyl compounds. Oxidation and disproportionation of α-hydroxycarbonyl precursors as well as the aldol condensation of low molecular weight carbonyl species (followed by subsequent reactions) play an important role in the formation of several α-dicarbonyl compounds.

Formation of carboxylic acids during degradation of monosaccharides

“…It is well known that alkaline solutions cause deep changes in sugar molecule and certain similarity with the degradation of sugars under high temperatures is often considered. Recently, we identified several α-hydroxycarbonyl and α-dicarbonyl compounds formed in the systems mentioned above (Novotný et al 2007). In the present work, we extended our study to another group of products, carboxylic acids, to elucidate the main reaction pathways of sugar transformations.…”

“…It was observed that the amount of acetic acid correlated well with the amounts of acetol and methylglyoxal (Novotný et al 2007). Both carbonyls are possible precursors of acetaldehyde (Figure 3) which seems to be the key intermediate for acetic acid forma-tion.…”

“…This pathway would be considered especially in systems with peroxodisulfate where an extensive formation of free radicals is expected. Additionally, acetic acid may be produced also by hydrolytic dicarbonyl cleavage from the corresponding low molecular α-and/or β-dicarbonyls previously identified in the reaction mixtures (Novotný et al 2007). Huyghues-Despointes and found acetic acid as the major (30%) product of methylglyoxal heated to 100°C.…”

“…Another alternative mechanism based on the recombination of free radicals ( • CH 3 , • CH 2 -COOH, • COOH) was suggested for the formation of acetic acid and other mono-and dicarboxylic acids during the degradation of amino acids in K 2 S 2 O 8 solutions (Rössner et al 2001). Recently, we reported on α-hydroxycarbonyl and α-dicarbonyl compounds formed during the degradation of selected monosaccharides in model systems (Novotný et al 2007). The work presented here is a continuation of our research on sugar degradation accomplished under the same experimental conditions employed previously.…”

“…Both aldehydes were identified as condensation products with o-phenylendiamine (OPDA) used for derivatisation of α-dicarbonyl compounds (Novotný et al 2007). (1H)-Benzimidazole and 2-methyl-(1H)-benzimidazole were identified as the OPDA condensation products of formaldehyde and acetaldehyde, respectively.…”