Formation, Characterization, and Occurrence of beta-Carboline Alkaloids Derived from alpha-Dicarbonyl Compounds and L-Tryptophan

beta-Carbolines (beta Cs) are naturally occurring bioactive alkaloids, whereas alpha-dicarbonyl compounds are reactive substances generated in foods and in vivo. In this work, L-tryptophan reacted with alpha-dicarbonyl compounds affording new beta-carbolines. Glyoxal afforded 1-hydroxymethyl-beta-carboline (HME-beta C) and its 3-carboxylic acid, and methylglyoxal afforded 1-(1-hydroxyethyl)-beta-carboline (HET-beta C) and its 3-carboxylic acid. 3-Deoxyglucosone afforded 1-(1,3,4,5-tetrahydroxypent-1-yl)-beta-carboline isomers (1a/b), 1-(1,4,5-trihydroxypent-1-yl)-beta-carboline (2), and 1-(1,5-dihydroxypent-3-en-1-yl)-beta-carboline (3). The formation of these beta Cs increased under acidic conditions and with increasing temperature. A mechanism is proposed explaining the conversion of a carbonyl into a hydroxy group based on tautomerism and cyclization to the dihydro-beta C-3-COOH intermediates, which were isolated and gave the beta Cs. These alpha-dicarbonyl-derived beta Cs occurred in model reactions of L-tryptophan with fructose or glucose incubated under heating and can be considered as advanced glycation end products (AGEs). They were also present in foods and formed during heating processes. HET-beta C appeared in processed foods, reaching up to 309 ng/g, with the highest amount found in dried tomato, fried onion, toasted bread, and Manuka honey. HME-beta C was only detected in some foods with lower amounts than HET-beta C. HET-beta C appeared in foods as a racemic mixture of enantiomers suggesting the same mechanism of formation as the synthetized product. alpha-Dicarbonyl-derived beta Cs (HET-beta C, HME-beta C, and 1a/b-3) occur in foods and food processing and, therefore, they are ingested during diet.