Sugar Fragmentation in the Maillard Reaction Cascade:  Isotope Labeling Studies on the Formation of Acetic Acid by a Hydrolytic β-Dicarbonyl Cleavage Mechanism

The formation of acetic acid was elucidated based on volatile reaction products and related nonvolatile key intermediates. The origin and yield of acetic acid were determined under well-controlled conditions (90−120 °C, pH 6−8). Experiments with various ¹³C-labeled glucose isotopomers in the presence of glycine revealed all six carbon atoms being incorporated into acetic acid:  C-1/C-2 (∼70%), C-3/C-4 (∼10%), and C-5/C-6 (∼20%). Acetic acid is a good marker of the 2,3-enolization pathway since it is almost exclusively formed from 1-deoxy-2,3-diulose intermediates. Depending on the pH, the acetic acid conversion yield reached 85 mol % when using 1-deoxy-2,3-hexodiulose (1) as a precursor. Hydrolytic β-dicarbonyl cleavage of 1-deoxy-2,4-hexodiuloses was shown to be the major pathway leading to acetic acid from glucose without the intermediacy of any oxidizing agents. The presence of key intermediates was corroborated for the first time, i.e., tetroses and 2-hydroxy-3-oxobutanal, a tautomer of 1-hydroxy-2,3-butanedione, also referred to as 1-deoxy-2,3-tetrodiulose. The hydrolytic β-dicarbonyl cleavage represents a general pathway to organic acids, which corresponds to an acyloin cleavage or a retro-Claisen type reaction. Although alternative mechanisms must exist, the frequently reported hydrolytic α-dicarbonyl cleavage of 1 can be ruled out as a pathway forming carboxylic acids. Keywords: Maillard reaction; acetic acid; 1-deoxy-2,3-hexodiulose; hydrolytic β-dicarbonyl cleavage; labeling studies; sugar fragmentation