Oxidative Fragmentation of Aspalathin Leads to the Formation of Dihydrocaffeic Acid and the Related Lysine Amide Adduct.
In the present study, the degradation of C-glucosidic dihydrochalcone aspalathin as the major phenolic compound in rooibos (Aspalathus linearis) was investigated. Analyses by gas chromatography-mass spectrometry of aqueous aspalathin-lysine incubations after silylation showed the formation of dihydrocaffeic acid [3-(3,4-dihydroxyphenyl)-propionic acid] under oxidative conditions as a novel degradation product up to 10 mol %. High-performance liquid chromatography analyses revealed the concurrent formation of the dihydrocaffeic acid lysine amide at about 30-fold lower concentrations, which was unequivocally verified by synthesis of an authentic reference standard. The amide was also verified in aspalathin-protein incubations after enzymatic hydrolysis by high-performance liquid chromatography-tandem mass spectrometry analyses. Thus, the covalent interaction of phenolic plant compounds with proteins under mild conditions (ambient temperatures and neutral pH) was confirmed for the first time. Acid and free amide were also quantitated in rooibos teas with significantly higher values in fermented varieties. The mechanism of formation was clarified to be initiated by singlet oxygen and to include a rearrangement-fragmentation mechanism with 1,2,3,5-tetrahydroxybenzene as the counterpart.
['Mertens|Nils|N|http://orcid.org/0000-0002-9769-1340', 'Heymann|Thomas|T|http://orcid.org/0000-0002-9472-8611', 'Glomb|Marcus A|MA|http://orcid.org/0000-0001-8826-0808']
D000577:Amides D036903:Aspalathus D002109:Caffeic Acids D047188:Chalcones D008239:Lysine D015394:Molecular Structure D010084:Oxidation-Reduction D010936:Plant Extracts
aspalathin dihydrocaffeic acid dihydrochalcones rooibos tea singlet oxygen high-performance countercurrent chromatography polyphenol−protein adduct