Isolation and quantification in food of 6-(2-formyl-5-methylpyrrol-1-yl)-l-norleucine (“rhamnolysine”) and its precursor 3,6-dideoxy-l-mannosone
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In the Maillard reaction (non-enzymatic browning), reducing carbohydrates react to 1,2-dicarbonyl compounds that may combine with the ε-amino group of lysine to form characteristic pyrrole compounds such as pyrraline and formyline in a Paal–Knorr reaction. In the present study, the Maillard reaction of the rare sugar l-rhamnose was studied. The l-rhamnose-derived dehydration product 3,6-dideoxy-l-mannosone (3,6-DDM) was synthesized and structurally characterized via NMR. Mainly the α-pyranose is formed in aqueous solution. 2-(2′(S),3′(S)-Dihydroxybutyl)quinoxaline was synthesized for the first time and used as a standard for quantification of 3,6-DDM in food. Analysis of 3,6-DDM after conversion to the respective quinoxaline via RP-HPLC–MS/MS revealed concentrations not exceeding 23 µg/kg in food items such as coffee and brewing malt. Moreover, the Paal–Knorr reaction product of 3,6-DDM with lysine, namely 6-(2-formyl-5-methylpyrrol-1-yl)-l-norleucine (rhamnolysine) was synthesized for the first time by reaction of N-α-Boc-lysine and 3,6-DDM and purified by semi-preparative ion-exchange chromatography followed by characterization by NMR and HPLC–MS/MS. Rhamnolysine was analysed in 42 food items by RP-HPLC–MS/MS in the MRM mode after enzymatic hydrolysis. The highest concentration of the compound was found in rye bread crust (4.9 mg/kg). It appears mainly in foods that are highly heat-treated, roasted or fried.
KeywordsLysine Maillard reaction Glycation Rhamnose Advanced glycation end product (AGE) 3,6-Dideoxy-l-mannosone Rare sugar Rhamnolysine
High-pressure liquid chromatography
Limit of detection
Limit of quantification
Multiple reaction monitoring
Nuclear magnetic resonance
We thank the members of the Chair of Inorganic Molecular Chemistry (TU Dresden), namely Dr. Sivathmeehan Yogendra and Dr. Kai Schwedtmann for recording the NMR spectra and Philipp Lange for performing the elemental analyses.
Compliance with ethical standards
Conflict of interest
The authors declare no competing financial interest.
Compliance with ethics requirements
This article does not contain any studies with human or animal subjects.
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