Alkylpyridiniums. 1. Formation in Model Systems via Thermal Degradation of Trigonelline

Trigonelline is a well-known precursor of flavor/aroma compounds in coffee and undergoes significant degradation during roasting. This study investigates the major nonvolatile products that are procured after trigonelline has been subjected to mild pyrolysis conditions (220−250 °C) under atmospheric pressure. Various salt forms of trigonelline were also prepared and the thermally produced nonvolatiles analyzed by thin layer chromatography, liquid chromatography−electrospray ionization tandem mass spectrometry, and 1H and 13C nuclear magnetic resonance. Results revealed the decarboxylated derivative 1-methylpyridinium as a major product of certain salts, the formation of which is positively correlated to temperature from 220 to 245 °C. Moreover, trigonelline hydrochloride afforded far greater amounts of 1-methylpyridinium compared to the monohydrate over the temperature range studied. Investigations into other potential quaternary amine products of trigonelline also indicate nucleophilic substitution reactions that lead to dialkylpyridiniums, albeit at concentration levels ∼100-fold lower than those recorded for 1-methylpyridinium. Keywords: Trigonelline; nicotinic acid; 1-methylpyridinium; 1,4-dimethylpyridinium; alkylpyridinium; liquid chromatography−mass spectrometry (LC-MS); nuclear magnetic resonance spectroscopy (NMR); model system studies; pyrolysis