Molecular characterization of wines nucleophilic potential by ultra-performance liquid chromatography high resolution mass spectrometry

Abstract

The knowledge about the molecular fraction associated to white wines oxidative stability is still poorly understood. However, the role of S- N- congaing compounds, like glutathione (GSH) and other peptides, as a source of reductant in many oxidation reactions protecting against heavy metals toxicity, or lipids and polyphenols oxidation as ROS-scavenger is today very well established. GSH is also reported being an important antioxidant, reacting as nucleophile substance that conjugates straightforwardly with reactive electrophiles resulting in foods and beverages chemical oxidative stability. It has been shown that, GSH efficiency against wines sensory oxidative stability is related to wines antioxidant metabolome consisting of N- and S- containing compounds like amino acids, aromatic compounds and peptides. These compounds present a strong nucleophilic character and their reactivity with wines electrophiles such as oxidized polyphenols, suggests the formation of stable adducts presenting lower oxidative potential. We consider that the knowledge behind the chemical composition of wines antioxidant metabolome is a key factor to estimate wines aging potential. 

In that respect, the present study introduces an original determination of the pool of nucleophilic compounds that can react with quinones in wine acidic conditions. One step derivatization of nucleophiles has been realized in wines with no pH adjustment by using 4‑methyl‑1,2‑benzoquinone (Q) as a nucleophilic probe. LC‑MS‑QToF analysis of 92 white followed by Multivariate analysis (PLS‑DA) and Wilcoxon test allowed to isolate up to 141 putative nucleophilic compounds. Only 20 of these compounds were detected without derivatization, showing an increase in detection level by quinone trapping, especially for thiols. Moreover, annotation using online database (Oligonet, Metlin and KEGG) as well as elementary formula determined by isotopic profile and MS² analysis allowed to show an important proportion of amino acids and peptides and to identify 4 compounds (GSH, Cys, homocysteine and Pro). The majority of the putative peptides can contain amino acids that are known to have antioxidant properties (Val, Leu, Ile, Pro, Trp, Cys and Met). 

These results show that derivatization of wines using Q allows to enhance thiol detection levels and to determine a pool of untargeted nucleophilic compounds that can be part of wines antioxidant metabolome