Investigating the Role of Antioxidant Compounds in Riboflavin-Mediated Photo-Oxidation of Methionine: A ¹H-NMR Approach

Abstract

Riboflavin (RF) is a well-known photosensitizer, responsible for the light-induced oxidation of methionine (Met) leading to the spoilage of wine. An NMR approach was used to investigate the role of gallic acid (GA) and sulfur dioxide (SO 2 ) in the RF-mediated photo-oxidation of Met. Water solutions of RF and Met, with and without GA or SO 2 , were exposed to visible light for increasing time in both air and nitrogen atmospheres. Upon light exposure, a new signal appeared at 2.64 ppm that was assigned to the S(O)CH 3 moiety of methionine sulfoxide. Its formation rate was lower in a nitrogen atmosphere and even lower in the presence of GA, supporting the ability of this compound in quenching the singlet oxygen. In contrast, SO 2 caused relevant oxidation of Met, moderately observed even in the dark, making Met less available in donating electrons to RF. The competition of GA versus Met photo-oxidation was revealed, indicating effectiveness of this antioxidant against the light-dependent spoilage of wine. A pro-oxidant effect of SO 2 toward Met was found as a possible consequence of radical pathways involving oxygen.

Light-Struck Taste in White Wine: Protective Role of Glutathione, Sulfur Dioxide and Hydrolysable Tannins

“…Differently, the decrease of Met was dependent on the additives added and ranged from −24% in MW+SO 2 and MW+SO 2 +GSH samples up to −100% in MW+GSH and MW+CT+GSH samples (Table 1). Among compounds expected to arise from the oxidation of Met [36], only Met sulfoxide was found, in accordance with the previous study carried out by NMR [21]. The absence of this compound in MW samples without additives allowed to exclude the possible oxidation of Met by the acidic environment or matrix components.…”

“…Met content decreased in WW samples containing both GSH and SO 2 (−21%) or SO 2 only (−38%) (Table 3), suggesting the influence of the antioxidants on photo-degradative mechanisms and their competition with Met in both Type I and Type II pathways [13,17,21].…”

“…Recently, the capability of hydrolysable tannins against LST occurrence was shown in model wine as they prevented the formation of sulfur compounds associated with LST [17]. More specifically, the protective effect of tannins can be ascribed to their competition with Met in donating electrons active in the reduction of RF [21]. In addition, Fracassetti and co-authors [17] hypothesized that singlet oxygen can oxidize tannins to quinones capable of binding MeSH; in this way, formation of DMDS is limited and LST resulted less perceived.…”

Riboflavin sensitized photodegradation of Furaneol in a β-cyclodextrin complex

“…Regarding the dark interaction Rf-FUR, steady-state fluorescence studies confirm that FUR is associated with Rf in the ground state. This dark complex possesses an association constant (Kas =1.8 ± 0.2 M -1 , process (19)). In addition to this, a 1 k q = 2.7 10 9 M -1 s -1 value for the dynamic component of 1 Rf* quenching by FUR was obtained by fluorescence lifetime experiments.…”

“…[16][17][18] For example, different studies have reported the Rf-photosensitization of antioxidants, flavorings, or J o u r n a l P r e -p r o o f 5 food-related substances. [19][20][21][22] Nevertheless, no previous study has reported the Rf-photosensitization of FUR in homogeneous media or CDs solutions. FUR, Rf, and -cyclodextrin (βCD) coexist in a wide variety of formulations.…”