# Decomposition of Flavonols in the Presence of Saliva
## 3 Results and Discussion  
### Oxidation of Flavonols with Hydrogen Peroxide  
As compared to the control, a noticeable decrease of quercetin concentration was observed during its incubation in the presence of H\({}_{2}\)O\({}_{2}\). As shown in Figure 2, rapid degradation of quercetin could be observed with the simultaneous formation of three other compounds with [M-H]\({}^{-}\) at _m_/2317 (t\({}_{\rm r}\) = 17.5 min) [M-H]\({}^{-}\) at _m_/2 =193 (t\({}_{\rm r}\) = 4.4 min) and [M-H]\({}^{-}\) at _m_/2 169 (t\({}_{\rm r}\) = 5.0 min). Oxidation product with [M-H]\({}^{-}\) at _m_/2 317, identified as depside characteristic for quercetin, was found as an intermediate product, the concentration of which increased in the first 80 min of the experiment. Further incubation led to the decrease of its concentration, whereas new peaks with [M-H]\({}^{-}\) at _m_/2 193 (t\({}_{\rm r}\) = 4.4) and [M-H]\({}^{-}\) at _m_/2 169 (t\({}_{\rm r}\) = 5.0) appeared. Final degradation products were identified as 2,4,6-trihydroxybenzoic acid (_m_/2 169) and protocatechuic acid (_m_/2 193). Such results suggest that the oxidation of quercetin with H\({}_{2}\)O\({}_{2}\) is based on its splitting by the opening the heterocyclic C-ring and realizing simpler aromatic compounds. Moreover, oxidation involves the initial oxidative step with subsequent changes in the flavonoid skeleton such as the formation of B-ring orthoquinone and rearrangement in the C-ring [33]. Detected oxidation products are similar to those obtained under other conditions such as oxygen [28], UVA and UVB [34], hydroxyl free radical [35], and presence of copper (II) [36; 37]. Thus, the hypothesis of [PERSON] et al. [30] that the oxidation of quercetin using different oxidations agents may yield, more or less, the same set of oxidized products seems to be particularly relevant. Decomposition pattern of myricetin leads throughout the characteristic depside as an intermediate at _m_/2 321. The LC-MS/MS measurements show that further oxidation led to its decomposition and formation of gallic acid as a corresponding hydroxybenzoic acid derivative at _m_/2 169 and t\({}_{\rm r}\) = 2.9 min. Besides, 2,4,6-trihydroxybenzoic acid (t\({}_{\rm r}\) = 4.9 min) was detected as a degradation product of myricetin. Although it shares the same mass as gallic acid, their separation was obtained in the established method. Unfortunately, neither for isorhamnetin nor for kaempfer corresponding depsides were detected. Nevertheless, transformations which involve initial oxidative steps with subsequent changes in the flavonols skeleton was observed. As a result of kaempfer oxidation, the formation of a compound with t\({}_{\rm r}\) = 7.1 min and SRM characteristic for 4-hydroxybenzoic acid (137/93) was observed. The breakdown product of isorhamnetin was identified as a vanillic acid with retention time 9.8 min and SRM pair (167/152). These results suggest that, as in the case of quercetin and myricetin, oxidation of kaempfer and isorhamnetin leads to C-ring cleavage in the flavonols' structure. As a result of this reaction, the corresponding hydroxybenzoic acid derivatives are formed. It should be mentioned that two reactive centers in the C-ring were identified as responsible for antioxidant activity in flavonols: the 2,3 double bond in conjugation with the 4-oxo function and the 3- and 5-hydroxyl groups with hydrogen bonding to the same 4-oxo function (for flavonols numbering, see Figure 1) [38; 39]. That is why all of the modifications which occur in that area could significantly alter the chemical and biological properties of flavonoids.