# Decomposition of Flavonols in the Presence of Saliva
## 4 Conclusions  
As noted in this study, the stability of flavonoids in the presence of saliva solution strongly depends on the number of hydroxyl groups attached to the B-ring. Indeed, flavonoid stability decreases with increasing B-ring substitution. This proves that saliva is a next oxidative agent, besides UVA and UVB radiation, air, enzymes, and free radicals, which leads to the formation of corresponding phenolic acids  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Kaempfer**} & \multicolumn{2}{c}{**Quercetin**} & \multicolumn{2}{c}{**Isorhamenti**} & \multicolumn{2}{c}{**Myricetin**} \\ \cline{2-7}  & **H\({}_{2}\)O\({}_{2}\)** & **Saliva** & **H\({}_{2}\)O\({}_{2}\)** & **Saliva** & **H\({}_{2}\)O\({}_{2}\)** & **Saliva** & **H\({}_{2}\)O\({}_{2}\)** & **Saliva** \\ \hline Depside 1 & – & – & + & + & – & – & – & – \\ Depside 2 & – & – & – & + & – & – & + & + \\ 2,4,6\(-\)trihydroxybenzoic acid & – & – & + & – & – & – & + & – \\ Benzoic acid derivative & + & + & + & + & + & + & + & + \\ \hline \multicolumn{7}{c}{\(-\)not detected; + detected.} \\ \end{tabular}
\end{table}
Table 2: Comparison of products detected with H\({}_{2}\)O\({}_{2}\) and saliva oxidation.  
Figure 5: General scheme of flavonoids’ degradation in the presence of saliva.  
as dead-end products of flavonoids. The obtained results indicate that myricetin is the most effective flavonol in the process of neutralizing free radicals formed in the oral cavity, due to its easy oxidation caused by the presence of three hydroxyl groups in the B-ring of this compound. Quercetin and isorhamnetin can also be regarded as quite effective antioxidants, capable of oxidizing in the presence of saliva. Nevertheless, their degradation products such as gallic acid and protocatechuic acid still possess reducing potential and are well-known antioxidant units. Rutin (quercetin 3-rhannoglucoside) and quercitin (quercetin 3-rhannoside), as well as, surprisingly, hyperoside (quercetin 3-galactoside) and spiraeoside (quercetin 4'-glucoside), were resistant to hydrolysis in the presence of saliva. Nevertheless, due to the interindividual fluctuations, further analyses should be elucidated on that issue. However, the most intriguing seems to be the relatively short residence time of most foods and their bioactive compounds in the oral cavity. Even so, it should be noticed that this time is sufficient for flavonoids decomposition, which clearly shows the process of flavonoids metabolic transformation starts in the oral cavity. Hence, studies on flavonoids metabolism should take under consideration that, in subsequent parts of the human digestive tract, they could be present not in their parent form but as phenolic acids. Consequently, information on the bioavailability and metabolic pathway of such dietary bioactive compounds is a key part in understanding their beneficial influence on human health.  
Conceptualization, M.R. and M.B.; methodology, M.R., M.B.; validation, M.R., M.B.; investigation, M.R., M.B.; writing--original draft preparation, M.R.; writing--review and editing, M.B.; supervision--M.B.; All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The presented studies were carried out with the approval of Ethics and Bioethics Committee (KEIB-5/2016, Cardinal Stefan Wyszynski University in Warsaw). LC-MS/MS measurements were performed at the Laboratory of Structural Research, Faculty of Chemistry, University of Warsaw, which was established under the European Regional Development Grant WPK_1/1.4.3/2004/72/72/165/2005/U. The authors thank prof. [PERSON] for her help in the microbiological part of the presented manuscript and prof. [PERSON] for her helpful advice on various technical issues examined in this paper.  
The authors declare no conflict of interest.