Identification of the Products of Oxidation of Quercetin by Air Oxygenat Ambient Temperature

Abstract

Oxidation of quercetin by air oxygen takes place in water and aqueous ethanol solutions under mild conditions, namely in moderately-basic media (pH ∼ 8-10) at ambient temperature and in the absence of any radical initiators, without enzymatic catalysis or irradiation of the reaction media by light. The principal reaction products are typical of other oxidative degradation processes of quercetin, namely 3,4-dihydroxy-benzoic (protocatechuic) and 2,4,6-trihydroxybenzoic (phloroglucinic) acids, as well as the decarboxylation product of the latter -1,3,5-trihydroxybenzene (phloroglucinol). In accordance with the literature data, this process involves the cleavage of the γ-pyrone fragment (ring C) of the quercetin molecule by oxygen, with primary formation of 4,6-dihydroxy-2-(3,4-dihydroxybenzoyloxy)benzoic acid (depside). However under such mild conditions the accepted mechanism of this reaction (oxidative decarbonylation with formation of carbon monoxide, CO) should be reconsidered as preferably an oxidative decarboxylation with formation of carbon dioxide, CO 2 . Direct head-space analysis of the gaseous components formed during quercetin oxidation in aqueous solution at ambient temperature indicates that the ratio of carbon dioxide/carbon monoxide in the gas phase after acidification of the reaction media is ca. 96:4 %. Oxidation under these mild conditi- Molecules, 2007, 12 655ons is typical for other flavonols having OH groups at C 3 (e.g., kaempferol), but it is completely suppressed if this hydroxyl group is substituted by a glycoside fragment (as in rutin), or a methyl substituent. An alternative oxidation mechanism involving the direct cleavage of the C 2 -C 3 bond in the diketo-tautomer of quercetin is proposed.

Does the Plant Matrix Type Have Impact on Rutin Transformation During Its Extraction?

“…There is no doubt about quercetin, isoquercitrin, and quercitrin which are truly native plant components involved in the rutin metabolic pathway, but the origin of DTB, OTA, CDA, DDA, DPD, DBT, DBD, and DBOT is less certain. As shown in Dawidowicz et al (2016), they are formed during the heating of methanolic and methanol/water rutin solution, and they are also known as rutin and/or quercetin degradation products forming during their high-temperature treatment, UV irradiation, or exposure to other aggressive factors (Dubber et al 2005;Rohn et al 2007;Wang et al 2011;Zenkevich et al 2007;Zvezdanović et al 2012). The presented data show that some of them are formed during the heating of ethanolic and ethanol/water rutin solution.…”

“…2. All of them were identified and confirmed on the basis of the retention data of their standards, the PDA, MS 2 , HRMS, and literature data (Dubber et al 2005;Wang et al 2011;Zenkevich et al 2007;Zvezdanović et al 2012). The MS 2 and HRMS data for the identified compounds are collected in Tables 1 and 2, respectively.…”

Rutin Transformation During Its Analysis Involving Extraction Process for Sample Preparation

“…1(A) shows that the methanolic rutin solution heated under reflux contains, besides the parent substance, 23 additional compounds formed as a result of the rutin transformation and degradation process. All of them were identified and confirmed on the basis of the retention data of their standards, the PDA, MS 2 , HRMS and literature data (Dubber et al 2005;Wang et al 2011;Zenkevich et al 2007;Zvezdanović et al 2012). The MS 2 and HRMS data for the identified compounds are collected in Tables 1 and 2, respectively.…”

“…2. Some of them were described in the literature as products of rutin and/or quercetin degradation resulting from high-temperature treatment, UV irradiation or the influence of aggressive environments: DTB, OTA, CDA, DDA, DBD, DBOT, DPD, quercetin, DBT, isoquercitrin and quercitrin (Dubber et al 2005;Rohn et al 2007;Wang et al 2011;Zenkevich et al 2007;Zvezdanović et al 2012). Yet our experiments have shown that these compounds are formed not only in aggressive conditions but also in the conditions typical for the extraction process.…”

“…Appropriate concentration axis is indicated by arrows show that at least 23 compounds (rutin transformation and degradation products and their reaction products with alcohol) can be formed from rutin during its simulated and real extraction from plants by methanol and methanol/water mixtures. Eleven of them are known from the literature as rutin and/or quercetin degradation products forming during their high-temperature treatment, during UV irradiation or in an aggressive environment (Dubber et al 2005;Rohn et al 2007;Wang et al 2011;Zenkevich et al 2007;Zvezdanović et al 2012). Twelve compounds have not yet been reported anywhere.…”