Experimental Determination of Octanol−Water Partition Coefficients of Quercetin and Related Flavonoids

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Abstract
Octanol−water partition coefficient (log P) values were determined for flavonoids from the flavone, flavonol, flavanone, and isoflavonoid subclasses. Each flavonoid was dissolved in an octanol−water system and allowed to equilibrate, and then both fractions were analyzed by high-performance liquid chromatography. log P was calculated as log[ratio of the concentration in the octanol phase to the concentration in the aqueous phase at pH 7.4]. The aglycons were more lipophilic than any conjugate. The conjugate moiety had a more significant effect on log P than the aglycon moiety. Quercetin was the least lipophilic aglycon (log P = 1.82 ± 0.32) and, together with kaempferol (log P = 3.11 ± 0.54), gave the most variable results. The isoflavones genistein and daidzein and the isoflavone metabolite equol gave relatively high log P values (3.04 ± 0.02, 2.51 ± 0.06, and 3.20 ± 0.13, respectively), while glycitein had an unexpectedly low value of 1.97 ± 0.05. The conjugation characteristics and hydroxylation pattern were the most important determinants of log P in general, and log P was highly variable within the flavonoid subclass. The results are discussed in terms of further understanding of the in vivo fate of the flavonoids as important dietary bioactives. Keywords: Partition coefficients; log P; flavonoids; isoflavones; quercetin; dietary nonnutrients; phytochemicals