Quercetin solubilisation in bile salts: A comparison with sodium dodecyl sulphate

Abstract

To understand the bioaccessibility of the flavonoid quercetin we studied its interaction with bile salt micelles. The environmental sensitivity of quercetin's UV-visible absorption spectrum gave information about quercetin partitioning. Two quercetin absorption peaks gave complementary information: Peak A (240-280nm) on the intermicellar phase and Peak B (340-440nm) on the micellar phase. Thus, by altering pH, we showed that only non-ionised quercetin partitions into micelles. We validated our interpretation by studying quercetin's interaction with SDS micelles. Pyrene fluorescence and the quercetin UV-visible spectra show that the adsorption site for pyrene and quercetin in bile salt micelles is more hydrophobic than that for SDS micelles. Also, both quercetin and pyrene reported a higher critical micelle concentration for bile salts than for SDS. Our method of using a flavonoid as an intrinsic probe, is generally applicable to other lipophilic bioactives, whenever they have observable environmental dependent properties.

Keywords: Bioavailability; Micelles; Pyrene (PubChem CID: 104853); Pyrene fluorescence; Quercetin (PubChem CID: 5280804); Quercetin’s pK(a1); Sodium dodecyl sulphate (PubChem CID: 3423265); Sodium glycodeoxycholate (PubChem CID: 3035026); Sodium taurocholate (PubChem CID: 3035026); UV–visible spectra.

Graphical abstract

Fig. 1

(A) The structure of Quercetin, and (B) the UV–visible absorption spectra of quercetin in DPBS at different pH’s at constant ionic strength. The pH values selected show the characteristic changes in spectra and in particular the peak at 270 nm due to Q⁻ at pH 7.88.The insert shows the data for one experiment for r = (A_{max(254–257)}/A₂₇₀) as a function of pH.

Fig. 2

Change in the pyrene fluorescence intensity ratio, F_R, for third and first vibronic peaks (F_III/F_I) as a function of bile salt (A) and SDS (B) concentration at different pH’s with and without quercetin (45 μM). ♦ pH 7.15 without quercetin, ♢ pH 7.15 with quercetin, ▴ pH 6.1 without quercetin, Δ pH 6.1 with quercetin. Solid line is for pH 7.15, dotted line for pH 6.1. Summary of analysis of pyrene fluorescence data (C), CMC, starting concentration of micelle formation; [S]_1/2, inflection/ midpoint of sigmoid curve for micelle formation; F_max, the maximum ratio of the intensity of the fine structure peaks at 383 and 372.5 nm; Slope at [S]_1/2, fitting parameter for calculation of [S]_1/2; results are for 2–3 experiments with SD for the values within 5% (cmc’s, slope, and [S]1/2) and 2% (F_max).

Fig. 3

UV–visible absorption spectra of quercetin in DPBS at pH 7.15 without (dashed line) and with (solid line) addition of 12 mM BS (A) or SDS (B).

Fig. 4

The effect of increasing BS (A–C) and SDS (D–F) concentration on the wavelength of maximum absorption (λ_max) of quercetin for peak A (A, D) and peak B (B, E) and the change in the ratio r (C, F) of peak A at different pH values. ♦ pH 7.15, □ pH 6.1, Δ pH 5.0; r = A_{max(254–257)}/A₂₇₀

Fig. 5

λ_max of peak B as a function of the dielectric constant ε for solvents of high polarity. 1, water; 2, water + 0.1% DMSO; 3, propylene carbonate; 4, DMSO; 5, glycerol; 6, dimethyl formamide; 7, ethylene glycol