Flavonoid–serum albumin complexation: determination of binding constants and binding sites by fluorescence spectroscopy

After a meal rich in plant products, dietary flavonols can be detected in plasma as serum albumin-bound conjugates. Flavonol–albumin binding is expected to modulate the bioavailability of flavonols. In this work, the binding of structurally different flavonoids to human and bovine serum albumins is investigated by fluorescence spectroscopy using three methods: the quenching of the albumin fluorescence, the enhancement of the flavonoid fluorescence, the quenching of the fluorescence of the quercetin–albumin complex by a second flavonoid. The latter method is extended to probes whose high-affinity binding sites are known to be located in one of the two major subdomains (warfarin and dansyl-l-asparagine for subdomain IIA, ibuprofen and diazepam for subdomain IIIA). Overall, flavonoids display moderate affinities for albumins (binding constants in the range 1–15×10⁴ M^?1), flavones and flavonols being most tightly bound. Glycosidation and sulfation could lower the affinity to albumin by one order of magnitude depending on the conjugation site. Despite multiple binding of both quercetin and site probes, it can be proposed that the binding of flavonols primarily takes place in subdomain IIA. Significant differences in affinity and binding location are observed for the highly homologous HSA and BSA.