A Multi-Spectroscopic and Molecular Docking Analysis of the Biophysical Interaction between Food Polyphenols, Urolithins, and Human Serum Albumin

Secondary polyphenol metabolites, urolithins (UROs), have anti-oxidative, anti-inflammatory, and antidiabetic properties. Therefore, their biological activity relies on blood transport via human serum albumin (HSA) and tissue distribution. The main goal we set was to investigate the interaction between HSA and different URO (URO A, URO B, URO C, URO D, and glucuronidated URO A and B) using a combination of multi-spectroscopic instrumental and in silico approaches. The fluorescence spectroscopy revealed that URO can quench the naturally occurring fluorescence of HSA in a concentration-dependent manner. The HSA fluorescence was quenched by both a static and dynamic mechanism. The results showed that free UROs bind to HSA with higher affinity than their conjugated forms. CD spectroscopy and FTIR revealed that the alpha-helical structure of HSA is preserved. The calculated Gibbs free energy change indicates that the URO-HSA complex forms spontaneously. There is a single binding site on the HSA surface. The molecular docking results indicated that unconjugated Uro binds to Sudlow I, while their conjugation affects this binding site, so in the conjugated form, they bind to the cleft. Docking experiments indicate that all UROs are capable of binding to both thyroxine recognition sites of ligand-bound HSA proteins. Examining interactions under the following conditions (298 K, 303 K, and 310 K, pH 7.4) is of great importance for determining the pharmacokinetics of these bioactive compounds, as the obtained results can be used as a basis for modulating the potential dosing regimen.