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Luteolin (LUT) is a polyphenolic compound, found in a variety of fruits, vegetables, and seeds, which has a variety of pharmacological properties. In the present contribution, binding of LUT to human serum albumin (HSA), the most abundant carrier protein in the blood, was investigated with the aim of describing the binding mode and parameters of the interaction. The application of circular dichroism, UV-Vis absorption, fluorescence, Raman and surface-enhanced Raman scattering spectroscopy combined with molecular modeling afforded a clear picture of the association mode of LUT to HSA. Specific interactions with protein amino acids were evidenced. LUT was found to be associated in subdomain IIA where an interaction with Trp-214 is established. Hydrophobic and electrostatic interactions are the major acting forces in the binding of LUT to HSA. The HSA conformations were slightly altered by the drug complexation with reduction of alpha-helix and increase of beta-turns structures, suggesting a partial protein unfolding. Also the configuration of at least two disulfide bridges were altered. Furthermore, the study of molecular modeling afforded the binding geometry.

Zn- and Cd-complexes of Quercus suber metallothionein (QsMT) were obtained by in vivo-synthesis, in order to obtain physiologically representative aggregates, and characterized by spectrometric and spectroscopic methods. The secondary structure elements and the coordination environments of the metal binding sites of the two aggregates were determined, as well as the main metal-containing species formed. The results obtained from the analysis of the Raman and IR spectra reveal that these metal-MT complexes predominantly contain beta-sheet elements (about 60%), whereas they lack alpha-helices. These structural features slightly depend on the divalent metal bound. In particular, Cd(II) binding to QsMT induces a slight increase of the beta-sheet percentage, as well as a decrease in beta-turn elements with respect to Zn(II) binding. Conversely, the in vivo capability of QsMT to inglobe metal and sulfide ions is metal-depending. Spectroscopic vibrational data also confirm the presence of sulfide ligands in the metal clusters of both Zn- and Cd-QsMT, while the participation of the spacer His residue in metal coordination was only found in Cd-QsMT, in agreement with the CD results. Overall data suggest different coordination environments for Zn(II) and Cd(II) ions in QsMT.