Binding of antioxidant flavone isovitexin to human serum albumin investigated by experimental and computational assays

• The quenching mechanism between ISO and HSA is a static process.
• ISO molecule binds to the site I of HSA interacting close to the Trp214 residue.
• A balance between enthalpic and entropic forces stabilizes the complexation.
• Binding density function describes more accurately the process of complex formation.
• Molecular modeling simulations agree with the fluorescence spectroscopy analysis.

The flavonoids are a large class of polyphenolic compounds which occur naturally in plants where they are widely distributed. Isovitexin (ISO) is a glycosylated flavonoid that exhibits a potential antioxidant activity. Some recent studies have shown the pharmacokinetic activity of isovitexin in rat blood plasma, however, without detailing the molecular target that is linked and what physicochemical forces govern the interaction. In mammalians, the most abundant protein in blood plasma is the albumin and is not unlike with human, which human serum albumin (HSA) is the major extracellular protein and functions as a carrier of various drugs. The interaction between HSA and ISO was investigated using fluorescence, UV–vis absorbance, circular dichroism (CD), Fourier transform infrared spectroscopy (FT-IR) together with, computational methods like ab initio and molecular modeling calculation. Fluorescence quenching indicated that ISO location is within the hydrophobic pocket in subdomain IIA (site 1) of HSA, close to the Trp214 residue. The Stern–Volmer quenching constants determined at 288, 298 and 308 K and its dependence on temperature indicated that the quenching mechanism was static. From the analysis of binding equilibrium were determined; the binding site number and binding constants, with the correspondent thermodynamic parameters, ΔH, ΔG and ΔS for HSA–ISO complex. Also, a second binding analysis, binding density function (BDF) method, which is independent of any binding model pre-established obtained similar results. The fluorescence resonance energy transfer estimated the distance between the donor (HSA–Trp214) and acceptor (ISO), while FT-IR and CD spectroscopy measured possible changes of secondary structure at the formation of the HSA–ISO complex. The optimized geometry of isovitexin calculation performed with its ground state by using DFT/B3LYP/6-311+G(d,p) method. The HSA–ISO complex interactions determined by molecular modeling tool corroborated with the thermodynamic analysis from the experimental data.

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