Probing the binding of morin to human serum albumin by optical spectroscopy

Morin [2-(2,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one], a member of flavonols, is an important bioactive compound by interacting with nucleic acids, enzymes and protein. Its binding to human serum albumin was investigated by fluorescence quenching, fluorescence anisotropy, and UV-vis absorbance under the simulative physiological condition. Fluorescence quenching data show that the interaction of morin with HSA forms a non-fluorescent complex with the binding constants of 1.394 x 10(5), 1.489 x 10(5), 1.609 x 10(5) and 1.717 x 10(5)M(-1) at 292, 298, 303 and 310 K, respectively. The thermodynamics parameters, enthalpy change (DeltaH) and entropy change (DeltaS) were calculated to be 8.97 kJ mol(-1) and 129.15 J mol(-1)K(-1) via van't Hoff equation. From the spectroscopic results and thermodynamics parameters, it is observed that van der Waals and hydrogen bonds are predominant intermolecular forces when forming the complex. The distance r=4.25 nm between donor (Trp214) and accepter (morin) was estimated based on the Förster theory of non-radiative energy transfer. The red shift of UV-vis absorbance shows that morin is bound to several amino acids on the hydrophobic pocket of HSA. Moreover, the competitive probes, such as warfarin and ibuprofen (site I and II probes, respectively), reveal that the binding location of morin to HSA in the site I of the hydrophobic pocket, which corresponds to the results of UV-vis absorbance, while morin also binds other lower affinity binding sites on HSA from the fluorescence anisotropy spectroscopy.