Combined multispectroscopic and molecular docking investigation on the interaction between strictosamide and human serum albumin.

The interaction between strictosamide (STM) and human serum albumin (HSA) was investigated by fluorescence spectroscopy, synchronous fluorescence spectroscopy, three-dimensional fluorescence spectroscopy, ultraviolet-visible absorption spectroscopy, circular dichroism spectroscopy and molecular modeling under physiological pH 7.4. STM effectively quenched the intrinsic fluorescence of HSA via static quenching. The binding site number n and apparent binding constant K(a) were determined at different temperatures by fluorescence quenching. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated as -3.01 kJ/mol and 77.75 J/mol per K, respectively, which suggested that the hydrophobic force played major roles in stabilizing the HSA-STM complex. The distance r between donor and acceptor was obtained to be 4.10 nm according to Förster's theory. After the addition of STM, the synchronous fluorescence and three-dimensional fluorescence spectral results showed that the hydrophobicity of amino acid residues increased and the circular dichroism spectral results showed that the α-helix content of HSA decreased (from 61.48% to 57.73%). These revealed that the microenvironment and conformation of HSA were changed in the binding reaction. Furthermore, the study of molecular modeling indicated that STM could bind to site I of HSA and the hydrophobic interaction was the major acting force, which was in agreement with the binding mode study.