Binding properties of the natural red dye carthamin with human serum albumin: Surface plasmon resonance, isothermal titration microcalorimetry, and molecular docking analysis.

The interaction between carthamin and human serum albumin (HSA) was investigated by multiple spectroscopic analyses, surface plasmon resonance (SPR), isothermal titration microcalorimetry (ITC), and molecular docking studies. Fluorescence lifetime measurements implied that carthamin quenched the intrinsic fluorescence of HSA with the formation of a new complex via static mode. Binding affinities regarding this interaction were obtained from SPR analysis. Results demonstrated that carthamin could form a 1:1 complex with HSA at the binding affinity of KD=8.726×10-5M and that a high temperature was unfavourable for the interaction. ITC analyses and molecular docking results illustrated that HSA shaped a proper cavity (site I) to embed the whole carthamin molecule and that the complex was formed depending on intermolecular forces, including hydrophobic interaction, hydrogen bonding, and electrostatic force. Moreover, circular dichroism and 3D fluorescence demonstrated that carthamin slightly disturbed the microenvironment of amino residues and affected the secondary structure of HSA.