[The Interaction Between a New Water-Soluble Meso-Tetrakis(Carboxyl) Zinc(Ⅱ) Porphyrin and Human Serum Albumin]

Porphyrin is a kind of photosensitizer for photodynamic therapy of cancer. Many porphyrin derivatives have been used in clinical treatment. Human Serum Albumin (HSA) is the carrier of drug transportation. Therefore, investigation on the interaction of porphyrin with HSA is very important to understand the pharmacokinetic of the porphyrin. In this paper, a new water-soluble carboxyl porphyrins, meso-tetrakis(carboxyl) zinc(Ⅱ) porphyrin (2-Zn), was synthesized and characterized. Its interaction with human serum albumin (HSA)was investigated by UV-Vis absorption spectra, fluorescence spectra, circular dichroism (CD) spectra and molecular modeling. The results indicated that the fluorescence quenching of HSA by 2-Zn was a static process with the quenching constants are 1.96×104 L·mol-1 (298 K) and 1.37×104 L·mol-1 (310 K) and the binding constants were calculated to be 1.93×104 L·mol-1 (298 K) and 1.50×104 L·mol-1 (310 K). According to the Van’t Hoff equation, the thermodynamic parameters were characterized by negative enthalpy (ΔH=-16.132 kJ·mol-1) and positive entropy (ΔS=27.905 J·mol-1·K-1), which indicated that 2-Zn binds with HSA mainly via electrostatic interaction along with the hydrogen bonding and hydrophobic interaction. Site marker competitive binding experiment confirmed that 2-Zn mainly binds at site Ⅱ. The distance between HSA and the receptor (2-Zn) and the efficiency energy transfer were obtained to be 4.01 nm and 0.163 respectively, based on the Forster theory on resonance energy transfer. Synchronous fluorescence, absorption and CD spectroscopy showed that the interaction of HSA with 2-Zn induced a conformational change of protein, and the amount of α-helical structures were decrease. Furthermore, the binding details between 2-Zn and HSA were further studied with the molecular docking, which was in good agreement with the site marker competitive binding experiments and thermodynamic parameters.