Comprehensive Study of the Interaction Between a Potential Antiprion Cationic Porphyrin and Human Prion Protein at Different pH by Using Multiple Spectroscopic Methods

Cationic porphyrins are potential antiprion drugs; however, the action mechanisms remain poorly understood. Herein, the interaction between a cationic porphyrin and recombinant human prion protein (rPrPC) was comprehensively studied by using surface plasmon resonance (SPR), fluorescence, resonance light scattering (RLS), and circular dichroism (CD) spectroscopy. The experimental results showed that the interaction between the cationic porphyrin and rPrPC was pH dependent. The equilibrium association constants obtained from SPR spectroscopy were 4.12 × 103 M− 1 at pH 4.0, 1.74 × 105 M− 1 at pH 6.0, and 5.98 × 105 M− 1 at pH 7.0. The binding constants at 298 K obtained from the fluorescence quenching method were 7.286 × 104 M− 1 at pH 4.0 and 1.45 7 × 105 M− 1 at pH 6.0. The thermodynamic parameters such as enthalpy change, entropy change, and free energy change were calculated, and the results indicated hydrogen bonds and van der Waals interactions played a major role in the binding reaction. The RLS experiment was performed to study the influence of porphyrin on the rPrPC aggregation at different pH values. The CD experiments were conducted to investigate the effects of porphyrin on the secondary structure and thermal stability of rPrPC. Finally, the comparison of SPR measurement and fluorescence quenching measurement was discussed.