Synthesis and Evaluation of Thiosemicarbazone Inhibitors for SARS-CoV-2 Mpro by Spectroscopy and Microscale Thermophoresis

The SARS-CoV-2 pandemic has spurred global efforts to develop therapeutic approaches. The main protease (Mpro) of SARS-CoV-2 is crucial for viral replication and a key target for therapeutic development. In this study, 22 thiosemicarbazone derivatives were synthesized. Enzyme activity assays showed compound 3c exhibited obvious inhibition of Mpro, with an IC₅₀ value of 3.89 μM. The interaction mechanisms between thiosemicarbazone derivatives and Mpro were investigated using synchronous fluorescence, three-dimensional fluorescence, and circular dichroism spectroscopy. The fluorescence results indicate that static quenching is predominant. The equilibrium dissociation constant (K_d) of 2.3 μM between 3c and Mpro was obtained by microscale thermophoresis. This K_d value demonstrates the binding affinity of 3c for Mpro, consistent with the spectral results. Meanwhile, the binding modes and stability of thiosemicarbazone derivatives with Mpro were evaluated through molecular docking and molecular dynamics simulations, which also confirmed the stable binding between compound 3c and Mpro. This study provides valuable insights into the interaction mechanism between thiosemicarbazone derivatives and Mpro and provides clues for the design of Mpro inhibitors.