A novel series of piperonal mesoionic derivatives (PMI 1-6) was synthesized. Tyrosinase inhibition in the presence of PMI-1, -2, -3, -4, -5 and -6 as well as human serum albumin (HSA) binding studies with PMI-5 and PMI-6 were done by spectroscopic and theoretical methods. The mesoionic compound PMI-5 is the most promising tyrosinase inhibitor with a noncompetitive inhibitory mechanism and an IC50=124μmolL-1. In accordance with the kinetic profile, molecular docking results show that PMI-5 is able to interact favorably with the tyrosinase active site containing the substrate molecule, L-DOPA, interacting with Val-247, Phe-263 and Val-282 residues. The spectroscopic results for the interaction HSA:PMI-5 and HSA:PMI-6 indicated that these mesoionic compounds can associate with HSA in the ground state and energy transfer can occur with high probability. The binding was moderate, spontaneous and can perturb significantly the secondary structure of the albumin. The molecular docking results suggest that PMI-5 and PMI-6 are able to be accommodated inside the Sudlow's site I in HSA, interacting with hydrophobic and hydrophilic amino acid residues.
