Antioxidant reactivity of nickel and zinc porphyrins revealed by electrochemical, spectroscopic and molecular modelling approaches.

The antioxidant properties of Nickel(II) 5,10,15,20-tetraphenylporphyrin (NiTPP) and Zinc(II) 5,10,15,20-tetrakis(4-methylphenyl)porphyrin (ZnTMPP) were examined using complementary experimental and computational approaches. Radical-scavenging activity was evaluated in the DPPH system by cyclic voltammetry and UV-visible spectroscopy, using α-tocopherol as a benchmark. Electrochemical measurements showed EC50 values of 139.1 ± 1.3 µM for NiTPP and 269.9 ± 4.5 µM for ZnTMPP, while α-tocopherol exhibited an EC50 of 90.99 ± 0.61 µM. In contrast, spectrophotometric analysis yielded EC50 values of 164.1 ± 2.3 µM for NiTPP and 142.1 ± 0.8 µM for ZnTMPP, indicating more efficient radical quenching by ZnTMPP in solution. Both metalloporphyrins displayed stronger binding affinities toward DPPH than α-tocopherol, with NiTPP showing the highest association constant (Kb = 7.49 × 104 L·mol-1; ΔG = -27.95 kJ·mol-1), followed by ZnTMPP (Kb = 3.14 × 104 L·mol-1; ΔG = -25.08 kJ·mol-1). Protein-level interactions relevant to antioxidant regulation were explored through molecular docking and 300 ns molecular dynamics simulations targeting the Keap1 Kelch domain. Both porphyrins occupied the canonical binding site, with NiTPP exhibiting the most favorable docking score (-9.4 kcal·mol-1). Molecular dynamics and MM-GBSA analyses confirmed greater structural stability and stronger binding for the NiTPP-Keap1 complex. These findings highlight a complementary antioxidant behavior, where ZnTMPP favors direct radical scavenging, while NiTPP shows enhanced protein recognition and stabilization.