Investigation of 4‑Hydrazinobenzoic Acid Derivatives for Their Antioxidant Activity: In Vitro Screening and DFT Study

Hydrazinobenzoic acid derivatives with isothiocyanate, benzylidene, and acid anhydride core units (1–13) were previously synthesized and fully characterized. Targets 1–13 were investigated for their antioxidant activities using different in vitro assays such as 1,1-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging, 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonate) (ABTS), ferric reducing antioxidant power (FRAP), and reducing power capability. All derivatives showed antioxidant properties in relation to the standard butylated hydroxylanisole (BHA). Superior antioxidant activities was observed for compounds 3 and 5–9 at a concentration of 20 μg/mL (70–72%) when tested by the DPPH method in comparison to BHA (92%), and compounds 1–10 showed the highest free radical quenching activity (80–85%) when examined by ABTS at 20 μg/mL in relation to BHA (85%). Density function theory (DFT) studies were carried out using the B3LYP/6-311G­(d,p) level of theory. Several antioxidant descriptors were calculated for targets 1–13 compared with BHA. Targets 1–13 were proposed to exhibit their antioxidant activities via the following three proposed antioxidant mechanisms: single electron transfer (SET), hydrogen atom transfer (HAT), and sequential proton loss electron transfer (SPLET). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies and electron levels for 1–13 were also determined.