Investigation of free radical scavenging activity of pyridine derivatives containing hydroxyl and amino functional groups: experimental and quantum chemical approaches

The free radical scavenging activities of three pyridine derivatives-isoniazid (ID), nicotinamide (NE), and pyridoxine (PE)-were evaluated using a combined theoretical and experimental approach. Density functional theory calculations were employed to optimize molecular geometries at the M06-2X/6-311++G(d,p) level, assess key thermodynamic parameters, and investigate antioxidant mechanisms, including hydrogen atom transfer (HAT), radical adduct formation (RAF), and single electron transfer (SET), in reactions with the hydroperoxyl radical (HOO˙). Theoretical results indicated that HAT was the dominant mechanism for ID and PE, while NE favored the RAF pathway. Among the studied compounds, ID exhibited the highest reactivity toward HOO˙, with calculated rate constants of 3.55 × 10⁵ M^-1 s^-1 in the gas phase and 6.48 × 10⁶ M^-1 s^-1 in aqueous solution. Experimental antioxidant assessments using DPPH˙ and ABTS˙⁺ assays further supported these findings. ID demonstrated the strongest radical scavenging activity, with IC₅₀ values of 7.50 × 10^-6 M (DPPH˙) and 1.60 × 10^-5 M (ABTS˙⁺), followed by PE with moderate activity, while NE showed the weakest performance and was inactive against ABTS˙⁺. These results identified ID as the most potent antioxidant among the compounds studied.