On the free radical scavenging and metallic ion chelating activities of pyridoxal - Could the pro-oxidant risk be competitive?

Primary and secondary antioxidant activities of pyridoxal have been investigated by using density functional theory (DFT) at the M05–2X level combined with 6–311++G(d,p) basis set for non-metallic atoms and LanL2DZ for metallic ions. The former has been examined by its free radical scavenging activity towards HOO●, HO●, and NO2●via different mechanisms including formal hydrogen transfer (FHT), proton transfer (PT), single electron transfer (SET), and radical adduct formation (RAF). The latter has been accomplished through its transition metal-chelating ability with Fe(III)/Fe(II) and Cu(II)/Cu(I) ions. The results show that pyridoxal illustrates as an efficient radical scavenger, especially, for HO● and NO2● in water. The overall rate constants (koverall) for the reactions with HOO●, HO●, and NO2● radicals are 1.30 × 104, 5.76 × 109, and 1.43 × 109 M−1s−1, respectively. The SET from the anionic state is the most dominant for the HOO● and NO2● scavenging reactions, while both RAF and SET contribute largely to the reaction with highly reactive HO● radicals. Moreover, the anionic form of pyridoxal demonstrates a better role as a metal chelator than the neutral. However, the pro-oxidant risks of the formed complexes could be observed if there are superoxide radical anion (O2●–) and ascorbate (Asc–) in aqueous media. An example of transition state (TS) of radical scavenging reactions and the optimized structure of metallic ions-pyridoxal complexes. [Display omitted] •Radical scavenging and metal chelating activities of pyridoxal (PL) have been studied.•PL is an efficient HO.●/NO2●-scavenger in water•Anionic PL can form more stable complexes with metallic ions than the neutral.•Cu(II)-neutral PL complexes show the most pro-oxidant risks when existing Asc– agents.