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 NOvia 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 NO in water. The overall rate constants (k) for the reactions with HOO, HO, and NO radicals are 1.30 × 10, 5.76 × 10, and 1.43 × 10 Ms, respectively. The SET from the anionic state is the most dominant for the HOO and NO 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 (O) and ascorbate (Asc) in aqueous media.