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 NO ₂ ^● 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 NO ₂ ^● in water. The overall rate constants (k _overall ) for the reactions with HOO ^● , HO ^● , and NO ₂ ^● radicals are 1.30 × 10 ⁴ , 5.76 × 10 ⁹ , and 1.43 × 10 ⁹  M ^-1 s ^-1 , respectively. The SET from the anionic state is the most dominant for the HOO ^● and NO ₂ ^● 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 ₂ ^●- ) and ascorbate (Asc ^- ) in aqueous media.
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