Abstract
This work aimed to explore the mechanism by which hydroxyl‐substituent Schiff bases scavenge free‐radicals. Thus, four Schiff bases, that is benzylidene aniline (BAN), 2‐(phenyliminomethyl)phenol (BAH), 4‐benzimidoylphenol (PBH) and 2‐benzimidoylphenol (OBH), were applied to protect human erythrocytes against 2,2′‐azobis(2‐amidinopropane hydrochloride) (AAPH)‐induced hemolysis. The results revealed that the OH attached to the ortho‐position of methylene in Schiff base scavenges 1.46 radicals per molecule, the OH attached to the para‐position of the N atom scavenges 2.94 radicals and the OH attached to the ortho‐position of the N atom scavenges 3.63 radicals. In addition, four Schiff bases were used together with some familiar antioxidants, such as 6‐hydroxyl‐2,5,7,8‐tetramethyl chroman‐2‐carboxylic acid (Trolox), L‐ascorbic acid (VC), α‐tocopherol (TOH) and L‐ascorbyl‐6‐laurate (VC‐12) in AAPH‐induced hemolysis of erythrocytes. It was found that, except for BAN+VC‐12, BAH + VC‐12, OBH + VC‐12 and PBH+TOH, all the other combinations protected erythrocytes more perfectly than when used individually. This result demonstrated that a promotive protection existed between Schiff base and other antioxidants and this improved their ability to scavenge free‐radicals. Finally, IC~50~ values of the aforementioned Schiff bases together with 2‐((o‐hydroxylphenylimino) methyl)phenol (OSAP) and 2‐((p‐hydroxylphenylimino)methyl)phenol (PSAP) were determined by reaction with two radical species, that is, 2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonate) radical (ABTS^+·^) and 2,2′‐diphenyl‐1‐picrylhydrazyl (DPPH). The results implied that the molecular framework of a Schiff base and an OH attached to the ortho‐position of methylene were apt to reduce radicals, but the OH attached to the aniline ring in a Schiff base was prone to scavenge radicals directly. Copyright © 2006 John Wiley & Sons, Ltd.