Evaluation of pH and concentration effects on the antioxidant and pro-oxidant activities of lysine- and lysine methyl ester-based antioxidant tetramers, each containing four units of either syringaldehyde or vanillin

Abstract

Previous studies have indicated that the antioxidant (AO) and pro-oxidant (PO) activities of antioxidants can depend on pH/concentration. To see if antioxidants carrying multiple hindered phenolic units are also affected by pH/concentration, we synthesized tetrameric antioxidants with varying solubilities: lysine-based hydrophilic antioxidants and lysine methyl ester-based hydrophobic antioxidants, each carrying four hindered phenolic units (ortho-methoxy containing syringaldehyde or vanillin). The AO activities of the synthesized antioxidants were measured using AAPH-derived radicals and pBR322 plasmid DNA. Hydrophilic sodium ascorbate and hydrophobic quercetin were used as comparison standards. Based on the study, all tetrameric antioxidants and the standards in PBS showed enhanced AO activities as concentrations increased within the tested range. In the study of pH effects on AO activities using pH 4, pH 7, and pH 9 buffers, the AO activities of all hydrophilic and hydrophobic antioxidants increased with increasing pH (pH 9 > pH 7 > pH 4), indicating that their AO activities are pH-dependent. In the PO assay employing Cu(II) ions and pBR322, sodium ascorbate and quercetin in PBS displayed strong PO effects, which intensified as their concentrations increased. In the buffers at pH 4, 7, and 9, sodium ascorbate and quercetin showed PO effects in the order of pH 9 > pH 7 > pH 4. At the same pH level, both sodium ascorbate and quercetin produced significantly stronger PO effects at high concentrations compared to low concentrations, suggesting that PO effects are concentration-dependent as well as pH-dependent. Our results with sodium ascorbate and quercetin show that both AO and PO activities increase with higher pH levels and concentrations. This indicates that a strong antioxidant can also function as a potent pro-oxidant. However, the tetrameric antioxidants synthesized with hindered phenolic building blocks did not exhibit any significant PO effects, regardless of pH levels and/or concentrations, even though they displayed strong AO activities. This suggests that sterically hindered phenolic units are beneficial in developing potent antioxidants without causing PO effects. Notably, none of our tetrameric antioxidants showed any concentration- or pH-dependent activity crossover between AO and PO effects.