Abstract
Direct and indirect antioxidant activities of rosmarinic acid (RA) based on HOO˙/CH3OO˙ radical scavenging and Fe(iii)/Fe(ii) ion chelation were theoretically studied using density functional theory at the M05-2X/6-311++G(2df,2p) level of theory. First, four antioxidant mechanisms including hydrogen atom transfer (HAT), radical adduct formation (RAF), proton loss (PL) and single electron transfer (SET) were investigated in water and pentyl ethanoate (PEA) phases. Regarding the free radical scavenging mechanism, HAT plays a decisive role with overall rate coefficients of 1.84 × 103 M−1 s−1 (HOO˙) and 4.49 × 103 M−1 s−1 (CH3OO˙) in water. In contrast to PL, RAF and especially SET processes, the HAT reaction in PEA is slightly more favorable than that in water. Second, the [Fe(iii)(H2O)6]3+ and [Fe(ii)(H2O)6]2+ ion chelating processes in an aqueous phase are both favorable and spontaneous especially at the O5, site-1, and site-2 positions with large negative ΔrG0 values and great formation constant Kf. Finally, the pro-oxidant risk of RA− was also considered via the Fe(iii)-to-Fe(ii) complex reduction process, which may initiate Fenton-like reactions forming reactive HO˙ radicals. As a result, RA− does not enhance the reduction process when ascorbate anions are present as reducing agents, whereas the pro-oxidant risk becomes remarkable when superoxide anions are found. The results encourage further attempts to verify the speculation using more powerful research implementations of the antioxidant activities of rosmarinic acid in relationship with its possible pro-oxidant risks.
Highlights
Oxidative stress (OS) resulting from free radical action is one of the reasons for the serious decline in human health.[1,2] Free radicals damage biological compounds that make up human cells or carry genetic information (i.e. DNA and RNA).[1,3,4] This causes several diseases such as cancers, heart diseases, and Alzheimer's disease.[5,6,7,8,9,10,11] Many methods have been used to protect human health from OS
Optimized structures of rosmarinic acid in neutral and monoanionic forms in the water phase at 298.15 K calculated at the M05-2X/6-311++G(2df,2p) level of theory are presented in Fig. S1 (ESI le†)
The non-polar pentyl ethanoate (PEA) solvent slightly enhances formal hydrogen transfer (FHT) reactions, while it is unfavorable to radical adduct formation (RAF) and especially to single electron transfer (SET) reactions
Summary
Oxidative stress (OS) resulting from free radical action is one of the reasons for the serious decline in human health.[1,2] Free radicals damage biological compounds that make up human cells (i.e. lipid and protein) or carry genetic information (i.e. DNA and RNA).[1,3,4] This causes several diseases such as cancers, heart diseases, and Alzheimer's disease.[5,6,7,8,9,10,11] Many methods have been used to protect human health from OS. The use of antioxidant compound supplementary provided from natural products or diets is one of the most effective ways.[1,12] Phenolic compounds that are ubiquitously distributed
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