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Free radical scavenging potency of ferulic acid phenoxyl radical - thermodynamic study

Ferulic acid is a ubiquitous plant constituent and it is more bioavailable than other dietary phenolics. In systemic circulation it could be present in sufficient concentrations to exert biological activities, such as direct free radical scavenging [1, 2]. Inactivation of free radicals (e.g., the most dangerous HO•) by H- atom donation from the phenolic OH group of ferulic acid results in the formation of the corresponding phenoxyl radical (PhR) (delta rG = -28.31 kcal/mol). In this DFT study at M06- 2X/6-31G(d) level of theory we are dealing with the thermodynamics of possible subsequent reactions (a-c) which PhR (1) may undergo (Figure 1). Another HO• free radical may be quenched by the OCH3 group of PhR resulting in the formation of 3, 4-(methylenedioxy)cinnamic acid (2) (delta rG = -72.04 kcal/mol) (a). Distribution of the unpaired electron in PhR (e.g., spin density and Fukui function for radical attack) indicates reactive sites. We found that the addition of HO• to C-5 of PhR (delta rG = -50.13 kcal/mol) is favoured by keto-enol tautomerization which is highly thermodynamically driven (delta rG = -25.35 kcal/mol) (b). Obtained 5-hydroxyferulic acid (3) via its catechol moiety has potential of inactivation of another two HO• free radicals. PhR may also undergo dimerization reactions (c). Our results indicate that the most favoured dimer results via C-5-C-5 linkage (delta rG = -6.49 kcal/mol) due to stabilization by keto-enol tautomerization (delta rG = -39.47 kcal/mol). C-5-C-5 dimer possesses two guaiacyl moieties and has potential to inactivate another four HO• free radicals. By comparison of obtained delta rG values of investigated processes (Figure 1) it appears that the inactivation of PhR by radical adduct formation (b) is thermodynamically most favoured.

ferulic acid ; DFT ; Fukui function ; HAT ; RAF

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