H-atom acceptor capacity of free radicals used in antioxidant measurements

Antioxidants are well-known for their beneficial effects on human health. Their capacity to scavenge free radicals mainly depends on their capacity to transfer H atoms to free radicals (R^•), and indirectly to labile H-atoms of the R^•-parent molecule (Ri-H). The aim of this study was to select an accurate method to estimate bond dissociation enthalpies (BDEs) of ROOi-H, LOOi-H and DPPHi-H. ROO^• and LOO ^• are radicals involved in lipid peroxidation and DPPH is a free radical used to measure antioxidant activities. BDE(ROOi-H) was systematically calculated with more than 20 different methods (HF, post-HF and DFT). This methodology was performed on three ROOi-H prototypes (HOOi-H, CH ₃OOi-H, and CH₃CH₂OOi-H) for which theoretical BDEs were compared to experimental BDEs. B3P86 and BHandH/6-31+G(d,p) appeared as the best compromises. The influence of R chain length and conjugation degree on BDEs was studied with DFT. Both parameters did not significantly change BDE(ROOi-H), except when intra H bonding was formed. The DPPHi-H BDE was accurately calculated with B3P86. This functional could definitely be considered as a good compromise since it also gave a reliable estimation of Oi-H BDE for polyphenols.