DFT/B3LYP calculated bond-dissociation enthalpies, radical-scavenging and antioxidant activities of natural-like coumarins

Bond dissociation enthalpies (BDEs) of O-H groups of a set of hydroxy- and dihydroxy-4-methylcoumarins have been calculated in gas phase and in acetone by means of density functional theory calculations at B3LYP/6-31+G(d,p) level. The study has been done to determine the capacity of bond-dissociation enthalpy to explain the observed radical-scavenging and chain-breaking antioxidant activities of the studied coumarins. DPPH radical scavenging activity (RSA) in acetone solution [as %RSA and stoichiometry coefficient (11) for the fast (2 min) and total (20 min) kinetics] and the chain-breaking antioxidant activity (as protection factor, PF) during bulk phase lipid autoxidation have been used in the experimental study. The experimental results for the studied compounds show that the two phenolic groups at ortho position work in tandem, while the same at meta position work independent of each other. According to the theoretical results, the substitution in the benzene ring of the coumarin system is very important for the chain-breaking antioxidant activity. At the same time, theoretical calculations reveal that the introduction of methyl group and/or various substituents at the C-3 and C-4 positions of the pyrone ring affects the BDEs insignificantly. interestingly, the radical scavenging activity towards DPPH radical of 7,8-dihydroxy-4-methylcoumarins are much higher than that of the 6,7-dihydroxy-4-methylcoumarins, 5,7-dihydroxy-4-methylcoumarin and 7-hydroxy-4-methylcoumarin. Differences in RSA of studied coumarins could be explained with the solvent effect of acetone. Our findings revealed that BDE can serve as a probe for radical scavenging and antioxidant activities and even have predictive capacity, but for some tiny effects a precise description of the solvent effects is required