Protective effects of 4-methylcoumarins and related compounds as radical scavengers and chain-breaking antioxidants.

The aim of this study is to determine, and to compare the protective effects of eight 4-methylcoumarins and four related compounds as radical scavengers and chain-breaking antioxidants. The main kinetic parameters of their radical scavenging activity (as % RSA, stoichiometry, n, and rate constants of reaction with DPPH radical, kRSA) and of chain breaking antioxidant activity (as antioxidant efficiency, PF and reactivity, ID), have been determined and discussed. The RSA study has been conducted at physiological temperature (37 °С) towards DPPH radical and the tested compounds are separated into three main groups: with strong activity (% RSA > 40%); with moderate activity (20% < % RSA > 40%) and with weak activity (% RSA < 20%). Chain-breaking antioxidant activities of the studied compounds have been evaluated during bulk phase lipid (triacylglycerols of sunflower oil, TGSO) autoxidation at 80 °C. All results obtained are compared with those for standard and known inhibitors of oxidation processes, e.g. caffeic and p-coumaric acids, α-tocopherol and butylated hydroxytoluene (BHT). On the basis of a comparative analysis with standard antioxidants, the differences in the radical scavenging and antioxidant abilities of the studied compounds have been discussed and reaction mechanisms proposed. All structures are optimized at UB3LYP/6-31 + G(d,p) level in gas phase and in acetone solution to study the solvation effects.