Development and Application of a Peroxyl Radical Clock Approach for Measuring Both Hydrogen-Atom Transfer and Peroxyl Radical Addition Rate Constants.

The rate-determining step in free radical lipid peroxidation is the propagation of the peroxyl radical, where generally two types of reactions occur: (a) hydrogen-atom transfer (HAT) from a donor to the peroxyl radical; (b) peroxyl radical addition (PRA) to a "C═C" double bond. Peroxyl radical clocks have been used to determine the rate constants of HAT reactions (), but no radical clock is available to measure the rate constants of PRA reactions (). In this work, we modified the analytical approach on the linoleate-based peroxyl radical clock to enable the simultaneous measurement of both and . Compared to the original approach, this new approach involves the use of a strong reducing agent, LiAlH, to completely reduce both HAT and PRA-derived products and the relative quantitation of total linoleate oxidation products with or without reduction. The new approach was then applied to measuring the and values for several series of organic substrates, including para- and meta-substituted styrenes, substituted conjugated dienes, and cyclic alkenes. Furthermore, the and values for a variety of biologically important lipids were determined for the first time, including conjugated fatty acids, sterols, coenzyme Q10, and lipophilic vitamins, such as vitamins D and A.