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 (kH),
but no radical clock is available to measure the rate constants of
PRA reactions (kadd). In this work, we
modified the analytical approach on the linoleate-based peroxyl radical
clock to enable the simultaneous measurement of both kH and kadd. Compared to the
original approach, this new approach involves the use of a strong
reducing agent, LiAlH4, 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 kH and kadd values for several series of organic substrates,
including para- and meta-substituted styrenes, substituted conjugated
dienes, and cyclic alkenes. Furthermore, the kH and kadd 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 D3 and A.