Study of lipid peroxyl radicals in urea clathrate crystals: oxygen-17 couplings and rotational averaging

Abstract

This work reports an electron spin resonance investigation of radicals formed after irradiation of a variety of fatty acid esters trapped in urea clathrate structures. Irradiation of saturated fatty acid esters results in carbon-centered radicals at the {alpha}-carbon site, whereas for unsaturated species the expected allylic and bisallylic radicals are found. Exposure of the carbon-centered radicals to oxygen results in their conversion to the corresponding peroxyl radicals. Orientation- and temperature-dependence studies show that at room temperature both the lipid carbon centered and peroxyl radicals rotate or oscillate in urea channels about their long axis. The rotational jump mechanism found for peroxyl radicals in this work usually follows the trigonal symmetry of the low-temperature distorted urea host channels. At room temperature the entrapped molecules rotate freely on their long axis, and as a consequence orientation of samples at the magic angle (54.7{degree}) gives isotropic couplings. Oxygen-17 labeling of the peroxyl radicals results in the first reported anisotropic oxygen hyperfine couplings for lipid peroxyl radicals.