New Approaches in Spin Labeling and Spin Trapping. Part Two: NMR Detects Free Radicals

Abstract

The significance of free radical chemistry in biochemical processes is increasingly appreciated. The technology available for detecting free radical reactions improved over the past 30 years and now includes a large variety of methods encompassing detection of chemical reaction products and fluorescent probes. However, none are as specific as electron spin resonance (ESR), particularly when coupled with spin trapping technology, which remains the gold standard for verification of the presence of free radical species. Furthermore, ESR still has the greatest potential for measurement in vivo in real time. Nevertheless, despite the specificity of ESR, it has significant limitations for in vivo applications particularly due to rapid reduction of the spin adducts in living cells and in the extracellular fluid to ESR silent products. The other obstacle for in vivo ESR particularly in any human application is RF penetration. Therefore, except for a few experiments (Liu et al., 1999;Timmins et al., 1999) in vivo measurements of ESR-detectable spin adducts from living systems are usually reported in buffer-perfused organs or cells. In whole animals, sensitivity is generally near the current limit of detection (10–100 µM). Those ESR applications which have proven the most useful have often relied on ex vivo samples, using X band.