Electron Spin Resonance. Part One: A Diagnostic Method in the Biomedical Sciences

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A review is presented of some of the ways in which electron spin resonance (ESR) spectroscopy may be useful to investigate systems of relevance to the biomedical sciences. Specifically considered are: spin-trapping in biological media; the determination of antioxidant efficiencies; lipid-peroxidation; the use of nitroxides as probes of metabolic activity in cells and as structural probes of cell-membranes; ESR coupled with materials for radiation-dosimetry; food-and drug-irradiation; studies of enzyme systems and of cyclodextrins; diagnosis of cancer and rheumatoid arthritis, measurement of oxidative stress in synovial tissue in preparation for joint replacement; determination of oxidative species during kidney dialysis; measurement of biological oxygen concentrations (oximetry); trapping in living cells of the endothelium-derived relaxing factor nitric oxide (NO); measurement of hydrogen peroxide; determination of drugs of abuse (opiates); ESR measurements of whole blood and as a means to determine the age of bloodstains for forensic analysis are surveyed, and also a determination of the aqueous volume of human sperm cells is described, among other topics.

Keywords: electron spin resonance, antioxidant efficiencies, lipid-peroxidation, structural probe of cell-membranes, diagnosis of cancer and rheumatoid arthritis, biological fluids

1. Introduction

Electron spin resonance (ESR)--also known as electron paramagnetic resonance (EPR) is often viewed as the Cinderella sister of nuclear magnetic resonance (NMR). This is partly because both the application and availability of NMR spectrometers are more apparent, and that not uncommonly ESR receives only passing mention in university courses, unless there is a specialist in the subject on the staff. In principle, NMR spectra may be recorded from dozens of different nuclei, whereas obtaining an ESR spectrum requires some of the sample molecules to contain one or more unpaired electrons, which might appear to be an oddity. Notwithstanding, here lies the beauty of ESR, since the method is completely specific for unpaired electrons, which are frequently formed in materials that have encountered one or more of a range of important energetic conditions for which a signature is supplied in the form of the consequent ESR spectrum. The unpaired electron bearing sites are usually termed "damage centres", "defects" or "trapped electrons" by physicists, geologists, archeologists and environmental scientists, but are normally referred to by chemists and biologists as "free radicals", whose detailed molecular structures may be revealed from the spectral parameters of hyperfine splitting, where it is observed, and of g-factor.

Paramagnetic transition-metal cations, e.g. [Fe.sup.3+], [Mn.sup.2+], [Cu.sup.2+], are commonly detected in environmental samples and in biological tissues using ESR. Paramagnetic materials, including metal cations and synthesised complexes or stable organic radicals (usually nitroxides), may be deliberately added to samples as probes of local molecular environments such as cell-membranes, and to determine their dynamic properties. Along with other stable "organic" radicals (e.g. carbon chars and lithium phthalocyanines), nitroxides may be used to measure oxygen tensions in whole tissues and in simpler cellular systems. Metabolic activities in such biological media may also be determined from the reduction kinetics of nitroxides. When it is desired to investigate various reacting systems for the intermediacy of free radicals, "spin-traps" are often added. These are frequently of a structural type designed to "trap" reactive radicals by addition to them, so forming nitroxides in situ, which rise to detectable concentrations in consequence of their relative stability. Clearly, there are many and varied important applications for ESR, and most importantly so in areas of the biological and environmental sciences.

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