Spin-Label Oximetry

Abstract

Molecular oxygen is paramagnetic and gives strong EPR signals in the gas phase. At sufficiently low pressure the number of observed lines is large indeed (see Figure 1 obtained at 180 micron pressure). Tinkham and Strandberg (1955a, 1955b) developed the theory for the spectrum of molecular oxygen and were able to assign 120 lines observed at X-band. As the pressure increases, the linewidths increase greatly. Even at atmospheric pressure, intense EPR signals can be detected from molecular oxygen (Figure 2). However, to the authors’ best knowledge no EPR spectra have been reported from oxygen dissolved in fluids near room temperature. Apparently lines are so broadened as to be undetectable. Thus there seems to be no possibility for the directdetection of oxygen in biological systems using magnetic resonance techniques. However, an indirect method does exist and is the subject of this chapter. Bimolecular collisions of oxygen with free radicals (and we consider particularly spin labels) alter the resonance characteristics of the radical. As will become apparent, it is a remarkable fact that effects can be detected at dissolved oxygen concentrations as low as 10−7M in a measurement that requires only a few seconds. This method has been called “spin-label oximetry.” The National Biomedical ESR Center has been active in the development of the field. A rigorous foundation has been laid down, and it is believed that the method can be applied with confidence to a wide range of biological systems.