A comparative ESR study of some paramagnetic materials as probes for the noninvasive measurement of dissolved oxygen in biological systems.

The ESR properties of three types of paramagnetic material, active charcoal, fusinite and a stable nitroxide radical 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPONE), were examined in order to evaluate their suitability as probes to measure dissolved intra- and extra-cellular oxygen. Although, with changes in oxygen concentration, a greater change in the linewidth of ESR signals was observed with fusinite or active charcoal, it took a long time (15 min for active charcoal and more than 6 h for fusinite) for equilibrium to be achieved. On the other hand, equilibrium was reached very rapidly in the case of the TEMPONE spectra although the sensitivity to changes in oxygen concentration was only moderate. Furthermore, since lipid bilayers are permeable to TEMPONE, this compound can be used to measure intracellular oxygen concentration when employed in combination with membrane-impermeable spin-broadening reagents which act on ESR signals arising from extracellular probes. A perdeuterated derivative of TEMPONE is useful in that it gives a greater signal-to-noise ratio and greater sensitivity to changes in oxygen concentration. In conclusion, active charcoal is suitable as a probe for extracellular oxygen in a system where changes are slow, while nitroxide is a versatile probe for measuring rapidly changing intra- and extra-cellular oxygen concentrations.