Recently, highly oxygen-sensitive solid-state paramagnetic to measure NO โข in biological systems. These methods inprobes have been developed from glucose chars which are capable clude a variety of optical-absorption-based and luminesof providing measurements of very low, millitorr, oxygen concencence-based measurements as well as the use of potentiomettrations in biological cells and tissues. With the increased interest ric electrodes (14-18).
and recognition of the presence and importance of the free-radical
Oxygen metabolism is also of great importance in mamgas nitric oxide, NO โข , in biology, studies were performed aimed malian cells and tissues, and, therefore, much effort has been at characterizing the effect of NO โข on this probe. It was observed expended in the development of techniques and instrumentathat NO โข exerts a similar concentration-dependent broadening as tion to measure oxygen in biological systems (19-27). Elecdoes molecular oxygen, O 2 . In anaerobic solutions, concentrations tron paramagnetic resonance spectroscopy with a variety of of NO โข could be detected down to 10-100 nM levels. Measurefree-radical labels or solid-state probes, including nitroxide ments of NO โข formation in aqueous solution were performed from a pharmacological NO โข donor, which produced micromolar con-radicals, particles of the natural coal fusinite, or crystals of centrations of NO โข . This technique, however, was not sufficiently lithium phthalocyanine, have been utilized to provide measensitive to measure physiological 1-10 nM concentrations. Thus, surements of oxygen in biological cells and tissues (20the glucose-char EPR probe could be used to detect pharmacologi-27). It has been known for some time that carbon chars cal levels of NO โข but was not significantly affected by the low have sharp linewidths which are oxygen-sensitive (28). Rephysiological levels which occur in normal biological tissues. These cently, we have shown that highly sensitive electron pararesults indicate that, under normal physiological conditions, the magnetic resonance oximetry probes can be formed from linewidth alterations measured using the glucose-char EPR probe chars of glucose heated under high pressures (29). The oxyare solely caused by oxygen.