Biological applications of spin pH probes

The determination of pH is one of the most important problems in the bioche mistry of living organisms, since many of the vital processes of cells and cellular organelles depend on the local pH value. Amongst currently used ex perimental approaches for the measurement of pH, the application of spin pH probes in combination with EPR spectroscopy is a comparatively new and rap idly developing field. In this article we describe the background, advantag es and limitations of the method of spin pH probes, and discuss its recent applications. Availability of a wide variety of pH-sensitive nitroxides wit h different ranges of pH-sensitivity, labeling group and lipophilicity faci litates their application to a variety of biological systems from subcellul ar organelles to complex organisms. The recent progress in low-field EPR-ba sed imaging and spectroscopy-based techniques allows spin pH probes to be u sed for non-invasive in vivo pH measurement and pH-sensitive imaging.