EPR imaging with natural spin probes

Abstract

Electron paramagnetic resonance imaging (EPRI) is a rapidly developing method for establishing spatial distribution of paramagnetic centers in a sample under study. This method has a wide field of potential applications, including biology and plant physiology. However, the concentration of native paramagnetic centers in such samples as a rule is insufficient for imaging experiments. The simple solution to this problem is to introduce into the object under study the necessary quantity of stable radicals as imaging substances. The most wide-spread type of such substance seems to be stable nitroxide radicals which have been applied recently to the study of spatial nonhomogeneity and diffusion in systems of chemical and biological interest (1-3). Nitroxide free radicals are very convenient imaging substances because the shapes of their EPR spectra are very sensitive to intermolecular conditions and can provide information about some important biological parameters, oxygen concentration, for example. In Ref. (4) the EPRI technique with nitroxide spin probes was used to investigate oxygen diffusion in models of biological systems. Nitroxide radicals were also used to study spatial characteristics of water penetration into wheat seeds during imbibition (5). In the latter work it was shown that nitroxide radicals may cause membrane destruction, thereby suppressing functions of cells. Much more attractive is the possibility of using some natural spin probes which demonstrate biological activity themselves and are compatible with biological tissues. In the present study we suggest the use, as natural probes, of the water-soluble fraction of soil humic substances (HS). These contain paramagnetic centers up to 10 ‘* spin/g, exhibiting at X band a single EPR signal with a linewidth about six gauss. The investigation of HS penetration into the seeds and their distribution is an important subject because of their biological effects on the seeds. It is well known that seeds are influenced by HS. It was reported that HS, which accelerated water uptake into the seeds (6)) could affect the rate of germination and initial seedling growth ( 7). Some observations indicate that humic substances act at the membrane level, but do not reveal the mode of action of these substances.