Electron Paramagnetic Resonance Spectroscopy

Abstract

Electron paramagnetic resonance (EPR, or ESR for electron spin resonance) spectroscopy is based on the fact that substances containing unpaired electrons are paramagnetic. This paramagnetism may be due to the presence of transition elements which have unfilled shells, or it may be due to the transitory presence of oxidized or reduced substances. In the case of photosynthetic materials, this oxidation and reduction (i.e., the transfer of an electron from one substance to another) is set in motion by light. The basic observation, made in the mid-1950s (Commoner et al., 1956), was that photosynthetic materials become paramagnetic when illuminated. Two prominent light-induced resonances in plants and a single one in bacteria were described in early papers, and numerous speculations on their origin and significance advanced. It is now generally accepted that the light-induced EPR signal at g = 2.002 (signal I) in plants is a direct measure of the oxidation state of the photosystem I reaction center, P-700. However, between the time when the signal was first described and a sense of certainty as to its significance, some 15 years elapsed. We now know that light-induced resonances in photosynthetic organisms or subcellular preparations of them are indeed probes into events essential to the overall process of photosynthesis.