OPTICALLY DETECTED MAGNETIC-RESONANCE OF TRIPLET-STATES IN PROTEINS

Triplet states, with few exceptions (notably dioxygen), are (photo) excited metastable states of usually aromatic, organic molecules. These types of molecules abound in all biological material, and the triplet state is an important probe of the molecular structure. Triplets have certain physical properties that make them eminently useful for identifying reactants in photoreactions, for probing structural environments, binding, and for serving as gentle perturbers. Studying the response of the system then gives important clues to its structure and function. This chapter discusses the physical characteristics of the triplet state and a variant of the technique of electron paramagnetic resonance (EPR)—namely, optically detected magnetic resonance (ODMR) in zero magnetic field, which is one of the most important spectroscopic tools to study the triplet state. Most attention is devoted to the absorbance-detected magnetic resonance (ADMR) technique, the use of which has greatly expanded since the early 1980s. By ADMR, it is possible to monitor low-temperature triplet absorbance difference spectra with unparalleled accuracy and sensitivity. For coupled pigment systems, such as encountered in photosynthetic membranes, this has opened up a new field for the study of pigment interaction and interpretation of optical spectra. © 1993, Elsevier Inc. All rights reserved.