General

In solving a number of important problems of enzyme catalysis it is necessary to know the distances between certain functional groups of enzymes and other bioobjects. Therefore, one must first decipher the arrangement of the groups present at the active site. One must also know the distance between the active site and the site of an allosteric transition in order to study allosteric transitions, the depth of immersion of paramagnetic centers, and other long-range effects. Experimental investigations of the structures of multicenter metal enzymes containing several metal atoms per macromolecule are beset with serious difficulties.

A technique making use of double paramagnetic labels has been proposed for deciphering the structures of biological materials, primarily enzymes (Likhtenshtein 1968, 1976a; Likhtenshtein & Bobodzhanov 1968; Taylor et al. 1969). The principle of this method is that various residues of protein molecules can be modified by spin labels of an identical or different structure. Under certain conditions, an analysis of the parameters of the ESR signals of spin-labeled proteins will allow determination of the distance between the modified groups. This method has been further developed by other investigators. The paramagnetic complexes of transition metals, Cu2+, Mn2+, Fe3+, Co2+, etc., which either give ESR signals or affect the ESR parameters of other paramagnetics, can serve as labels as well.

Under certain circumstances, the ESR spectra of paramagnetic centers will respond suitably to the approach of other centers.