The application of perturbed directional correlation of gamma rays to the study of protein-metal interactions.

Abstract

Several branches of nuclear spectroscopy—magnetic resonance and the Mossbauer effect in particular—have been the subject of considerable interest to obtain structural and motional information about biological macromolecules. Recently, a third technique— perturbed gamma-gamma directional correlation—is being tested for the study of rotational correlation times, internal rotations and conformational changes in biological macromolecules. When two gamma rays from a radioactive nucleus are emitted in a cascade and detected with a coincidence spectrometer, the coincidence counting rate depends strongly on the angle between their directions of propagation. This directional correlation can be perturbed by the interaction of the nucleus with fields existing in macromolecules. A study of such perturbed directional correlation (PDC) has the potential to provide information on protein-metal interactions. The use of a radioactive nucleus as a rotational tracer to label macromolecules offers the possibility of obtaining information on protein structure with the sensitivity and instrumental simplicity of radioactive tracer techniques. Work to date has centered on measurement of internal electric field gradients of metalloproteins, molecular correlation times and the correlation time of water associated with proteins. This paper is a brief review of the theory and experimental technique of perturbed directional correlation of gamma rays in comparison to Mossbauer spectroscopy and the methods of magnetic resonance. Particular emphasis will be placed on the isotopes which can be used in PDC especially those which may occupy sites in biological macromolecules.