Physics and chemistry of spin labels

Abstract

Biological systems provide the physical chemist with an abundance of interesting, challenging and significant problems. One example is the problem of the molecular basis of co-operative or allosteric interactions between distant ligand or substrate binding sites in hemoglobin and in enzymes. This problem has been discussed recently in This Journal by Eigen (1968) and by Wyman (1968). Another particularly challenging problem is the molecular organization of biological membranes. Such problems tend to be particularly resistant to solution by the straight-forward application of most spectroscopic techniques, in large part because of the enormous chemical and spectroscopic complexity of biological macromolecules. This spectroscopic complexity has stimulated the use of various ‘probes’ that can be introduced into selected sites in complex systems to provide spectroscopic signals that are comparatively free from interference. The use of heavy metal atoms (‘isomorphous replacement’) in X-ray studies of protein crystals (Green, Ingram & Perutz, 1954), and fluorescent dyes in the study of proteins in solutions (Weber, 1953; Steiner & Edelhoch, 1962) are early examples. Spin labels represent a new member of the family of spectroscopic structural probes. A spin label is a synthetic paramagnetic organic free radical, usually having a molecular structure and/or chemical reactivity that results in its attachment or incorporation at some particular target site in a biological macromolecule, or assemblage of macromolecules (Ohnishi & McConnell, 1965; Stone et al.1965). This type of probe is being used in our laboratory to study allosteric interactions in proteins, and molecular dynamics and organization in membranes.