ABSTRACT
In optical spectroscopy the reporter group may be a chromophore or fluorophore; in nuclear magnetic resonance a molecule containing a unique nucleus. With the electron spin resonance (ESR) technique we call this group a spin label or spin probe, which is a molecule containing a stable paramagnetic group. While some proteins contain their own intrinsic spin probe, such as the paramagnetic Fe(III) heme of met-myoglobin or met-hemoglobin, they are a minority of all proteins and enzymes for the general application of ESR to biochemistry. The “labeling” technique may be adopted in physical studies of biological systems by incorporating specific physical probes at an intended target site in an enzyme or protein. The most sensitive property of the spin label technique relative to protein structure-function studies is its ability to detect small structural changes. The ESR spectrum is strongly immobilized implying the possibility that the label is rigidly held, perhaps uniquely, by the enzyme at its active site.
