Advances in the spin labeling method

Abstract

The spin labeling technique, coined by Harden McConnell in the early 1960s, was modeled after the "reporter group" approach of placing a spectroscopic probe in a biological system such as an enzyme or a protein in order to monitor spectroscopic variables to which the biosystem alone is transparent. This might be a fluorophore, NMR isotope such as 19F or 13C, resonance Raman probe, a stable paramagnetic nitroxyl molecule, etc. The spin label technique has flourished since then with several texts devoted specifically to the subject. Spin labels have now enjoyed a history of close to twenty-five years in demonstrating the applicability of paramagnetic nitroxides to biochemical problems of structure and function in enzymes, membranes, cells and animals. These have been employed traditionally as reporter groups; that is, the nitroxide spin label serves as a physical probe that reports aspects of its structure and environment at a localized molecular site. It has been quoted many times that this reporter group must “report the news”' not “make the news.” It is important to insure that the sometimes bulky nitroxide spin label, does not perturb the macromolecular system under study. Although the method is technically not restricted to just nitroxyl compounds (most authors use the term nitroxide for these structures) it is fair to say that more than 99.5% of all papers using ESR reporter group techniques employ nitroxyl (nitroxide) spin labels. These molecules are classified as spin labels, when a covalent linkage to the biological system is employed (i.e. with proteins, polymers and nucleic acids) while a spin probe involves nonce valent interactions (cells, membranes, liquid crystals and some polymer systems).