Patricia C. Jost

[17] The spin-labeling technique

Publisher Summary The purpose of this chapter is to describe the applications of spin labeling in enzymology. This technique refers to the use of stable free radicals as reporter groups or labels. Spin labels (stable free radicals) are usually molecules containing the nitroxide moiety that contains an unpaired electron localized on the nitrogen and oxygen atoms. Most applications of nitroxide–nitroxide interactions fall into two categories: (1) pairwise interactions in an otherwise magnetically dilute system and (2) multiple interactions in a system with high local concentrations of spin labels. Pairwise interactions occur when dinitroxide spin labels are used, or when two spin labels occupy binding sites that are very close together. The adjacent methyl or alkyl groups are necessary to stabilize the free radical. The interactions between spin labels give rise to a variety of interesting effects. Pairs of spin labels can provide sensitive molecular rulers for determining distances and geometries in biological systems.

SPIN LABELING AND MEMBRANE STRUCTURE

Publisher Summary This chapter discusses spin labeling and membrane structure. The molecular structure of membranes and model membranes is being investigated with a variety of spectroscopic approaches. One of the interesting and potentially important spectroscopic methods is the use of electron spin resonance (ESR) in conjunction with paramagnetic reporter molecules (spin labels). The N—O group contains the unpaired electron that renders the molecule paramagnetic and, hence, suitable for detection by ESR. The alkyl side chains are necessary to stabilize the free radical. The choice of phase and scan direction results in four possible permutations of the same spectrum. Although all four can be found in the spin labeling literature, the presentation most consistent with current trends in spectroscopy involves positive phase and increasing from left to right. The shape of an ESR spectrum is markedly dependent upon the rotational mobility of the nitroxide. Anisotropic motion in the broadest sense refers to any molecular reorientation that does not occur with equal probability in all directions.