Spin label

(physical chemistry) A molecule which contains an atom or group of atoms exhibiting an unpaired electron spin that can be detected by electron spin resonance (ESR) spectroscopy and can be bonded to another molecule.

A molecule which contains an unpaired electron spin which can be detected with electron spin resonance (ESR) spectroscopy. Molecules are labeled when an atom or group of atoms which exhibits some unique physical property is chemically bonded to a molecule of interest. Groups containing unpaired electrons include organic free radicals and a variety of types of transition-metal complexes (such as vanadium, copper, iron, and manganese). Through analysis of ESR spectra, rates of molecular motion whose motion is restrained by surrounding molecules can be determined.

Analysis of the rate and type of motion of a spin label is important for a wide variety of biological problems. The type of label used in these studies is generally a nitroxide free radical. Spin-labeling studies provide a powerful technique for the study of the geometry and dimensions of receptors in enzymes. Spin labels have been used extensively to study the structure of membranes, and can provide important information about the organization and rates of motion in membranes. Spin labels have also been used to study the structure and organization of synthetic polymers and to study phase transitions. See also Electron paramagnetic resonance (EPR) spectroscopy; Enzyme.

EPR spectrum of a spin label

A spin label (SL) is an organic molecule which possesses an unpaired electron, usually on a nitrogen atom, and the ability to bind to another molecule. Spin labels are normally used as tools for probing proteins or biological membrane-local dynamics using EPR spectroscopy. The site-directed spin labeling (SDSL) technique allows one to monitor a specific region within a protein. In protein structure examinations, amino acid-specific SLs can be used.

The goal of spin labeling is somewhat similar to that of isotopic substitution in NMR spectroscopy. There one replaces an atom lacking a nuclear spin (and so is NMR-silent) with an isotope having a spin I 0 (and so is NMR-active). This technique is useful for tracking the chemical environment around an atom when full substitution with an NMR-active isotope is not feasible. Recently, spin-labelling has also been used to probe chemical local environment in NMR itself, in a technique known as Paramagnetic Relaxation Enhancement (PRE).

References

Berliner, L.J. (1976). Spin labeling I : theory and applications, Academic Press, New York.

Berliner, L.J. (1979). Spin labeling II : theory and applications, Academic Press, New York.

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