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{{no footnotes|date=December 2010}}
[[Image:Epr spectrum.jpg|thumb|right|250px|[[Electron paramagnetic resonance|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 [[protein]]s or [[biological membrane]]-local dynamics using [[Electron paramagnetic resonance|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 <math>\neq</math> 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.

[[Category:Spectroscopy]]