What is "spin label"

n. (context physics chemistry English) An organic compound, containing an unpaired electron spin, that can be bonded to a molecule under investigation so as to be analysed using electron spin resonance spectroscopy

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 electron paramagnetic resonance 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).

Recent developments in the theory and experimental measurement of PREs have enabled the detection, characterization and visualization of sparsely-populated states of proteins and their complexes. Such states, which are invisible to conventional biophysical and structural techniques, play a key role in many biological processes including molecular recognition, allostery, macromolecular assembly and aggregation.