Spin label

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

Spin labeling is a similar technique in NMR spectroscopy, where an atom whose element whose naturally abundant isotope is NMR silent, is replaced by an NMR-active isotope of the same element. This is useful for tracking the chemical environment around this atom when full substitution with an NMR-active isotope is too expensive or results in too much noise.