MTSL (S-(1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)methyl methanesulfonothioate) is an organosulfur compound that is used as a nitroxide spin label.

Methanesulfinate (CH3SO2−) is the leaving group: The heterodisulfide bond to the cysteine residue is robust, enabling site-directed spin labelling.

In Nuclear magnetic resonance the introduction of the paramagnetic group increases the relaxation rate of nearby nuclei.

A major advantage of this method over traditional methods for obtaining distance restraints in protein NMR is the increased length, as paramagnetic relaxation enhancement can detect distances up to 25 Å (2.5 nm) as opposed to about 6 Å (0.6 nm) using the nuclear Overhauser effect.

Spin labelling with MTSL is frequently used in investigation of residual structure in intrinsically unstructured proteins.