MTSL (''S''-(1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)methyl methanesulfonothioate) is an

organosulfur compound Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

that is used as a

nitroxide Aminoxyl denotes a radical functional group with general structure R2N–O•. It is commonly known as a nitroxyl radical or a nitroxide, however IUPAC discourages the use of these terms, as they erroneously suggest the presence of a nitro group. ...

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 dynami ...

.Christian Altenbach, Kyoung-Joon Oh, René J. Trabanino, Kálmán Hideg, Wayne L. Hubbell "Estimation of Inter-Residue Distances in Spin Labeled Proteins at Physiological Temperatures: Experimental Strategies and Practical Limitations" Biochemistry, 2001, volume 40, pp 15471–15482. MTSL is

bifunctional In chemistry, bifunctionality or difunctionality is the presence of two functional groups in a molecule. A bifunctional species has the properties of each of the two types of functional groups, such as an alcohol (), amide (), aldehyde (), nitrile ...

, consisting of the nitroxide and the thiosulfonate ester

functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

s. The nitroxide label is sterically protected, so it is relatively unreactive.

Labeling

MTSL is attached to proteins by reaction with thiol groups. The reaction exploits standard reactivity of thiosulfate esters. Methanesulfinate (CH₃SO₂⁻) is the leaving group: :RSO₂S-''nitroxide'' + ''protein''-SH → ''protein''-S-S-''nitroxide'' + RSO₂H The heterodisulfide bond to the

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

residue is robust, enabling site-directed spin labelling.Berliner, L.J., Grunwald, J., Hankovszky, H.O., Hideg, K. (1982). A novel reversible thiol-specific spin label: papain active site labeling and inhibition. Analytical Biochemistry 119, 450-455. The MTSL moiety will add 184.3 daltons to the mass of the

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

to which it is attached. The cysteine can be introduced using

site-directed mutagenesis Site-directed mutagenesis is a molecular biology method that is used to make specific and intentional mutating changes to the DNA sequence of a gene and any gene products. Also called site-specific mutagenesis or oligonucleotide-directed mutagenes ...

, and hence most positions in a protein can be labelled.

Spectroscopy

Nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

the introduction of the paramagnetic group increases the relaxation rate of nearby nuclei. Its presence can be detected as peak broadening and loss of intensity in peaks corresponding to nearby nuclei. Hence proximity can be inferred for all nuclei, that are affected. 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 The nuclear Overhauser effect (NOE) is the transfer of spin polarization, nuclear spin polarization from one population of Spin (physics), spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via Relaxation (NMR), cross-relaxation. A phenomenolog ...

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

intrinsically unstructured proteins In molecular biology, an intrinsically disordered protein (IDP) is a protein that lacks a fixed or ordered three-dimensional structure, typically in the absence of its macromolecular interaction partners, such as other proteins or RNA. IDPs ran ...

References

{{DEFAULTSORT:Mtsl Pyrrolines Amine oxides