A spin label (SL) is an
organic molecule
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-cont ...
which possesses an unpaired
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
, usually on a
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
atom, and the ability to bind to another molecule. Spin labels are normally used as tools for probing
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 ...
s or
biological membrane
A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of th ...
-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
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
-specific SLs can be used.
The goal of spin labeling is somewhat similar to that of isotopic substitution in
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...
. There one replaces an atom lacking a nuclear spin (and so is NMR-silent) with an isotope having a spin (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
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
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.
Applications of spin EPR
Spin labelled fatty acids have been extensively used to understand dynamic organization of lipids in bio-membranes and
membrane biophysics Membrane biology is the study of the Biology, biological and Biochemistry, physiochemical characteristics of Biological membrane, membranes, with applications in the study of cellular physiology.
Membrane bioelectrical impulses are described by the ...
. For example, stearic acid labelled with
aminoxyl spin label moiety at various carbons (5, 7, 9, 12, 13, 14 and 16) with respect to first carbon of carbonyl group have been used to study the ''flexibility gradient'' of membrane lipids to understand
membrane fluidity
In biology, membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane or a synthetic lipid membrane. Lipid packing can influence the fluidity of the membrane. Viscosity of the membrane can affect the rotation and diffusion ...
conditions at different depths of their lipid bilayer organization.
See also
*
Spin trapping
References
{{reflist
*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.
Electron paramagnetic resonance
Free radicals