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Review
. 2014 Oct 23;19(10):16998-7025.
doi: 10.3390/molecules191016998.

New developments in spin labels for pulsed dipolar EPR

Alistair J Fielding  1 , Maria Grazia Concilio  2 , Graham Heaven  2 , Michael A Hollas  2
Affiliations
Review

New developments in spin labels for pulsed dipolar EPR

Alistair J Fielding et al. Molecules. .

Abstract

Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique and has certain advantages over X-ray crystallography; which requires high quality crystals. The technique relies on the design of binding probes that target a functional group, for example, the thiol group of a cysteine residue within a protein. The unpaired electron is typically supplied through a nitroxide radical and sterically shielded to preserve stability. Pulsed electron paramagnetic resonance (EPR) techniques allow small magnetic couplings to be measured (e.g., <50 MHz) providing information on single label probes or the dipolar coupling between multiple labels. In particular, distances between spin labels pairs can be derived which has led to many protein/enzymes and nucleotides being studied. Here, we summarise recent examples of spin labels used for pulse EPR that serve to illustrate the contribution of chemistry to advancing discoveries in this field.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Structures of radical spin labels commonly used for SDSL of proteins.
Figure 2
Figure 2
Spin labelled RNA bases.
Scheme 1
Scheme 1
Spin labelling of uracil (12).
Scheme 2
Scheme 2
Spin labelling of the 2' position on the sugar backbone of RNA.
Scheme 3
Scheme 3
Spin labelling of phosphate backbone of RNA.
Figure 3
Figure 3
Parent structures of nitroxides; (a) a piperidine, (b) a pyrroline, (c) an isoindoline, (d) an imidazoline.
Figure 4
Figure 4
Structures of common nitroxide linkers.
Figure 5
Figure 5
tructures of nitroxides with various steric groups on the 2, 6 positions.
Figure 6
Figure 6
Structures of pyrrolines with rigid substituents on the 4 position.
Figure 7
Figure 7
A tetrathiatriarylmethyl radical 41.
Figure 8
Figure 8
A cysteine specific TAM spin label CT02-TP 42.
Figure 9
Figure 9
(a) TAM-labelled oligonucleotide 43, (b) structure of the doubly-labelled nucleic acid duplex.
Figure 10
Figure 10
Chemical structure of the bis-labelled peptide TPP-(Ala-Aib)4-Ala-TOAC-Ala-(Aib-Ala)2-OH 44.
Figure 11
Figure 11
Gadolinium (III) spin labels DOTA chelate 45 and C1 46 with phenylethylamine substituents.
Figure 12
Figure 12
The Gd(III) spin label 47; chelated by maleimido mono amide 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), which can attach to the thiol group of cysteine residues.
Figure 13
Figure 13
Paramagnetic unnatural amino acids 48 and 49.
Figure 14
Figure 14
Spin labelling of unnatural amino acids (a) p-acetyl phenylalanine 52 and (b) p-azidophenylalanine 55.
Figure 15
Figure 15
Spin labels used for in-cell DEER. The cysteine binding 3-maleimido-proxyl 9, and nucleic acid base binding labels TEMPA 57 and TPA 11.
Figure 16
Figure 16
Histidine tag spin label proxyl-trisNTA 58. X = histidine coordination site.
See this image and copyright information in PMC

References

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    1. Bowman M.K. Pulsed Electron Paramagnetic Resonance. In: Brustolon M., Giamello E., editors. Electron Paramagnetic Resonance: A Practitioner’s Toolkit. John Wiley & Sons, Inc.; Hoboken, NJ, USA: 2009. pp. 159–194.
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