Research Guides

The E680 pulsed Electron Spin Resonance spectrometer (X-band 9.5 GHz and W-band 95 GHz) located in the Centre for Advanced Electron Spin Resonance at Oxford University.

Pulsed Electron Spin Resonance (ESR) is presently undergoing an unprecedented methodological revolution; as Arbitrary Waveform Generators (AWGs) are becoming commercially available. AWGs can produce shaped pulses (arbitrary waveforms) at the microwave frequencies (GHz) and time frames (ns) required for ESR. My research is based in the Centre for Advanced Electron Spin Resonance (CAESR), as shown in the picture on the right, which is located in the Inorganic Chemistry Laboratory. I aim to develop the use of AWGs in pulsed ESR methodology with a focus on the investigation of proteins and protein complexes.

ESR is an important tool for studying large, complex systems, such as proteins. It relies on the presence of unpaired electrons (electronic spin centres) and, by using different experiments, the coupling between different electronic spin centres or between an electronic spin centre and the surrounding spin active nuclei can be probed. This provides information about long range structure or local environment of the spin centre, respectively. The presence of only one or two electronic spin centres in a molecular structure often makes data interpretation somewhat more straightforward than for other spectroscopic techniques (such as NMR). Furthermore, for large biological molecules, ESR has an additional advantage over crystallographic techniques in that the data is recorded for samples in frozen solution negating the need for crystallisation of the protein or protein complexes.

In protein systems electronic spin centres can be added via site directed mutagenesis and spin labelling, using spin active moieties such as nitroxides, NO•, (e.g. methanesulfonothioate spin label; MTSL) or trityl, that are chemically bound to selected amino acids (usually cysteine). Alternatively, electronic spin centres may take the form of a metal centre or metal based cluster with unpaired electrons. These can either be bound into inherent binding sites within the protein (e.g. Iron Sulphur clusters) or an artificial binding site can be introduced in the form of a binding loop formed of several amino acids or a binding tag associated with single amino acid. The ESR spectra of these different moieties differ in form, as coupling of the electron spin to nuclei (hyperfine coupling) and/or g-tensor anisotropy broadens the ESR spectrum.

In the absence of an AWG only rectangular time-domain pulses can be generated, yielding sinc-shaped frequency responses. For these pulses the width of the frequency bandwidth excited by the pulse is limited by the pulse length, and it is generally not possible to excite the entire width of a NO• spectrum with these pulses. AWG units afford complete control of pulse shapes and phases used in experiments and thus the frequency profiles of AWG generated pulses can be carefully engineered to excite a specific bandwidth, corresponding to either full spectral excitation (e.g. NO• spectra are ca. 0.2 GHz broad at X-band; 9 GHz) or excitation of a well-defined region. These technologies present many exciting opportunities for improving and expanding upon existing methodologies.

• A Structural Model of a P450-Ferredoxin Complex from Orientation-Selective Double Electron–Electron Resonance Spectroscopy

A.M. Bowen, E.O.D. Johnson, F. Mercuri, N.J. Hoskins, R. Qiao, J.S.O. McCullagh, J.E. Lovett, S.G. Bell, W. Zhou, C.R. Timmel, L.L. Wong, J.R. Harmer.  J. Am. Chem. Soc., 2018, vol. 140 (7), pp 2514–2527.

DOI: 10.1021/jacs.7b11056

• Orientationally selective DEER measurements between two copper centres in a protein and model systems.

A.M. Bowen,* M.W. Jones,* J.E. Lovett, T.G. Gaule, M.J. McPherson, J.R. Dilworth, C.R. Timmel, J.R. Harmer. Phys. Chem. Chem. Phys., 2016, vol. 10, pp. 5981-5994.

DOI: 10.1039/c5cp06096f

• Engineering coherent interactions in molecular nanomagnet dimers.  

A.Ardavan, A.M. Bowen, A. Fernandez, A.J. Fielding, D. Kaminski, F.Moro, C.A. Muryn, M.D. Wise, A. Ruggi, E.J.L. McInnes, K. Severin, G.A. Timco, C.R. Timmel, F. Tuna, G.F.S. Whitehead, R.E.P. Winpenny. npj Quantum Information, 2015, vol. 1, pp. 15012.

DOI: 10.1038/npjqi.2015.12

arXiv:1510.01694

• Orientation-selective DEER using rigid spin labels, cofactors, metals, and clusters.

A.M. Bowen, C.E. Tait, C.R. Timmel, and J.R. Harmer. Struct. Bond., 2013, pp. 152.

DOI: 10.1007/430_2013_115

Print ISBN: 978-3-642-39124-8

• Characterisation of the paramagnetic [2Fe-2S] centre in Palustrisredoxin-B (PuxB) from Rhodopseudomonas palustris CGA009: magnetic moment and g-matrix determination.

J.A.B. Abdalla, A.M. Bowen, S.G. Bell, L.L. Wong, C.R. Timmel, J.Harmer. Phys. Chem. Chem. Phys., 2012, vol.14, pp. 6526–6537.

DOI: 10.1039/C2CP24112A

• Structural information from orientationally selective DEER spectroscopy

J.E. Lovett,* A.M. Bowen,* C.R. Timmel, M.W. Jones, J.R. Dilworth, D. Caprotti, S.G. Bell, L.L. Wong, J. Harmer. Phys. Chem. Chem. Phys., 2009, vol. 11, pp. 6840–6848.

DOI: 10.1039/B907010A