Jeffrey Harmer

Electron Paramagnetic Resonance (EPR) spectroscopy in biological, medical, chemical and physical sciences.

My main research field is Electron Paramagnetic Resonance (EPR) spectroscopy, a technique that probes the interaction of unpaired electrons with their surroundings. Paramagnetic centres are intrinsic to many systems and materials, for example biomolecules may contain metal clusters (e.g. [2Fe-2S]), transition metals (e.g. Cu, Fe, Mn, Ni) or organic radicals. Paramagnetic centres can also be attached to specific points in diamagnetic materials, as for example with the MTLS molecule that contains a nitroxide radical which is extensively used in site-directed spin labelling of biomolecules. A powerful technique of modern EPR is dipolar spectroscopy which is utilised in structural studies of biomolecules, for example with soluble and membrane proteins and their oligomers, DNA and RNA. Here dipolar spectroscopy refers to the measurement of electron-electron couplings with techniques such as Double Electron-Electron Resonance (DEER) or synonymously pulsed electron double resonance (PELDOR), double-quantum EPR (DQ-EPR), and related EPR methods. These EPR techniques can very accurately measure the dipole interaction between unpaired electron spins which enables the distance between them and their relative orientation to be determined. Owing to the large magnetic moment of the electron, the technique delivers information in the distance range from ca. 15-80 Å. From a set of such measurements a structural model of the system under investigation can be developed. For example DEER studies deliver information on protein conformational changes on ligand binding, and enable the investigation of protein-protein complexes and oligomers in frozen solution. The standard paramagnetic spin-label for dipolar spectroscopy is MTLS which is covalently attached to a protein via a disulfide bond with a cysteine residue, although there are a number of other organic labels and a number employing Cu2+ and Gd3+ ions for example. Possibilities also exist to attach spin-labels via other amino acids. DNA and RNA studies are also readily amendable to dipolar spectroscopy technologies. My area of research encompasses the characterization of structure-function relationships of biomolecules and their complexes, which includes development of the methodologies to measure electron-electron couplings and distances, the development of improved data analysis algorithms, and the development of modelling the sparse set of EPR constrains into 3D structures (for example using rigid-body docking, molecular dynamic simulations, etc.). Unpaired electrons are also coupled to nearby nuclear spins (e.g. 1H, 14N, 13C, 31P) and these couplings provide information in the distance range ca. <10 Å from the unpaired electron(s). Structural and electronic information of the paramagnetic centre from experiments is obtained with multi-frequency continuous wave (CW) EPR, and multi-frequency pulse EPR techniques such as electron nuclear double resonance (ENDOR), electron spin-echo envelope spectroscopy (ESEEM), and hyperfine sublevel correlation spectroscopy (HYSCORE). The experimentally measured EPR couplings describe the samples electronic structure as they relate in a direct way to the spin density distribution and thus single occupied molecular orbital. EPR couplings allow for example the identification of the type of nucleus, provide a description of the coordination environment in metal complexes, in metalloenzyme locate a substrate bound too or near the active site, and enable the identification of organic radicals. To aid in the interpretation of the experimental data extensive use of quantum chemistry calculations is used to further characterise the system under investigation.