Meeting the Sensitivity Grand Challenges in Pulsed Electron Magnetic Resonance

Summary

This instrument development project seeks to substantially and dramatically increase the sensitivity and time resolution and capability of electron paramagnetic resonance (EPR) spectrometers and to demonstrate a major impact across biology, chemistry, physics and materials science.

One of the fundamental quantum mechanical rules governing the basic structure and organisation of matter, is that electrons like to pair up. However, in many materials there are unpaired electrons left over from this pairing process. Such systems are known as paramagnetic and examples include radicals, many types of metal atoms, and defects in crystals. The reactivity of any given unpaired electron strongly depends on its local atomic environment. Some radicals are so reactive that they are able to tear electrons from any nearby molecules and initiate a destructive cascade of reactions. Indeed, it is the accumulated damage from such free radicals within the body that is believed to underlie our aging process, despite the body evolving many defense mechanisms. Measurements of free radicals in the blood can be health indicators. Other paramagnets can be relatively stable and highly beneficial. Transient paramagnetic species are involved in closely regulated reactions in huge numbers of biological processes. Much of the UK's chemical industry depends on the use of radicals and transition metals to initiate and promote catalytic reactions. Paramagnetic defects in crystals, thin films or at interfaces can determine or strongly affect a material's electronic, magnetic, optical, chemical and mechanical properties and are hugely important in the UK's material science and electronics industries. The sensitivity of NMR or MRI experiments can be dramatically increased by making electron spins interact with local nuclei.

Even in systems where there are no naturally occurring unpaired electrons, molecular biologists have developed ways to routinely add free radical (electron) spin labels at specific sites within biomolecules, which can be used as "molecular spies" to understand reactions, interactions, large-scale structure and fast dynamics with a precision not possible with other techniques. Characterisation of such structures and processes can underpin the understanding of the mechanisms behind disease and the development of new drugs.

The most important tool in studying and understanding these systems is pulsed electron paramagnetic resonance. This technique involves placing a paramagnetic sample in a large magnetic field and illuminating it with a carefully controlled sequence of rapid high power microwave pulses and monitoring the response of the sample. Until relatively recently, it was widely believed there was little scope to significantly improve the sensitivity of pulsed EPR instruments. Yet ten years ago we demonstrated a significant increase by a factor of between 15 and 30 in concentration sensitivity for common measurements. Today, commercial instruments have nearly but still not caught up. This project now seeks to further increase sensitivity, by another factor of 30. This increase will be achieved by taking advantage of recent advances in fast electronics and by modifying an existing state-of-the-art system using techniques that we have already demonstrated in many proof-of-principle experiments. This would be a major advance, particularly for molecular biology, as for the first time it would allow spin-labeled protein systems to be investigated at natural (in-cell) protein concentrations using electron magnetic resonance. There are also many important electronic, materials and catalytic systems, which involve paramagnetic centres in thin films or at interfaces where sensitivity is paramount.

To maximise the impact of the instrument development, the project is linked to a large number of applications and methodology development programmes, with a wide range of local collaborators and co-investigators.

Impact Summary

By making a breakthrough in sensitivity and time resolution we aim to transform pulsed EPR and optimise pulsed DNP strategies.

One of the grand challenges in molecular biology is to develop techniques to understand the structure and interactions of large biomolecules, supramolecular assemblies and metallo-proteins, at physiological protein concentrations, both in-vitro and in the cell environment. A major challenge in the development of new electronic materials and devices is the identification of paramagnetic defects at critical interfaces or within thin film systems, as these frequently determine a device's performance. A similar challenge is related to understanding catalytic processes on surfaces or electrodes. A challenge in more highly concentrated systems is the accurate measurement of very fast relaxation and correlation times to gain an insight into electron or molecular dynamics. A major challenge in DNP is how to speed up the enhancement in solid state and dramatically increase sample volumes for liquid (aqueous) state DNP.

This proposal seeks to improve the sensitivity and time resolution of EPR measurements to a point where these challenges can now be met. Spin label techniques are commonly used to understand molecular dynamics, and to accurately determine long (and short) range structure and spatial distribution of biomolecules. If spin label measurements can be routinely made in biomolecules at physiological protein concentrations (10 -1000 nM) this would mark a major change in performance for pulsed EPR measurements. It would make EPR a powerful characterisation tool for the many complex, large, biomolecular machines that are only available to low concentrations, either because systems cannot be efficiently expressed or because systems suffer from severe aggregation effects. It would pave the way for in-cell EPR measurements at physiological concentrations.

EPR is also used to understand the catalytic/enzymatic and other properties of metal ions, nanoclusters, radicals and metallo-organic frameworks as well as to determine intermediate reactions in complex chemical processes. It is used to study electron and/or ion transport in solar cells, battery materials, and other electronic devices and determine the effect of paramagnetic defects on the physical, electronic, optical and chemical properties of new materials (e.g. oxide and organic electronics). It is used to measure oxidative and nitrosative stress in blood. It is a common characterisation technique in studies of molecular magnetism, and is currently one of the more promising avenues of research into quantum computing. EPR techniques are also used in dynamic nuclear polarisation (DNP) to dramatically improve the sensitivity of NMR measurements. In many cases, it would allow defects and paramagnetic intermediates to be identified at interfaces and in thin films, which is important both for new electronics industries and for understanding catalytic processes. It will allow coherent pulsed EPR techniques to be evaluated for both liquid and solid-state DNP and for critical EPR parameters to be evaluated in-situ. These advances will impact across physics, biology, chemistry, healthcare and materials science.

A primary beneficiary of the technical development will be our commercial partner Thomas Keating Ltd. who were also the commercial partner on the previous Basic Technology HIPER project. Thomas Keating Ltd is arguably the world's leading supplier of mm-wave quasi-optical instrumentation. After the previous grant (HIPER) Thomas Keating sold multiple EPR and DNP sub-systems to groups around the world. They also supplied the major optics for contracts with two major US National laboratories. Many of the components developed under HIPER also contributed to sales in other fields associated with mm-wave technology including instrumentation for astronomy, metrology, earth resource studies and security applications.