A novel dual mode X-band EPR resonator for rapid in situ microwave heating.

Jaafar Harari,Michael Barter,Damien M Murphy,Andrea Folli,Emma Richards,Heungjae Choi,Daniel Slocombe,Adrian Porch

doi:10.1016/j.jmr.2019.106644

Jaafar Harari, Michael Barter + Show 6 more

Open Access

https://doi.org/10.1016/j.jmr.2019.106644

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Abstract

A unique dual mode X-band Continuous Wave (CW) EPR resonator designed for simultaneous EPR measurement and rapid microwave (MW) induced sample heating is described. Chemical reactions subjected to a flow of energy and matter can be perturbed away from the thermodynamic equilibrium by imposing a rapid shock or physical change to the system. Depending on the magnitude of the perturbation, these changes can dictate the subsequent evolution of the entire system, allowing for instance to populate non-equilibrium reactive intermediate states. Temperature jump (T-jump) experiments are a common method to achieve such perturbations. Most T-jump experiments are based on Joule Heating methods or IR lasers. Here we demonstrate the principle of rapid sample heating based on microwaves. The benefits of MW heating include (i) rapid and efficient heating (i.e. using a tuned resonant cavity, >99% efficient power transfer to the sample can be achieved), and (ii) volumetric heating (i.e. the entire sample volume rises in temperature at once, since heat is generated in the sample instead of being transferred to it). Accordingly, the key concept of the design is the use of a cavity resonator allowing EPR detection (at 9.5GHz) and simultaneous sample heating (at 6.1GHz). Temperature increments of 50°C within a few seconds are possible. This is evidenced and illustrated here by probing the temperature-induced variation of the rotational dynamics of 16-doxyl stearic acid methyl ester (16-DSE) spin probe grafted on the surface of sodium dodecyl sulphate (SDS) micelles in water, as well as copper (II) acetylacetonate in chloroform. Rapid changes in the rotational dynamics of the paramagnetic centres provide direct evidence for the in situ and simultaneous EPR measurement-heating capabilities of the resonator. Improvements afforded by the use of pulsed MW sources will enable faster heating time scales to be achieved. In the longer term, this current study demonstrates the simple and direct possibilities for using MW heating as a means of performing T-jump experiments.

Highlights

All chemical reactions subject to a flow of energy and matter can be perturbed away from thermodynamic equilibrium by imposing a rapid shock or physical change to the system
This is the essential basis of temperature jump (T-jump) experiments, and as a result, a considerable amount of research has been conducted over the years into new methods of generating homogeneous T-jumps, in the study of reaction kinetics or conformations in biophysical/biochemical [2,3,4,5,6,7] or chemical reaction systems [8]
We have described a novel dual-mode EPR resonator capable of performing standard X-band EPR measurements whilst simultaneously enabling the in situ dielectric volumetric heating of the sample using microwaves

Summary

Introduction

All chemical reactions subject to a flow of energy and matter can be perturbed away from thermodynamic equilibrium by imposing a rapid shock or physical change to the system. If the population of any intermediate in the reaction is increased during this process, this may be studied spectroscopically This is the essential basis of temperature jump (T-jump) experiments, and as a result, a considerable amount of research has been conducted over the years into new (and faster) methods of generating homogeneous T-jumps, in the study of reaction kinetics or conformations (i.e. protein folding dynamics) in biophysical/biochemical [2,3,4,5,6,7] or chemical reaction systems [8]. Owing to the above two major benefits offered by MWs, for rapid heating (potentially creating T-jump capabilities) and for enhancing the rates of some chemical reactions, we sought to develop a unique dual-mode EPR resonator enabling rapid in situ heating, in order to study chemical (catalytic) reactions in solution. We describe the basic principles of the resonator, designed for simultaneous heating and EPR experiments, and demonstrate how rapid heating can be achieved using a series of radicals and paramagnetic metal centres

Resonant cavity design

Helmoltz coils

Sample preparation for EPR

Results and discussion

Perspective for T-jump EPR experiments

Conclusions