HFSS Modeling in Aqueous Biological Samples for EPR

Medical College Of Wisconsin, Milwaukee WI

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Abstract

DESCRIPTION (provided by applicant): This proposal is device-design driven. Two of the aims focus on development of novel sample resonators for electron paramagnetic resonance (EPR) spectroscopy that provides substantially higher signal-to-noise ratios (SNR) than those currently used. EPR resonators are designed to enhance dynamic molecular structure determination studies using nitroxide radical spin labels at physiological temperatures. The third aim focuses on development of a novel bimodal resonator for nuclear magnetic resonance (NMR) signal enhancement by dynamic nuclear polarization (DNP). The goal of Aim 3 is to open up new opportunities in high-resolution NMR. This first competitive renewal proposal is very strongly based on progress in the initial funding period. The methodology utilizes finite-element modeling of electromagnetic fields for resonator design and both electric discharge machining (EDM) and laser milling for fabrication. Aim 1 proposes development of a second generation loop-gap resonator (LGR) at X-band (10 GHz) to replace the one that has been in widespread usage for site-directed spin labeling (SDSL) for over 20 years. The discovery of the Uniform Field (UF) LGR in the previous funding period is the primary technological driver for this project. Another driver is the long-slot iris, which enables direct coupling to a waveguide replacing the previous coaxial-coupler configuration. The goal of this aim is increase of SNR by a factor of 5. Aim 2 proposes to develop a UF TE011 cavity resonator tailored to optimize concentration-sensitivity at Q-band (35 GHz). This is a novel design objective at Q-band. In addition to UF cavity technology, experience in custom fabrication of polytetrafluoroethylene (PTFE) extruded sample cuvettes is a technology driver. Resonators will be tailored for a specific extrusion utilizing as much as 10 <l of sample. A sub-aim will explore an additional opportunity for enhanced concentration-sensitivity utilizing as much as 60 <l of aqueous sample. Aim 3 is part of an international collaboration with Dr. Thomas Prisner of Frankfurt University, Germany, to extend liquid phase DNP technology to 260 GHz for the microwave pump frequency and 400 MHz for the NMR frequency. This is an ambitious high-risk, high-payoff aim. The proposed bimodal resonator is a cavity for the microwaves and an LGR for the radiofrequency. Considerable analysis has already been carried out using finite-element modeling, but much remains to be done, including development of precision fabrication methods. In EPR there is great excitement in the use of site-specific mutagenesis to introduce spin labels to proteins as a way, perhaps unique, to obtain dynamic structural information on a time scale that is relevant to function. The overall goal of the DNP project is improved NMR that could impact many of the nearly 1,000 NIH grants that include NMR as a keyword.