Resonator Approach to Pulsed Dynamic Nuclear Polarization of Membrane Proteins

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Nuclear Magnetic Resonance (NMR) is an exceptionally versatile and informative spectroscopic technique for atomic-level structure-function studies of biological macromolecules in their native-like environments. In particular, solid-state NMR allows one to study membrane proteins in lipid bilayers under the conditions approaching those encountered in the biological cells. Membrane proteins are of particular interest for biomedicine being implicated in numerous biological processes and diseases and constituting nearly 50% of the modern drug targets. However, low polarization of the nuclear spins limits NMR sensitivity and represents the major roadblock for expanding its use in structural biology. Dynamic nuclear polarization (DNP) can potentially boost sensitivity of NMR by up to several hundred times via irradiating the sample with mm-waves at matching frequencies. Despite significant progress, DNP NMR of biological samples above the freezing temperatures remains to be a challenge mainly because of short relaxation times of the nuclear and electron spins at higher temperatures and excessive sample heating by mm-waves. We propose to overcome these fundamental problems by constructing a novel 200 GHz/300 MHz DNP spectrometer which will be based on resonant mm-wave structures and will operate in a pulse mode for DNP transfer vs. the continuous mode currently in use. The key innovation is our recently invented mm-wave photonic band-gap resonators which increase the sample volume by approximately 1-2 orders of magnitude as compared to the existing resonator cavity designs. We propose to increase the quality factors of such resonators from Q=200 as demonstrated for the prototype to at least Q=1,000 in order to boost mm-wave field at the sample. Achieving these higher mm- wave fields will be essential for enabling advanced pulse schemes for DNP that will provide maximum NMR signal enhancements while minimizing sample heating. The spectrometer development will be guided by computer simulations of mm-wave fields and pulse DNP sequences, and will be based on the existing low- power prototype operating in a continuous DNP mode yielding record-breaking preliminary data obtained at room temperature. The spectrometer will operate over a broad temperature range (100-330 K), and multi- resonance probeheads will be optimized for hydrated biological samples above the freezing point. The new DNP technology will be applied to a series of biological samples including hydrated membrane proteins aligned by nanoporous substrates. Success of the project will be built upon the extensive expertise of the two collaborating PIs (Nevzorov and Smirnov) in designing and constructing a room temperature DNP NMR spectrometer prototype based on solid-state mm-wave components. The new pulsed DNP spectrometer will open up unexplored perspectives with regard to developing novel pulse methodologies for DNP-enhanced solid-state NMR of membrane proteins. This is a high-gain high-risk project where the risk is leveraged by the extensive experience of the investigators and the highly encouraging preliminary results. 1