Advancing the Magnetic Resonance Frontiers for the Study of Complex Molecules and Active Materials
Nuclear Magnetic Resonance (NMR) is a spectroscopic method that provides atomically resolved structural and dynamical information of systems from a vast category including organic and inorganic chemistry, materials and biology. The technique offers unique capabilities to study complex molecules that are insoluble and non-crystalline such as amyloid fibrils, membrane proteins, amorphous polymers, catalytic compounds, and battery materials etc. A major challenge in the applications of solid-state NMR spectroscopy is the intrinsically low sensitivity of the technique that practically restricts its use to a limited number of NMR active isotopes such as 1H, 19F, 31P, 13C, and 15N. Therefore, a number of practically relevant systems that in principle can be studied using solid-state NMR, remain out of its reach.
I will present two approaches to address the problem of sensitivity in solid-state NMR. The first approach is development of new methods, i.e., radio frequency (RF) pulse sequences to probe NMR isotopes like 2H, 14N, 7Li etc. that are difficult due to their less sensitivity (low gyromagnetic ration and/or less natural abundance) and large anisotropic interaction like quadrupolar coupling and chemical shift anisotropy. A new RF pulse sequence and its applications to biological and material samples with various challenging NMR isotopes will be discussed, demonstrating the versatility of the new method and its impact.
In the second approach, I will discuss a rapidly emerging hyperpolarization technique known as dynamics nuclear polarization (DNP), which can enhance the sensitivity of NMR by orders of magnitude. DNP has already made a paradigm shift in solid-state NMR spectroscopy by enabling studies of systems like proteins in physiological conditions and materials with extremely insensitive NMR isotopes like 17O. However, the state-of-the-art DNP methods rely on exogenously added source of paramagnetic centers (typically stable nitroxide or carbon-based radicals) for the polarization. The scope of DNP can be further expanded by performing endogenous DNP using paramagnetic metal centers as the source of polarization to enhance sensitivity of the surrounding nuclear spins. Such metal centers are intrinsically present in systems like metalloproteins, metalloenzymes, energy harvesting materials and battery compounds. More importantly, endogenous DNP is a critical step towards performing “hyperfine DNP spectroscopy” to extract the local structural information directly from the electron-nuclear hyperfine interaction while detecting sensitivity enhanced NMR signal. Recent results using V4+ centers for the first time to enhance polarization of protons in the sample will be shown to illustrate the concept of hyperfine DNP spectroscopy.