Dynamic Nuclear Polarization enhanced NMR _for biomedical and biological applications:_current work at 10 GHz and perspectives for higher frequencies

Song-i Han

UCSB Chemistry

My research team is inventing methods that utilize the principle of dynamic nuclear polarization (DNP), which is a marriage between NMR—direct molecular fingerprint, non-invasive detection—and electron paramagnetic resonance (EPR)—high sensitivity and selectivity—. The 600 to 6000 fold higher magnetization of unpaired electrons, e.g. as part of spin-labels, can be transferred to neighboring, ¹H or ¹³C nuclei, whose NMR signals are then selectively enhanced. This effort is challenging because instrumentation and theories needs to be first developed, and important and approachable problems to be formulated, however, is well justified by the potential of DNP to revolutionize the molecular analysis of dynamic biomolecular assemblies in situ.

We developed a 0.35 Tesla (10 GHz) DNP instrument, and highly enhanced the ¹H NMR signal of water (up to 130 fold) via mobile spin labels. The most direct use of this hyper-polarized water was its introduction as a perfectly non-toxic and authentic contrast agent for magnetic resonance imaging (MRI) without the use of foreign tracers. We also introduced spin labels as sensor molecules into membranes and protein aggregation intermediates with the purpose of directly illuminating the sensor’s local environment. We were able to demonstrate on a well-defined micelle/vesicle system that the DNP-induced NMR signal enhancement of water reflects the water permeability at the location of the spin label, and thus decreases with increasing vesicle fraction with significant hydrophobic character. The important potential of our analysis tool is proven by the fact that such basic information already presents a novel and exciting insight. The application of related approaches to the in situ study of complex coacervates and aggregation intermediates of tau proteins is in progress. Common to these examples is that water exclusion is a key event. What I envision next is to illuminate not only water, but other molecular details of the spin label reporter’s local environment, which will shed light on molecular organization and packing of the supra-molecules. This requires introducing superconducting magnets with superior homogeneity at 0.35 Tesla (under construction), and the development of DNP at high magnetic fields (>7 Tesla). As the latter builds on yet to be developed high power, 200 GHz EPR technology, I pursue this challenge in collaboration with my colleagues at UCSB and the NHMFL utilizing UCSB’s FEL source.