High-field DNP for the study of materials and interfaces: DNP is a powerful technique that has gained prominence in the magnetic resonance community for its significant NMR signal enhancements by several orders of magnitude. In the past few years, high field DNP has made great progress toward general applicability. Our group focuses on developing DNP instrumentation and methods geared towards materials characterization with site- and surface- or interfacial- specificity. We believe that solid state DNP-amplified NMR has great potential for the detailed study of the microstructure and heterogeneities of materials, especially for detecting dilute phenomena at domain boundaries, surfaces, and interfaces of multi domain polymers or heterogeneous inorganic solids, which is the focus of our group’s research. We have developed a state of the art experimental setup for both DNP and EPR detection at 7 Tesla using a quasi-optical bridge for propagation of the 200 GHz beam. Typical operation temperatures are between 4 and 30 Kelvin, where high nuclear spin polarization of few to tens of % is achieved. The quasi-optical bridge allows the polarization of the microwave beam to be changed from linear to circular, which gains effective microwave power by a factor of two for DNP operation. The current focus is on the mechanistic understanding of solid state DNP below 20 Kelvin, as well as the development of applications for the study of materials microstructure.
We have developed Overhauser DNP as a novel tool for the site-specific quantification of hydration water diffusion dynamics on biomolecular surfaces, including (membrane) proteins, lipid membranes and nucleic acids. Applications of interest of ODNP and EPR spectroscopy have been described under the section chemical systems of interest to the Han lab.
The Overhauser DNP instrumental capability is built in by extending a commercial EPR bridge with higher power amplifies and a circulator, and by implementing home-built DNP-NMR probes inside a high Q EPR cavity. This is now a routine operation in the Han lab, and the same capability has been added to the shared facility of the Materials Research Laboratory for access to all UCSB researchers on a recharge basis. We have developed customized hardware and software that permit automated measurement of hydration dynamics via Overhauser DNP, so that undergraduate or visiting researchers with short training periods can carry out ODNP measurements.
- John Franck,
- Chiyuan Cheng,
- Jinsuk Song,
- Kuo-Ying Huang,
- Sunyia Hussain,
- Neil Eschmann,
- Yasar Akdogan
A pulsed and cw EPR spectrometer operating at 240 GHz is developed by and located in the lab of Mark Sherwin (Physics, UCSB). The Han lab is collaborating with the Sherwin lab on various aspects of the development of techniques and methods, but especially focuses on the development of new biophysics and biochemistry applications using this unique instrumentation. This state of the art high field pulsed EPR spectrometer operates in dual mode, one powered by a low power (~50 mW), solid state, source, and the other by a high power (>300 W), UCSB Free Electron Laser, source. The latter is the only FEL-powered EPR spectrometer to date, whose first operation has been reported in a 2012 Nature article. A similar pulsed EPR spectrometer operating at 200 GHz is being built in the Han lab, to be used in conjunction with the 200 GHz DNP instrument. One important focus is the development of Gd3+-chelate based novel spin probes (in collaboration with D. Goldfarb, Weizmann Institute, and M. Sherwin, UCSB) to carry out distance measurements at closer to physiological temperatures and at longer nanometer scale distances. Gd3+-based spin probes reveal extraordinarily beneficial features, in particular at high magnetic fields above 3 Tesla.
We developed an EPR spectrometer featuring an arbitrary waveform generator (AWG) operating at 8–10 GHz (X-band) that is based on a 1 GHz digital-to-analog converter (DAC) board with a 42 dB (i.e. 14-bit) dynamic range. With this AWG-capable spectrometer, we pursue to widen the scope of pulsed EPR and enable new experiments. It generates shaped X-band pulses with precise amplitude and phase control and can specify inter-pulse delays with a time resolution of ≤250 ps. Among the demonstrated capabilities of the spectrometer include spin-echo measurements that implement an entirely digitally controlled and calibrated 16-step phase cycle and by measuring the excitation profiles seen by the spins in the microwave resonator as they respond to various pulse shapes, including rectangular, triangular, Gaussian, sinc, and adiabatic rapid passage waveforms. Another application is the deadtime reduction by custom designed pulse shapes. Potential applications of these capabilities, and their implementation in commercial instrumentation, are being pursued.