Electron Paramagnetic Resonance is a powerful technique for studying the local environment near a paramagnetic species. When coupled with the ability to introduce stable paramagnetic labels in a variety of biological systems, EPR becomes an important tool to unravel questions of structure and dynamics in biological systems. Because these paramagnetic species are introduced site specifically, EPR offers unusual spatial precision to address specific questions that evade other techniques. As with other kinds of magnetic resonance, EPR becomes more powerful at higher magnetic fields. However, in EPR expansion to high fields has been slowed by technical challenges of working at hundreds of GHz. This work expands the capabilities of high-field EPR through dual approaches. The first approach is to develop new methods for biological studies exclusive to high magnetic fields.

For instance, a distance measurement technique is developed based on the temperature dependence of electron-spin phase memory times at high fields. Further, \Gd is demonstrated as a spin-label exceptionally well-suited for use at high magnetic fields. In particular, cw EPR with \Gd allows measurements of interspin distances up to 4 nm under less severe conditions than are typically necessary for pulsed EPR distance measurements. Additionally, pulsed EPR distance measurements with Gd3+ are used to elucidate the oligomeric structure of a membrane protein. The results strengthen the case for Gd3+ as a particularly useful probe for targeting complex, oligomeric systems, which tend to be difficult to study in other ways. The second approach is to eliminate the power restrictions of high-field, pulsed EPR through the use of UCSB's Free Electron Lasers as a radiation source to enable the highest power, pulsed EPR at frequencies over 100 GHz.

The ability to manipulate and measure spins 50-100x faster than with other sources is introduced and the technical approach described. The intrinsic phase instability of the FEL source can then be eliminated through a post-processing routine, which recovers the capability for phase cycling using the FEL. As phase cycling can be used to dramatically reduce artifacts, and is a common technique in both EPR and NMR, this greatly expands the detection capabilities of the spectrometer. Together these approaches have allowed new capabilities for studying biological systems, particularly the ability to measure more complex systems, and closer to physiological conditions than otherwise possible. Beyond this, it is hoped these developments continue to spur efforts to realize mature, high-field EPR techniques and technology.