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National Biomedical Center
for Advanced Electron Spin Resonance Technology

Our research is supported by a grant from the National Institute of General Medical Sciences (NIGMS), part of the National Institutes of Health.

  Core R & D at ACERT  
      HFHF ESR: continuous-wave

The extension of ESR to sub-terahertz frequencies (the millimeter wavelength-end of the far-infrared region), has been one of the most important instrumental advancements at ACERT:

  • The primary advantage of going to sub-THz is increased signal-to-noise ratio (SNR) and improved spectral resolution in many cases.
  • The second, higher frequency ESR acts as a faster "snapshot" of the motional dynamics. That is, for a given diffusional rate the spin label motion appears to become slower as one utilizes higher frequencies. For the same motional rate, at low or conventional frequencies (e.g. 9 GHz) one may observe motionally narrowed spectra, whereas at high frequencies (e.g. 250 GHz) the spectra may display very slow motion features, almost the rigid limit character. This "snapshot" feature enables a multi-frequency ESR approach to study the complex modes of motion of proteins, DNA, and other polymers, which leads to decomposition of the motion into modes according to their different time scales
  • The third feature is the increase of the orientational resolution of the nitroxide spectrum, due to the dominant role of the g-tensor, as the ESR frequency increases. For rigid limit spectra at 250 GHz, one can clearly distinguish the well-separated spectral regions corresponding to those nitroxide spin labels with their x-axes parallel to B0, their y-axes parallel to B0, and their z-axes parallel to B0. Then as motion is introduced (e.g. by warming the sample) one can discern the axis (or axes) about which the motion occurs. Because of this enhanced resolution, the 250 GHz slow-motional spectra are much more sensitive to the details of the motional dynamics than are those at microwave frequencies.

Our general aim in this area is to continue the development of quasioptical techniques for specific instrumentation needs of high-field-high-frequency ESR (HFHF-ESR). The demonstrated benefits of a multifrequency approach to ESR for unravelling the complicated dynamics in biological systems  provides the impetus for upgrading and improving the sensitivity of ACERT existing spectrometers of 95, 170 and 250 GHz. Our extensive accomplishments in HFHF-ESR summarized in seminal publications are indicative both of the commitment and success of our group in exploiting the rich interplay between quasioptical techniques and ESR instrumentation needs. In order to further develop HFHF-ESR for studies on biological systems, we proposed four specific areas of development that are ideally tailored to our strengths and resources, and which significantly improve the applicability of HFHF-ESR.

The ACERT‘s specific goals are:

  • Improve the achievable signal to noise for samples with broad lines in our HFHF-ESR spectrometers through the development of new spectral coding techniques, e.g. circular dichroism ESR. This will be particularly important for the study of metallo-proteins, and for nitroxide spin labels in the slow motional regime, which is common for spin labeled biomolecules.
  • Improve the performance of the resonant structures used in HFHF-ESR resonators and sample holders by reducing the insertion loss and increasing the flexibility of coupling, as well as reducing the effective cavity volume VC to enhance the achievable B1 at the sample. These developments will greatly facilitate studies of protein and DNA dynamics, as well as studies of molecular dynamics in membranes. These developments will also be useful for the planned improvements to our 95 GHz pulsed spectrometer.
  • Develop the necessary quasioptical instrumentation for testing the performance of novel probeheads and other quasioptical circuits that will be needed for implementing the first two specific aims of this project, as well as the low Q resonators necessary for pulsed HFHF-ESR.
  • Develop and build on our experience with cryogenic goniometers for HFHF-ESR in order to have multi-axis cryogenic capability. Our goal here is to facilitate single crystal studies of, e.g., metallo-enzymes.

Highlights of recent HFHF ESR: continuous-wave developments.