This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Pulse techniques at 95GHz and higher have been constrained by technical limitations in the recent past to bandwidth limited pulses of 50ns and longer. While these selective pulses are useful for performing a variety of low temperature experiments in the solid state, they are not suitable for studying dynamics in fluid systems at physiological temperatures, due to the rapid relaxation that characterizes these systems. We have, therefore, built a new, high-power, pulsed 95GHz spectrometer based on a 1kW extended interaction klystron amplifier that overcomes the limitations of conventional 95GHz pulsed spectrometers. The spectrometer uses a dedicated 6T magnet allowing it to be operated independently. We have achieved to date pi pulses of 5ns, full phase cycling capability for the suppression of unwanted coherences, 150MHz coverage and dead times of 30ns in favorable cases. The ability to perform 2D-FT-ESR experiments within the context of HFHF-ESR can be extremely useful, particularly due to the demonstrated enhanced sensitivity to details of dynamics compared to ESR at conventional frequencies. This is particularly important for studying the dynamic structure of lipid membranes. At conventional ESR frequencies, the details of the ordering tensors are ambiguous due to the lower orientational resolution of conventional ESR. We have successfully performed a variety of time-domain experiments on aqueous samples and oriented lipid membrane samples in order to exploit the enhanced resolution of our spectrometer to dynamic structure in these heterogeneous systems. The in-house development of a novel Fabry-P?rot resonator with continuously variable coupling was crucial for the success of these experiments. We are currently extending our studies to oriented lipid membranes deep in the slow-motional regime. Although the currently available spectral coverage of our short pulses is not optimum for exploring all of the details of the dynamics, nevertheless, we have successfully recorded 2D-ELDOR spectra with a spectral extent of approximately 500MHz. We are also making substantial improvements to the timing system that are nearly complete and which will be crucial for building on the success of our initial slow-motional 2D-ELDOR experiments.
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