This
work demonstrates the feasibility of making sensitive nanometer
distance measurements between Fe(III) heme centers and nitroxide spin
labels in proteins using the double electron–electron resonance
(DEER) pulsed EPR technique at 94 GHz. Techniques to measure accurately
long distances in many classes of heme proteins using DEER are currently
strongly limited by sensitivity. In this paper we demonstrate sensitivity
gains of more than 30 times compared with previous lower frequency
(X-band) DEER measurements on both human neuroglobin and sperm whale
myoglobin. This is achieved by taking advantage of recent instrumental
advances, employing wideband excitation techniques based on composite
pulses and exploiting more favorable relaxation properties of low-spin
Fe(III) in high magnetic fields. This gain in sensitivity potentially
allows the DEER technique to be routinely used as a sensitive probe
of structure and conformation in the large number of heme and many
other metalloproteins.
The EU FP7 project CONSORTIS (Concealed Object Stand-Off Real-Time Imaging for Security) is developing a demonstrator system for next generation airport security screening which will combine passive and active submillimeter wave imaging sensors. We report on the development of the 340 GHz 3D imaging radar which achieves high volumetric resolution over a wide field of view with high dynamic range and a high frame rate. A sparse array of 16 radar transceivers is coupled with high speed mechanical beam scanning to achieve a field of view of ~ 1 x 1 x 1 m 3 and a 10 Hz frame rate.
The need for improved security at airports with high detection performance, high throughput rates and an improved passenger experience is motivating research into new sensing technologies. The European Union funded CONSORTIS project is addressing these aims by demonstrating a system which combines a submillimeter wave radar, a dual-band passive submillimeter wave camera and automatic anomaly detection software for reliable detection while ensuring passenger privacy. In this paper we describe the 340 GHz 16-channel FMCW radar which produces 3D maps of the subject with ~1 cm 3 voxel resolution over a 1 m 3 sense volume at multi-hertz frame rates. The radar combines advanced transceiver electronics with high speed mechanical beam steering and parallelized processing to achieve this level of performance. I.
The sensitivity of pulsed electron paramagnetic resonance (EPR) measurements on broad-line paramagnetic centers is often limited by the available excitation bandwidth. One way to increase excitation bandwidth is through the use of chirp or composite pulses. However, performance can be limited by cavity or detection bandwidth, which in commercial systems is typically 100-200MHz. Here we demonstrate in a 94GHz spectrometer, with >800MHz system bandwidth, an increase in signal and modulation depth in a 4-pulse DEER experiment through use of composite rather than rectangular π pulses. We show that this leads to an increase in sensitivity by a factor of 3, in line with theoretical predictions, although gains are more limited in nitroxide-nitroxide DEER measurements.
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