Study of Stable L-Alanine Radicals by W-Band EPR Spectroscopy

Stable L-alanine radicals, SAR1 and SAR2, induced by γ-irradiation of the L-alanine crystal have been investigated by electron paramagnetic resonance (EPR) technique at W-band (94 GHz) frequency. The study provides assignment of radical centers detected by continuous-wave EPR, saturation transfer mode and echo-detected field-swept EPR at W-band frequencies. The phase memory time, T _m, which was measured simultaneously at X-band (9.5 GHz) and W-band frequencies for different spectral components has been employed to estimate rotation correlation times of CH₃ protons and an effective correlation time related to the local dynamics of the entire SAR1 center at room temperature.     

A Rapid Calibration Technique for Scanning Line-Structured Laser Sensor

A novel procedure to calibrate the scanning line-structured laser sensor is presented. A drone composed of two orthogonal planes is designed, with the result that camera parameters and light-plane equation parameters is achieved simultaneously.     

Rapid frequency scan EPR

In rapid frequency scan EPR with triangular scans, sufficient time must be allowed to insure that the magnetization in the x, y plane decays to baseline at the end of the scan, which typically is about 5T(2) after the spins are excited. To permit relaxation of signals excited toward the extremes of the scan the total scan time required may be much longer than 5T(2). However, with periodic, saw-tooth excitation, the slow-scan EPR spectrum can be recovered by Fourier deconvolution of data recorded with a total scan period of 5T(2), even if some spins are excited later in the scan. This scan time is similar to polyphase excitation methods. The peak power required for either polyphase excitation or rapid frequency scans is substantially smaller than for pulsed EPR. The use of an arbitrary waveform generator (AWG) and cross loop resonator facilitated implementation of the rapid frequency scan experiments reported here. The use of constant continuous low B(1), periodic excitation waveform, and constant external magnetic field is similar to polyphase excitation, but could be implemented without the AWG that is required for polyphase excitation.     

A W-Band Frequency Tracing Transceiver System

In millimeter-wave, phase-locked oscillators are often used because the frequency drift of oscillators is very large. However, they are very complex. In this paper, a W-band frequency tracing transceiver system is presented and tested, which is used for digital communication. Although the communication distance is less than that of phase-locked systems, the scheme is simple and the cost is low.     

Design,Implementation, Simulation,and Visualization of a Highly Efficient RIM Microfluidic Mixer for Rapid Freeze-Quench of Biological Samples

Abstract  

Rapid freeze-quench (RFQ) trapping of short-lived reaction intermediates for spectroscopic study plays an important role in the characterization of biological reactions. Recently, there has been considerable effort to achieve sub-millisecond reaction deadtimes. We present here a new, robust, high-velocity microfluidic mixer that enables such rapid freeze-quenching. It is a based on the mixing method of two impinging jets commonly used in reaction injection molding of plastics. This method achieves efficient mixing by inducing chaotic flow at relatively low Reynolds numbers (Re = 140). We present the first mathematical simulation and microscopic visualization of mixing in such RFQ micromixers, the results of which show that the impinging solutions efficiently mix within the mixing chamber. These tests, along with a practical demonstration in an RFQ setup that involves copper wheels, show that this new mixer can in practice provide reaction deadtimes as low as 100 μs.     

A program for simulation of powder-type EPR spectra

An algorithm of a computer program for simulation of powder type EPR spectra (S=1/2,I≠0) is described. For determination of the energy levels, the perturbation approach is used and corrections up to second order due to hyperfine, quadrupole, and nuclear Zeeman coupling are taken into account. No restrictions on the mutual orientations of the eigenvectors ofg, hyperfine, and quadrupole tensors are imposed. Simulated spectra for a model paramagnetic system (S=1/2,I=3/2) with axial symmetry and noncoinciding directions ofg _‖ andA _‖ are presented.     

A Fixed-Tuned W-Band Waveguide SIS Mixer with 4.0-7.5 GHz IF

The design and performance of a fixed-tuned W-band SIS mixer with a wide band IF of 4.0-7.5 GHz is presented. Waveguide-to-stripline transition of the SIS mixer is designed using the lumped-gap-source port provided by HFSS^TM. Measured receiver noise temperature is less than 25 K in the frequency range of 95-120 GHz, with a minimum value of around 19 K achieved. Mixer noise temperature is determined to be about 8.5 K, which is around twice the quantum limit (i.e., 2hw/k). In spite of the high IF frequencies (f ₀ = 6 GHz), the performance of the SIS receiver is comparable or even superior to those of the best mechanically-tunable waveguide SIS receivers at low IF frequencies (f ₀ = 1.5 GHz). This result suggests that it is easy to design waveguide-to-stripline transitions without scale-model measurements.     

Millimeter wave pulsed gunn oscillator for low chirp radar application at W-Band

Pulsed Gunn oscillator at 94 GHz has been developed using GaAs CW Gunn diode, by choosing a proper operating point and resonant circuit. Peak power output of 25 mw at 94 GHz with a pulse width of 2 microseconds and duty factor of 2% is achieved. Bias circuit oscillations are suppressed by rising the operating voltage alongwith other circuit considerations.     

A virtually nondestructive EPR technique accounting for diagnostic X-rays

We have implemented the method proposed by Aldrich and Pass (Aldrich, J.E., Pass, B., 1988. Determining radiation exposure from nuclear accidents and atomic tests using dental enamel. Health Phys. 54, 469–471.) to measure both the interior and exterior portions of a tooth in order to determine the diagnostic X-ray component of the total dose in the teeth. This was done using a modified nondestructive approach of Haskell et al. (Haskell, E.H., Hayes, R.B., Romanyukha, A.A., Kenner, G.H., 2000. Preliminary report on the development of a virtually non-destructive additive dose technique for EPR dosimetry Appl. Radiat Isot. 52, 1065–1070.) given in Hayes et al. ( Hayes, R.B., Haskell, E.H., Barrus, J.K., Kenner, G.H., Romanyukha, A.A., 2000. Accurate EPR radiosensitivity calibration using small sample masses. Nucl. Instr. Meth. A. 441, 535–550). Here only 5% of the total tooth enamel sample from an enamel crown was used for individual sample sensitivity calibration. A cavity response correction for the low mass samples was also used ( Hayes et al., 2000). The teeth were American wisdom teeth having unknown doses applied by the IAEA in the range of 0 to 1 Gy as part of the second international intercomparison ( Wieser, A., Mehta, K., Amira, S., Aragno, S., Bercea, S., Brik, A., Bugai, A., Callens, F., Chumak, V., Ciesielski, B., Debuyst, R., Dubovsky, S., Duliu, O., Fattibene, P., Haskell, E., Hayes, R., Ignatiev, E., Ivannikov, A., Kirillov, V., Kleschenko, E., Nakamura, N., Nathe, M., Nowak, J., Onori, S., Pass, B., Pivovarov, S., Romanyukha, A., Scherbina, O., Shames, A., Sholom, S., Skvortsov, V., Stepanenko, V., Tikounov, D., Toyoda, S., 2000. The 2nd international intercomparison on EPR tooth dosimetry. Radiat. Meas. in press). Final dose reconstruction estimates of the IAEA applied values showed a very high correlation (R=0.996). The approximate excess dose measured on the outside of the teeth relative to the inside was 49±35 mGy which is attributed to the diagnostic X-ray exposure given to the America wisdom teeth prior to extraction. The method used showed only a small dependence on the accelerating voltage of the X-ray source.     

Mössbauer and EPR Study of Reaction Intermediates of Cytochrome P450

Hyperfine Interactions - We present a complementary Mössbauer and EPR study on reaction intermediates of substrate-free and substrate-bound cytochrome P450cam from Pseudomonas putida prepared...     

EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer

In photosynthetic bacteria, light-induced electron transfer takes place in a protein called the reaction center (RC) leading to the reduction of a bound ubiquinone molecule, Q_B, coupled with proton binding from solution. We used electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) to study the magnetic properties of the protonated semiquinone, an intermediate proposed to play a role in proton coupled electron transfer to Q_B. To stabilize the protonated semiquinone state, we used a ubiquinone derivative, rhodoquinone, which as a semiquinone is more easily protonated than ubisemiquinone. To reduce this low-potential quinone we used mutant RCs modified to directly reduce the quinone in the Q_B site via B-branch electron transfer (Paddock et al. in Biochemistry 44:6920–6928, 2005). EPR and ENDOR signals were observed upon illumination of mutant RCs in the presence of rhodoquinone. The EPR signals had g values characteristic of rhodosemiquinone (g _x  = 2.0057, g _y  = 2.0048, g _z  ~ 2.0018) at pH 9.5 and were changed at pH 4.5. The ENDOR spectrum showed couplings due to solvent exchangeable protons typical of hydrogen bonds similar to, but different from, those found for ubisemiquinone. This approach should be useful in future magnetic resonance studies of the protonated semiquinone.     

A bridged loop-gap S-band surface resonator for topical EPR spectroscopy.

The design and structure of a bridged loop-gap surface resonator developed for topical EPR spectroscopy and imaging of the distribution and metabolism of spin labels in in vivo skin is reported. The resonator is a one-loop, one-gap bridged structure. A pivoting single loop-coupling coil was used to couple the microwave power to the loop-gap resonant structure. A symmetric coupling circuit was used to achieve better shielding and minimize radiation. The frequency of the resonator can be easily adjusted by trimming the area of the capacitive foil bridge, which overlaps the gap in the cylindrical loop. The working frequency set was 2.2 GHz and the unloaded Q was 720. The B1 field of this resonator was measured and spatially mapped by three-dimensional EPR imaging. The resonator is well suited to topical measurements of large biological subjects and is readily applicable for in vivo measurements of free radicals in human skin.     

Planar microresonators for EPR experiments

EPR resonators on the basis of standing-wave cavities are optimised for large samples. For small samples it is possible to design different resonators that have much better power handling properties and higher sensitivity. Other parameters being equal, the sensitivity of the resonator can be increased by minimising its size and thus increasing the filling factor. Like in NMR, it is possible to use lumped elements; coils can confine the microwave field to volumes that are much smaller than the wavelength. We discuss the design and evaluation of EPR resonators on the basis of planar microcoils. Our test resonators, which operate at a frequency of 14 GHz, have excellent microwave efficiency factors, achieving 24 ns pi/2 EPR pulses with an input power of 17 mW. The sensitivity tests with DPPH samples resulted in the sensitivity value 2.3 x 10(9) spins.G(-1) Hz(-1/2) at 300 K.     

A new method for the determination of oxygen solubilities by EPR

Free radicals dissolved in oxygen-containing solutions give rise to EPR spectra characterized by very broad lines due to Heisenberg spin exchange. In the method herein proposed oxygen is consumed at a constant rate, within the cavity of an EPR spectrometer, by alkyl radicals generatedin situ by thermal decomposition of an aliphatic azo compound in the presence of a nitroxide probe. The effect of decreasing the oxygen concentration is to reduce the width, and therefore to increase the height of the spectral lines of the nitroxide, which reach a maximum when oxygen has been completely consumed. From the knowledge of the rate of generation of the alkyl radicals, the oxygen solubility in a given solvent can be easily determined.     

A novel loop-gap resonator probehead for EPR and ENDOR at X-band

An EPR and ENDOR probehead with a loop-gap resonator for X-band is described. The novel feature of the construction is that an iris-type coupling of the resonator is used instead of the conventional antenna coupling. The ENDOR coil combines the role of creating the radio frequency field and that of a shield for the microwave loop-gap structure. Hence, in order to accommodate the iris and waveguide, a pair of RF coils is used in conjunction with a reduced waveguide with dielectric filling. This arrangement simplifies matching the resonator to the microwave bridge, and standard EPR cryostats can be used making sample manipulation more convenient.     

A field-sweep/field-lock system for superconducting magnets--Application to high-field EPR

We describe a field-lock/field-sweep system for the use in superconducting magnets. The system is based on a commercially available field mapping unit and a custom designed broad-band 1H NMR probe. The NMR signal of a small water sample is used in a feedback loop to set and control the magnetic field to high accuracy. The current instrumental configuration allows field sweeps of +/-0.4 T and a resolution of up to 10(-5) T (0.1 G) and the performance of the system is demonstrated in a high-field electron paramagnetic resonance (EPR) application. The system should also be of utility in other experiments requiring precise and reproducible sweeps of the magnetic field such as DNP, ENDOR or PELDOR.     

A scheme for secure direct communication using EPR pairs and teleportation

A novel scheme for secure direct communication between Alice and Bob is proposed, where there is no need for establishing a shared secret key. The communication is based on Einstein-Podolsky-Rosen (EPR) pairs and teleportation between Alice and Bob. After insuring the security of the quantum channel (EPR pairs), Bob encodes the secret message directly on a sequence of particle states and transmits them to Alice by teleportation. In this scheme teleportation transmits Bobs message without revealing any information to a potential eavesdropper. Alice can read out the encoded messages directly by the measurement on her qubits. Because there is not a transmission of the qubit which carries the secret message between Alice and Bob, it is completely secure for direct secret communication if perfect quantum channel is used.Received: 17 March 2004, Published online: 30 September 2004PACS: 03.67.Dd Quantum cryptography - 03.67.Hk Quantum communicationF.L. Yan: Present address: Department of Physics, Hebei Normal University, Shijiazhuang 050016, P.R. China