The world as viewed by and with unpaired electrons

Abstract

Recent advances in electron paramagnetic resonance (EPR) include capabilities for applications to areas as diverse as archeology, beer shelf life, biological structure, dosimetry, in vivo imaging, molecular magnets, and quantum computing. Enabling technologies include multifrequency continuous wave, pulsed, and rapid scan EPR. Interpretation is enhanced by increasingly powerful computational models.

Fig. 1

Comparison of rapid scan (1.8 MG/s) and conventional CW EPR spectra of the low-field nitrogen hyperfine line of ¹⁵N-mHCTPO. (A) As-recorded sinusoidal rapid scan signal obtained with a scan rate of 1.8 MG/s and microwave power about 80 mW (B₁= 0.14 G). 12,288 averages were recorded in about 0.9 sec. B) Slow-scan absorption spectrum obtained by deconvolution of signal in A. C) First derivative spectrum obtained by pseudomodulation of the signal in B. D) Single scan of a conventional field-modulated first-derivative CW EPR spectrum of the same sample, obtained in 0.9 sec using 5 mW power, 10 kHz modulation frequency and 0.13 G modulation amplitude. Modulation amplitude, power, and filter were chosen to maximize signal amplitude with less than 2% broadening.

Fig. 2

Comparison of the signal amplitudes obtained by conventional field-modulated phase-sensitive detection and by direct detection. (A) In phase sensitive detection the amplitude is a fraction of the total signal, which is determined by the magnitude of the modulation. (B) In direct detection the full signal amplitude is detected.

Fig. 3

The percent improvement in signal to noise, κ, that is achieved by combining the absorption and dispersion channels from a quadrature detector, is a function of the phase difference between the two channels. When the two channels are exactly 90° out of phase the improvement is √2. The calculations assume that the noise floor is defined prior to the detector.

Fig. 4

Slice of a 250 MHz pulsed EPR image of pO₂ in a tumor-bearing mouse leg. The pO₂ was obtained from 5 electron spin echo images of the trityl radical OX063 injected into the mouse intravenously. Images were obtained at 5 different echo times from 0.7 to 2.4 μs to obtain relaxation rates in each voxel. pO₂ is proportional to the relaxation rate 1/T_2e. The tumor contour is obtained from a co-registered T₂-weighted MRI. (H. J. Halpern, unpublished results)

Fig. 5

Comparison of X- and Q-band DEER measurements. Positions of the pump (solid arrows) and observe (dashed arrows) pulses are shown on the field-swept echo-detected spectra for an enzymatically reduced, spin-labeled protein (top). Contributions from MTSL [241] (short dashes) and FAD SQ^−• [242] (long dashes) to the field-swept echo-detected spectra were simulated using a locally written program [243] and are shown, with arbitrary y-axis scales. Time domain DEER dipolar evolution signals for enzymatically reduced, spin-labeled protein (95 μM MTSL, ~45 μM FAD SQ^−•) were recorded at 80 K, averaging 23 scans overnight, followed by a 2^nd degree polynomial background subtraction (bottom). X- and Q-band data are shown in grey and black, respectively. Signal-to-noise at Q-band is enhanced by approximately 10 times relative to X-band. The sample within the active volume of the resonators was approximately 40 μL for X-band and 5 μL for Q-band [107].

Fig. 6

Easy axis EPR data for a single crystal of a tetrameric nickel complex at 172 GHz as a function of temperature. All 8 transitions within the S = 4 state are observed. Peaks that are highlighted with vertical dashed lines correspond to transitions between states in higher lying (S < 4) multiplets [110].

Fig. 7

HYSCORE spectra at X-band (9.77 GHz) obtained for Anabaena flavodoxin neutral semiquinone. Spectra with different tau values are shown in different colors: red, τ = 96 ns; blue, τ = 128 ns; green, τ = 168 ns; purple, τ = 208 ns. The field position is 348 mT. Some level curves have been eliminated to make the picture clearer. The four regions identified as containing specific spectral features are marked with dashed contour lines. A, B, C, and D regions correspond to H(N5), other protons, N10, and N1–N3 features, respectively. Details are given in the original text [125].

Fig. 8

Davies ENDOR spectrum of the E₂′ center measured at 60 K. The pair of peaks centered at 14.7 MHz unambiguously characterizes the coupling to a proton. Reproduced with permission from [174]