Spin-label CW microwave power saturation and rapid passage with triangular non-adiabatic rapid sweep (NARS) and adiabatic rapid passage (ARP) EPR spectroscopy

Abstract

Non-adiabatic rapid passage (NARS) electron paramagnetic resonance (EPR) spectroscopy was introduced by Kittell, A.W., Camenisch, T.G., Ratke, J.J. Sidabras, J.W., Hyde, J.S., 2011 as a general purpose technique to collect the pure absorption response. The technique has been used to improve sensitivity relative to sinusoidal magnetic field modulation, increase the range of inter-spin distances that can be measured under near physiological conditions, and enhance spectral resolution in copper (II) spectra. In the present work, the method is extended to CW microwave power saturation of spin-labeled T4 Lysozyme (T4L). As in the cited papers, rapid triangular sweep of the polarizing magnetic field was superimposed on slow sweep across the spectrum. Adiabatic rapid passage (ARP) effects were encountered in samples undergoing very slow rotational diffusion as the triangular magnetic field sweep rate was increased. The paper reports results of variation of experimental parameters at the interface of adiabatic and non-adiabatic rapid sweep conditions. Comparison of the forward (up) and reverse (down) triangular sweeps is shown to be a good indicator of the presence of rapid passage effects. Spectral turning points can be distinguished from spectral regions between turning points in two ways: differential microwave power saturation and differential passage effects. Oxygen accessibility data are shown under NARS conditions that appear similar to conventional field modulation data. However, the sensitivity is much higher, permitting, in principle, experiments at substantially lower protein concentrations. Spectral displays were obtained that appear sensitive to rotational diffusion in the range of rotational correlation times of 10−3 to 10−7 s in a manner that is analogous to saturation transfer spectroscopy.

Autobiography of James S. Hyde

“…The papers by Prof. D. D. Thomas and his students apply this methodology to the muscle-protein system [164, 167, 168, 171, 172]. More recently, my graduate student, Aaron W. Kittell [76; see also, Topic XIV], used the technique of non-adiabatic rapid sweep (NARS) to study the early evidence of adiabaticity. Kittell’s work describes the gradual transition from stationary CW EPR to the onset of free induction decay (FID) and the appearance of passage phenomena.…”

“…A recent paper [76] applied NARS spectroscopy to a spin-label sample undergoing very slow rotational diffusion. Turning points revealed the onset of FID response, while regions between turning points responded normally.…”

Rapid-scan EPR imaging

“…Weger [15] and others have given special meaning to rapid, fast, etc., and our initial papers defined rapid-scan relative to T 2 [5, 11]. Hyde and co-workers proposed the designation non-adiabatic rapid scan (NARS) for similar experiments in which the scans were wider than spectral features and in which direct detection was used, but scan rates were slow enough that rapid-scan oscillations were not observed [16–20]. However, as the technique becomes more generally applied we propose to use rapid-scan to describe the detection method rather than a particular scan rate regime.…”

Rapid-Scan EPR of Nitroxide Spin Labels and Semiquinones

“…After combining segments, the S/N was about 4 times higher than with conventional CW EPR. Extension of the NARS method to spin-labeled T4-lysozyme (Kittell & Hyde, 2015) included conditions under which the spin label signal was partially saturated and under which adiabatic rapid passage effects were observed. A key characteristic of these experiments was that there was little overlap of successive spectral segments, which caused discontinuities between segments.…”