A comparison of pulse and CW EPR T2-relaxation measurements of an inhomogeneously broadened nitroxide spin probe undergoing Heisenberg spin exchange

Barney L Bales; Miroslav Peric; Daniel Kinzek; Robert N Schwartz

doi:10.1016/j.jmr.2023.107456

A comparison of pulse and CW EPR T₂-relaxation measurements of an inhomogeneously broadened nitroxide spin probe undergoing Heisenberg spin exchange

J Magn Reson. 2023 Jun:351:107456. doi: 10.1016/j.jmr.2023.107456. Epub 2023 Apr 17.

Authors

Barney L Bales¹, Miroslav Peric¹, Daniel Kinzek¹, Robert N Schwartz²

Affiliations

¹ Department of Physics and Astronomy, California State University at Northridge, Northridge, CA 91330, USA.
² Electrical and Computer Engineering Department, University of California, Los Angeles, Los Angeles CA90095, USA.

PMID: 37121021
DOI: 10.1016/j.jmr.2023.107456

Abstract

Nitroxide spin probes are inhomogeneously broadened (IHB) by intramolecular hyperfine interactions with protons (deuterons) producing lines of Voigt shape. Thus, to study T₂ relaxation by continuous wave (CW) EPR, the Voigt must be deconvoluted to find the Lorentzian component. For homogeneously broadened lines, T₂ is obtained directly from the Lorentzian line widths ΔH_pp^L; however, for IHB lines finding T₂ from ΔH_pp^L is more complicated. It has been known for many years that values of ΔH_pp^L of high precision may be obtained from IHB lines; however, direct, accurate comparison of spin exchange frequencies obtained from electron spin echo decay and CW EPR data has been lacking. It is demonstrated here that despite complications in the interpretation of experiments, these two techniques yield the same spin exchange rate constant for spin probes that are the most difficult to treat.

Keywords: Heisenberg spin exchange; Nitroxide spin probes; Pulse versus CW EPR; Spin relaxation.