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Two-Dimensional Electron Spin Resonance and Slow Motions

Saxena, Sunil and Freed, Jack H. (1997) Two-Dimensional Electron Spin Resonance and Slow Motions. Journal of Physical Chemistry A, 101 (43). pp. 7998-8008. ISSN 1089-5639. doi:10.1021/jp9717047.

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The slow rotational dynamics of a polyproline peptide with a nitroxide labeled at one end in a glassy medium is probed using two-dimensional (2D) electron spin resonance (ESR). The contributions to the homogeneous relaxation time, T_2, from the overall and/or the internal rotations of the nitroxide is elucidated from the COSY spectra. The use of pure absorption spectra allows the variation of T_2 across the spectrum to be monitored. It is shown from simulations that the model of anisotropic Brownian diffusion provides semiquantitative agreement with such a variation. In the 2D ELDOR experiment several mechanisms can lead to spectral diffusion, which yields a broadening of the hyperfine (hf) auto-peaks with mixing time. We call these spectral diffusion (SD) cross-peaks. It is shown that at higher temperatures the principal mechanism for the formation of SD cross-peaks is the slow reorientation of the molecule, which modulates the ^(14)N hf and g tensor interactions. A procedure is shown for extracting a correlation time, τ_c, by monitoring this growth of SD cross-peaks, which is in good agreement with theory. An anomalous temperature dependence of the experimental τ_c, at very low temperatures, is tentatively attributed to the fast internal rotations of the methyl groups on the nitroxide, which leads to spin-flips of the protons on these methyl groups. The use of pure absorption spectra in 2D ELDOR enhances the sensitivity to these cross-peaks.

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Additional Information:© 1997 American Chemical Society. Received: May 22, 1997; In Final Form: August 12, 1997. Publication Date (Web): October 23, 1997. This work was supported by NIH Grants RR 07126 and GM 25862 and NSF Grant CHE 9615910. We would like to thank Dr. J. P. Barnes for providing the 250 GHz rigid limit spectrum of the monoradical.
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Issue or Number:43
Record Number:CaltechAUTHORS:20171121-083940170
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Official Citation:Two-Dimensional Electron Spin Resonance and Slow Motions. Sunil Saxena and Jack H. Freed. The Journal of Physical Chemistry A 1997 101 (43), 7998-8008 DOI: 10.1021/jp9717047
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:83386
Deposited By: Tony Diaz
Deposited On:21 Nov 2017 17:01
Last Modified:15 Nov 2021 19:57

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