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Dipolar Coupling between Nitroxide Spin Labels: The Development and Application of a Tether-in-a-Cone Model

Eric J. Hustedt ^*, Richard A. Stein ^*, Latsavongsakda Sethaphong ^*, Suzanne Brandon ^*, Zheng Zhou ^* and Susan C. DeSensi 

^* Department of Molecular Physiology and Biophysics, and Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37232

Correspondence: Address reprint requests to Eric J. Hustedt, 735B Light Hall, Vanderbilt University, Nashville, TN 37232. Tel.: 615-322-3181; Fax: 615-322-7236; E-mail: eric.hustedt{at}vanderbilt.edu.

A tether-in-a-cone model is developed for the simulation of electron paramagnetic resonance spectra of dipolar coupled nitroxide spin labels attached to tethers statically disordered within cones of variable halfwidth. In this model, the nitroxides adopt a range of interprobe distances and orientations. The aim is to develop tools for determining both the distance distribution and the relative orientation of the labels from experimental spectra. Simulations demonstrate the sensitivity of electron paramagnetic resonance spectra to the orientation of the cones as a function of cone halfwidth and other parameters. For small cone halfwidths (<40°), simulated spectra are strongly dependent on the relative orientation of the cones. For larger cone halfwidths, spectra become independent of cone orientation. Tether-in-a-cone model simulations are analyzed using a convolution approach based on Fourier transforms. Spectra obtained by the Fourier convolution method more closely fit the tether-in-a-cone simulations as the halfwidth of the cone increases. The Fourier convolution method gives a reasonable estimate of the correct average distance, though the distance distribution obtained can be significantly distorted. Finally, the tether-in-a-cone model is successfully used to analyze experimental spectra from T4 lysozyme. These results demonstrate the utility of the model and highlight directions for further development.

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