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Biophys. J. BioFAST: First Published October 7, 2005. doi:10.1529/biophysj.105.068544
© 2005 by the Biophysical Society.

A more recent version of this article appeared on January 1, 2006.
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SPECTROSCOPY, IMAGING, OTHER TECHNIQUES

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

Eric J. Hustedt 1*, Richard A. Stein 1, Latsavongsakda Sethaphong 1, Suzanne Brandon 1, Zheng Zhou 1 and Susan C. DeSensi 1

1 Vanderbilt University

* To whom correspondence should be addressed. E-mail: eric.hustedt{at}vanderbilt.edu.

Submitted on June 10, 2005
Revised on September 1, 2005
Accepted on 26 September 2005


  Abstract
A tether-in-a-cone model is developed for the simulation of electron paramagnetic resonance (EPR) spectra of dipolar coupled nitroxide spin labels attached to tethers statically disordered within cones of variable half width. 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 EPR spectra to the orientation of the cones as a function of cone half width and other parameters. For small cone half widths (less than approximately 40°), simulated spectra are strongly dependent on the relative orientation of the cones. For larger cone half widths, 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 half width 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.

Key Words: Convolution, Dipolar Coupling, Distance Measurement, EPR, Orientation, T4 Lysozyme




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Copyright © 2005 by the Biophysical Society.