Orientation selective DEER measurements on vinculin tail at X-band frequencies reveal spin label orientations

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Orientation selective DEER measurements on vinculin tail at X-band frequencies reveal spin label orientations


Publication year: 2012
Source:Journal of Magnetic Resonance, Volume 216

Christoph Abé, Daniel Klose, Franziska Dietrich, Wolfgang H. Ziegler, Yevhen Polyhach, Gunnar Jeschke, Heinz-Jürgen Steinhoff

Double electron electron resonance (DEER) spectroscopy has been established as a valuable method to determine distances between spin labels bound to protein molecules. Caused by selective excitation of molecular orientations DEER primary data also depend on the mutual orientation of the spin labels. For a doubly spin labeled variant of the cytoskeletal protein vinculin tail strong orientation selection can be observed already at X-band frequencies, which allows us to reduce the problem to the relative orientation of two molecular axes and the spin–spin axis parameterized by three angles. A full grid search of parameter space reveals that the DEER experiment introduces parameter-space symmetry higher than the symmetry of the spin Hamiltonian. Thus, the number of equivalent parameter sets is twice as large as expected and the relative orientation of the two spin labels is ambiguous. Except for this inherent ambiguity the most probable relative orientation of the two spin labels can be determined with good confidence and moderate uncertainty by global fitting of a set of five DEER experiments at different offsets between pump and observer frequency. The experiment provides restraints on the angles between the z axis of the nitroxide molecular frame and the spin–spin vector and on the dihedral between the two z axes. When using the same type of label at both sites, assignment of the angle restraints is ambiguous and the sign of the dihedral restraint is also ambiguous.
Graphical Abstract

Graphical abstract Highlights

? Orientation selective DEER was applied to a spin labeled variant of vinculin. ? Strong orientation selection could be observed at X-band frequencies. ? Parameter space could be reduced to the interspin distance and three angles. ? A grid search of the parameter space reveals symmetry, distance and angles.





Source: Journal of Magnetic Resonance