Intermediate Dipolar Distances from Spin Labels
 

Intermediate Dipolar Distances from Spin Labels


Publication date: Available online 16 November 2013
Source:Journal of Magnetic Resonance</br>
Author(s): Derek Marsh</br>
Methods for determining inter-spin distances between nitroxide spin labels from dipolar couplings in the intermediate range (r 12 ? 1.1-2 nm) by CW-EPR are addressed. For nitroxide powder patterns, the assumption of unlike spins is a better approximation than assuming strong coupling between like spins. Methods that determine the average splitting in dipolar deconvolutions yield the mean value ? 1 / r 12 3 ? , and those that determine the dipolar contribution to the spectral second moment correspondingly yield ? 1 / r 12 6 ? . To relate these ensemble averages to the mean inter-spin distance ? r 12 ? requires knowledge of the distribution in r 12. Values of the inverse roots r eff , n = ? 1 / r 12 n ? - 1 / n always lie below ? r 12 ? . Consistent comparisons of literature data on double-labelled ? -helical peptides support this view. Evaluations of reff,n for Gaussian distributions in r 12 yield calibrations to determine ? r 12 ? for a given distribution width, ? r 12 .
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Source: Journal of Magnetic Resonance