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Free keywords:
Inter-spin distances; Dipolar interactions; Second moment; Absolute first moment; Distance distribution; Spin label
Abstract:
Methods for determining inter-spin distances between nitroxide spin labels from dipolar couplings in the intermediate range (r(12) approximate to 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 alpha-helical peptides support this view. Evaluations of r(eff,n) for Gaussian distributions in r(12) yield calibrations to determine < r(12)> for a given distribution width, sigma(r12).
