title: Intermediate dipolar distances from spin labels.
abstract: Methods for determining inter-spin distances between nitroxide spin labels from dipolar couplings in the intermediate range (r12≈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/r12(3)〉, and those that determine the dipolar contribution to the spectral second moment correspondingly yield 〈1/r12(6)〉. To relate these ensemble averages to the mean inter-spin distance 〈r12〉 requires knowledge of the distribution in r12. Values of the inverse roots reff,n=〈1/r12(n)〉(-1/n) always lie below 〈r12〉. Consistent comparisons of literature data on double-labelled α-helical peptides support this view. Evaluations of reff,n for Gaussian distributions in r12 yield calibrations to determine 〈r12〉 for a given distribution width, σ(r12).
authors: Max-Planck-Institut für biophysikalische Chemie, 37070 Göttingen, Germany. Electronic address: dmarsh@gwdg.de.
authors: Derek
authors: D
authors: Marsh
mesh_terms: D000465:Algorithms; D003545:Cysteine / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005583:Fourier Analysis; D006801:Humans; D010455:Peptides / Q000737:chemistry; D017433:Protein Structure, Secondary; D043562:Receptors, G-Protein-Coupled; D013113:Spin Labels
chemical_list: C121670:GPR17 protein, human; D010455:Peptides; D043562:Receptors, G-Protein-Coupled; D013113:Spin Labels; D003545:Cysteine
keywords: Absolute first moment; Dipolar interactions; Distance distribution; Inter-spin distances; Second moment; Spin label