title: Nuclear spin-lattice relaxation in nitroxide spin-label EPR.
abstract: Nuclear relaxation is a sensitive monitor of rotational dynamics in spin-label EPR. It also contributes competing saturation transfer pathways in T1-exchange spectroscopy, and the determination of paramagnetic relaxation enhancement in site-directed spin labelling. A survey shows that the definition of nitrogen nuclear relaxation rate Wncommonly used in the CW-EPR literature for14N-nitroxyl spin labels is inconsistent with that currently adopted in time-resolved EPR measurements of saturation recovery. Redefinition of the normalised14N spin-lattice relaxation rate, b=Wn/(2We), preserves the expressions used for CW-EPR, whilst rendering them consistent with expressions for saturation recovery rates in pulsed EPR. Furthermore, values routinely quoted for nuclear relaxation times that are deduced from EPR spectral diffusion rates in14N-nitroxyl spin labels do not accord with conventional analysis of spin-lattice relaxation in this three-level system. Expressions for CW-saturation EPR with the revised definitions are summarised. Data on nitrogen nuclear spin-lattice relaxation times are compiled according to the three-level scheme for14N-relaxation: T1n=1/Wn. Results are compared and contrasted with those for the two-level15N-nitroxide system.
authors.affiliation: Max-Planck-Institut für biophysikalische Chemie, 37070 Göttingen, Germany; University of Southern Denmark, MEMPHYS-Centre for Biomembrane Physics, Campusvej 55, 5230 Odense M, Denmark. Electronic address: dmarsh@gwdg.de.
authors.forename: Derek
authors.initials: D
authors.lastname: Marsh
keywords: (15)N-, (14)N-nitroxides; ELDOR; Electron-nuclear dipolar (END); Saturation recovery