Magnetic Relaxation Dispersion of7Li

19F and 1H Magnetic Relaxation Dispersion Determination of the Translational Encounter between Ionic Salts and Nitroxide Free Radicals in Aqueous Solution

“…The magnetic induction fields generated by the presence of unpaired electron spin density provide much more efficient relaxation mechanisms, particularly for those nuclei in close proximity to the paramagnetic center. In the event that a direct chemical bond is formed between the nuclear probe and the paramagnetic spin, as has been observed for the interaction between 7 Li(H 2 O) n + and nitroxides, the existence of a fixed intermoment vector induces a Lorentzian contribution to the magnetic dipole−dipole interaction that is correlated by a well-defined molecular rotation. Should a significant steady state concentration of such complexes be maintained, relaxation will likely be dominated by the spatially invariant Fermi contact interaction that is correlated by the lifetime of the covalently bonded complex.…”

“…Our recent reports of the magnetic relaxation dispersion (MRD) of 7 Li have revealed some interesting and unexpected solution chemistry based on an analysis of the contributions to the observed relaxation derived from models of rotational and translational diffusion. For example, our strategy to characterize translational diffusion of lithium at macromolecular and membrane surfaces by exploiting the relaxation induced by nitroxides localized at these surfaces was thwarted by the formation of a complex between Li + and aqueous nitroxide radicals, with the resulting MRD curve dominated by the Fermi contact interaction . With the existence of this direct interaction between the paramagnetic center and the nuclear spin probe, it was impossible to resolve the subtle effect of charge localized on the free-radical moiety.…”

“…For example, our strategy to characterize translational diffusion of lithium at macromolecular and membrane surfaces by exploiting the relaxation induced by nitroxides localized at these surfaces was thwarted by the formation of a complex between Li + and aqueous nitroxide radicals, with the resulting MRD curve dominated by the Fermi contact interaction. 4 With the existence of this direct interaction between the paramagnetic center and the nuclear spin probe, it was impossible to resolve the subtle effect of charge localized on the free-radical moiety. Subsequently, we reported a bimolecular association constant for the formation of a Li + -Mn 2+ outer sphere cation-cation complex and interpreted the relaxation data in terms of the relevant time scales and geometric constraints.…”

Oxygen Accessibility to Ribonuclease A: Quantitative Interpretation of Nuclear Spin Relaxation Induced by a Freely Diffusing Paramagnet

“…(2) T 1e is short compared with translational diffusion correlation times. The first case is relevant to nitroxide centers and some metal centers that have been used most often in the past; ,,,,− ,,− the second is important to the present discussion where we use dioxygen as the paramagnetic relaxation agent. ,,, The paramagnetic contribution to the k th proton in solution originating from a dipolar coupling to the oxygen center is given by Freed 16 where [ C ] is the concentration of the paramagnetic molecule, while S is the electron spin, 1 in this case, ω is the Larmor frequency for the nuclear spin, I , or the electron spin, S , P is a factor discussed below that equals 1 for a hydrogen atom, b is the distance of closest approach between the electron and nuclear spin, D is the relative diffusion constant, and 〈 r 2 〉 is the mean-square jump length of the small solute. In the present case, the motion of the protein is slow so that the effective diffusion constant becomes that for the paramagnet, oxygen.…”

High-Resolution Magnetic Relaxation Dispersion Measurements of Solute Spin Probes Using a Dual-Magnet System

“…The low-field relaxation dispersion may be identified with complex formation between the lithium ion and the nitroxide, leading to a contact or hyperfine coupling between the unpaired electron and the lithium nucleus (14). The inflection point corresponds to the correlation time for this coupling which is either the electron relaxation time, T 1 , or the residence time for the lithium ion in the complex.…”