1H relaxation dispersion in solutions of nitroxide radicals: Influence of electron spin relaxation

Abstract

The work presents a theory of nuclear ((1)H) spin-lattice relaxation dispersion for solutions of (15)N and (14)N radicals, including electron spin relaxation effects. The theory is a generalization of the approach presented by Kruk et al. [J. Chem. Phys. 137, 044512 (2012)]. The electron spin relaxation is attributed to the anisotropic part of the electron spin-nitrogen spin hyperfine interaction modulated by rotational dynamics of the paramagnetic molecule, and described by means of Redfield relaxation theory. The (1)H relaxation is caused by electron spin-proton spin dipole-dipole interactions which are modulated by relative translational motion of the solvent and solute molecules. The spectral density characterizing the translational dynamics is described by the force-free-hard-sphere model. The electronic relaxation influences the (1)H relaxation by contributing to the fluctuations of the inter-molecular dipolar interactions. The developed theory is tested against (1)H spin-lattice relaxation dispersion data for glycerol solutions of 4-oxo-TEMPO-d16-(15)N and 4-oxo-TEMPO-d16-(14)N covering the frequency range of 10 kHz-20 MHz. The studies are carried out as a function of temperature starting at 328 K and going down to 290 K. The theory gives a consistent overall interpretation of the experimental data for both (14)N and (15)N systems and explains the features of (1)H relaxation dispersion resulting from the electron spin relaxation.

Translational Dynamics of Water at the Phospholipid Interface

“…In spite of the uncertainty in the translational correlation time, it differs fundamentally from that measured from nitroxide spin-labeled phospholipids reported earlier . Because the field dependence for these paramagnetic cases was not logarithmic, a three-dimensional model − was used to interpret the data and yielded a translational correlation time for the interface that was 3–4 times slower than bulk water. In contrast, the logarithmic field dependence implies a two-dimensional translational model and the much weaker dipolar coupling localizes more strongly the depth of the interfacial region that drives the spin relaxation.…”

Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion

“…27,28 The rotational component can be included phenomenologically with a Lorentzian spectral density J rot (ω n ,τ c ), with τ c as the rotational correlation time. 13,29 As previously predicted for water 10,31 and observed in organic solvents, 32 the coupling factor ξ 1 H at room temperature for small organic radicals is mainly dependent on translational diffusion, and including a rotational contribution introduces an overparameterization. 33,34 Therefore, as a first approximation, we assume k rot = 0 for TP, leaving only the translational diffusion component in eq 2.…”

“…The rotational contribution of the dipolar relaxation becomes larger for slower rotations, as previously observed in high-viscosity liquids. 29,36 This is the case in FN-na (where n is the number of adducts), with rotational correlation times τ c EPR larger than τ D (Table 1). The rotational dynamics of the solvent molecules are much faster, being τ c < 2 ps for both toluene 37 and chloroform, 38 meaning that their contribution can be neglected at this magnetic field (τ c ω n ≪ 1).…”

Quantitative Interpretation of Solvent Paramagnetic Relaxation for Probing Protein–Cosolute Interactions

“…For the sPRE, the Lorentzian spectral density is flawed because in the low frequency limit, as a result of translational diffusion, the spectral density behaves as: 43,44,[49][50][51][52][53][54] lim ω 0…”