Dissecting the Molecular Origin of g-Tensor Heterogeneity and Strain in Nitroxide Radicals in Water: Electron Paramagnetic Resonance Experiment versus Theory

Abstract

Nitroxides are common EPR sensors of microenvironmental properties such as polarity, numbers of H-bonds, pH, and so forth. Their solvation in an aqueous environment is facilitated by their high propensity to form H-bonds with the surrounding water molecules. Their g- and A-tensor elements are key parameters to extracting the properties of their microenvironment. In particular, the g xx value of nitroxides is rich in information. It is known to be characterized by discrete values representing nitroxide populations previously assigned to have different H-bonds with the surrounding waters. Additionally, there is a large g-strain, that is, a broadening of g-values associated with it, which is generally correlated with environmental and structural micro-heterogeneities. The g-strain is responsible for the frequency dependence of the apparent line width of the EPR spectra, which becomes evident at high field/frequency. Here, we address the molecular origin of the g xx heterogeneity and of the g-strain of a nitroxide moiety (HMI: 2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl, C9H19N2O) in water. To treat the solvation effect on the g-strain, we combined a multi-frequency experimental approach with ab initio molecular dynamics simulations for structural sampling and quantum chemical EPR property calculations at the highest realistically affordable level, including an explicitly micro-solvated HMI ensemble and the embedded cluster reference interaction site model. We could clearly identify the distinct populations of the H-bonded nitroxides responsible for the g xx heterogeneity experimentally observed, and we dissected the role of the solvation shell, H-bond formation, and structural deformation of the nitroxide in the creation of the g-strain associated with each nitroxide subensemble. Two contributions to the g-strain were identified in this study. The first contribution depends on the number of hydrogen bonds formed between the nitroxide and the solvent because this has a large and well-understood effect on the g xx -shift. This contribution can only be resolved at high resonance frequencies, where it leads to distinct peaks in the g xx region. The second contribution arises from configurational fluctuations of the nitroxide that necessarily lead to g-shift heterogeneity. These contributions cannot be resolved experimentally as distinct resonances but add to the line broadening. They can be quantitatively analyzed by studying the apparent line width as a function of microwave frequency. Interestingly, both theory and experiment confirm that this contribution is independent of the number of H-bonds. Perhaps even more surprisingly, the theoretical analysis suggests that the configurational fluctuation broadening is not induced by the solvent but is inherently present even in the gas phase. Moreover, the calculations predict that this broadening decreases upon solvation of the nitroxide.

Unraveling the Isotropic Hyperfine Coupling Constants of Nitroxide Radicals via Molecular Electrostatic Potential Analysis

“…The correlation diagrams presented above signify the robustness of V N in representing the HFCCs of nitroxide radicals. It is worth mentioning that factors such as solvent polarity, noncovalent interactions, temperature, pH, and physical state of the sample can influence the HFCC values, − which are not considered in the present study. Nevertheless, our study demonstrated the usefulness of V N values to capture subtle changes associated with the steric and electronic nature of the substituents in nitroxide radicals.…”

The accuracy limit of chemical shift predictions for species in aqueous solution

“…While EC-RISM already yielded improved NMR shift results compared to PCM calculations on optimized structures, 27 its performance advantage was particularly evident in the context of calculating isotropic hyperfine coupling constants for electron paramagnetic resonance (EPR) spectroscopy, as demonstrated recently. 34,35 In a similar spirit as in the present work, we worked out the accuracy limit for such calculations on the basis of AIMD-generated ensembles of a spin probe in water. By comparing to fully explicit solvation with a quantum-mechanical/molecular-mechanical (QM/MM) approach, we showed that EC-RISM as a hybrid solvation model is capable of replacing the short-range explicit solvation effects, unlike PCM.…”