Solvation of Small Disulfonate Anions in Water/Methanol Mixtures Characterized by High-Field Pulse Electron Nuclear Double Resonance and Molecular Dynamics Simulations

Abstract

The solvation of Fremy's salt, the paramagnetic nitrosodisulfonate anion ON(SO(3)(-))(2), in binary solvent mixtures was investigated by means of pulse (Mims- and Davies-type) electron nuclear double resonance (ENDOR) spectroscopy and molecular dynamics (MD) simulations. (1)H and (2)H pulse ENDOR measurements were performed on small Fremy's salt radicals in isotope-substituted solvent mixtures of methanol and water in frozen solution. We were able to obtain well-resolved, orientation-selective, high-field/high-frequency pulse ENDOR spectra of methyl protons from the alcohol moiety and exchangeable protons from the alcohol-hydroxyl group and water. In the studied solvent systems (volume ratio v/v = 30:70, 50:50, 70:30), the solvation of 2.5 mM Fremy's salt by methyl protons was found to be almost identical. From the analysis of the dependence of pulse ENDOR spectra on the observer field position and spectral simulations, we obtained the principal components of the hyperfine coupling (hfc) tensor for each class of protons. The combination of Mims- and Davies-type pulse ENDOR measurements was necessary to obtain blind spot free information on hfc that spans a broad range of 0.25-6 MHz. Using the point-dipole approximation, the dipolar hfc component yields a prominent electron-nuclear distance of 3.5 A between Fremy's salt and methyl protons, which was found along the molecular z-axis (perpendicular to the approximate plane spanned by ON(S)(2)) of the probe molecule. Exchangeable protons were found to be distributed nearly isotropically, forming a hydrogen-bonded network around the sulfonate groups. The distribution of exchangeable and methyl protons found in MD simulations is in very good agreement with the pulse ENDOR results, and we find that solvation is dominated by an interplay of H-bond (electrostatic) interactions and steric properties. The elucidation of the microscopic solvation of a small probe molecule in binary solvent mixtures represents the first step for understanding the interactions in more complex biochemical systems. In particular, this includes the potential perturbation of the H-bond network due to the presence of a spin probe or other polar molecules.