Rationalizing Overhauser DNP of nitroxide radicals in water through MD simulations

The recently introduced methodology (Sezer, Phys. Chem. Chem. Phys., 2013, 15, 526) for calculating dynamic nuclear polarization (DNP) coupling factors through synergistic use of molecular dynamics (MD) simulations and the analytical model of diffusing hard spheres with spins at their centers (HSCS) is applied to several nitroxides in water. Computations with one TEMPONE and one TEMPOL in water agree with experiments at 0.35 T and 3.4 T, respectively. At 9.2 T the predicted coupling factors are larger by about 50% than the experimental numbers obtained with 1 M TEMPOL solution. MD simulations at this elevated concentration reveal nanoscopic TEMPOL clusters and qualitatively explain the lower experimental values. Comparing the dynamics from the MD simulations with those of the HSCS model, the assumption of centered spins is shown to be too limiting even for small molecules like TEMPOL and water. Using the available extension of the HSCS model to off-centered spins, the current procedure for analyzing hydration water dynamics from Overhauser DNP measurements on spin-labeled proteins is revisited.