Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1‑Ethyl-3-Methylimidazolium

Temperature-dependent ²H longitudinal spin relaxation times (<i>T</i>₁) of dilute benzene-<i>d</i>₆ in 1-butyl-3-methylimidazolium tetrafluoroborate ([Im₄₁]­[BF₄]) and two deuterated variants of the 1-ethyl-3-methylimidazolium cation (Im₂₁⁺-<i>d</i>₁ and Im₂₁⁺-<i>d</i>₆) in 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([Im₂₁]­[Tf₂N]), measured at multiple Larmor frequencies, were used to probe rotational dynamics in ionic liquids. Rotational correlation times significantly faster than predicted by slip hydrodynamic calculations were observed for both solutes. Molecular dynamics simulations of these systems enabled extraction of more information about the rotational dynamics from the NMR data than rotation times alone. The multifrequency ²H <i>T</i>₁(<i>T</i>) data could be fit to within uncertainties over a broad region about the <i>T</i>₁ minimum using models of the relevant rotational time correlation functions and their viscosity/temperature dependence derived from simulation. Such simulation-guided fitting provided confidence in the semiquantitative accuracy of the simulation models and enabled interpretation of NMR measurements to higher viscosities than previously possible. Simulations of the benzene system were therefore used to explore the nature of solute rotation in ionic liquids and how it might differ from rotation in conventional solvents. Whereas “spinning” about the <i>C</i>₆ axis of benzene senses similarly weak solvent friction in both types of solvents, “tumbling” (rotations about in-plane axes) differs significantly in conventional solvents and ionic liquids. In the sluggish environment provided by ionic liquids, orientational caging and the presence of rare but influential large-amplitude (180°) jumps about in-plane axes lead to rotations being markedly nondiffusive, especially below room temperature

The unravelling of the complex pattern of tyrosinase inhibition

Tyrosinases are responsible for melanin formation in all life domains. Tyrosinase inhibitors are used for the prevention of severe skin diseases, in skin-whitening creams and to avoid fruit browning, however continued use of many such inhibitors is considered unsafe. In this study we provide conclusive evidence of the inhibition mechanism of two well studied tyrosinase inhibitors, KA (kojic acid) and HQ (hydroquinone), which are extensively used in hyperpigmentation treatment. KA is reported in the literature with contradicting inhibition mechanisms, while HQ is described as both a tyrosinase inhibitor and a substrate. By visualization of KA and HQ in the active site of TyrBm crystals, together with molecular modeling, binding constant analysis and kinetic experiments, we have elucidated their mechanisms of inhibition, which was ambiguous for both inhibitors. We confirm that while KA acts as a mixed inhibitor, HQ can act both as a TyrBm substrate and as an inhibitor