Conformational change induced by binding of Mn ions activates SloR transcription factor in Streptococcus mutans.

Streptococcus mutans, a bacterium commonly found in the human oral cavity, is considered the primary causative agent of dental caries. A key player in the pathophysiology of S. mutans is SloR, a 25-kDa metalloregulatory protein.

Effect of Charge on the Rotation of Prolate Nitroxide Spin Probes in Room-Temperature Ionic Liquids.

We have studied the rotational diffusion of two prolate nitroxide probes, the doubly negatively charged peroxylamine disulfonate (Frémy's salt - FS) and neutral di--butyl nitroxide (DTBN), in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons using electron paramagnetic resonance (EPR) spectroscopy. Though the size and shape of the probes are reasonably similar, they behave differently due to the charge difference. The rotation of FS is anisotropic, and the rotational anisotropy increases with the alkyl chain length of the cation, while the rotation of DTBN is isotropic.

Radical Diffusion Crossover Phenomenon in Glass-Forming Liquids.

We studied the diffusivities of a nitroxide radical at various temperatures in six glass-forming molecular liquids by electron spin resonance. By comparing the radical diffusivities and solvent self-diffusivities, we found that the radical diffusivities are lower than the self-diffusivities at high temperatures and approach them at low temperatures in all liquids. This crossover behavior was considered as evidence that a single-molecule diffusion process transforms into a collective process with temperature lowering.

Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids.

X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO moiety. The Stokes-Einstein-Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe.

An analysis of radical diffusion in ionic liquids in terms of free volume theory.

The Heisenberg spin exchange-dipole-dipole separation method was used to measure the translational diffusion coefficients of the N-labeled perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (N-pDTEMPONE) nitroxide spin probe as a function of temperature in two methylimidazolium ionic liquid series, one based on the tetrafluoroborate (BF) anion and another one on the bis(trifluoromethane)sulfonimide (TFSI, Tf2N) anion. The obtained translational diffusion coefficients of N-pDTEMPONE were analyzed in terms of the Cohen-Turnbull free volume theory. It was found that the Cohen-Turnbull theory describes, exceptionally well, the translational diffusion of N-pDTEMPONE in all the ionic liquids in the measured temperature range.

Continuous Diffusion Model for Concentration Dependence of Nitroxide EPR Parameters in Normal and Supercooled Water.

Electron paramagnetic resonance (EPR) spectra of radicals in solution depend on their relative motion, which modulates the Heisenberg spin exchange and dipole-dipole interactions between them. To gain information on radical diffusion from EPR spectra demands both reliable spectral fitting to find the concentration coefficients of EPR parameters and valid expressions between the concentration and diffusion coefficients. Here, we measured EPR spectra of the N- and N-labeled perdeuterated TEMPONE radicals in normal and supercooled water at various concentrations.

Study of nanostructural organization of ionic liquids by electron paramagnetic resonance spectroscopy.

The X-band electron paramagnetic resonance spectroscopy (EPR) of a stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO) has been used to study the nanostructural organization of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids (ILs) with alkyl chain lengths from two to eight carbons. By employing nonlinear least-squares fitting of the EPR spectra, we have obtained values of the rotational correlation time and hyperfine coupling splitting of pDTO to high precision. The rotational correlation time of pDTO in ILs and squalane, a viscous alkane, can be fit very well to a power law functionality with a singular temperature, which often describes a number of physical quantities measured in supercooled liquids.

Hydrodynamic and nonhydrodynamic contributions to the bimolecular collision rates of solute molecules in supercooled bulk water.

Bimolecular collision rate constants of a model solute are measured in water at T = 259-303 K, a range encompassing both normal and supercooled water. A stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl, is studied using electron paramagnetic resonance spectroscopy (EPR), taking advantage of the fact that the rotational correlation time, τ(R), the mean time between successive spin exchanges within a cage, τ(RE), and the long-time-averaged spin exchange rate constants, K(ex), of the same solute molecule may be measured independently. Thus, long- and short-time translational diffusion behavior may be inferred from K(ex) and τ(RE), respectively.

Rotation of Four Small Nitroxide Probes in Supercooled Bulk Water.

Using a precise method of least-squares nonlinear electron paramagnetic resonance (EPR) line fitting, we have obtained experimental evidence of a decoupling of the rotational motion of four nitroxide spin probes from the viscosity of bulk water at 277 K. This decoupling is about 50 K higher than another such phenomenon observed in interstitial supercooled water of polycrystalline ice by Banerjee et al. (Proc Natl Acad Sci USA 106 (2009) 11448-11453).

Effect of glassy modes on electron spin-lattice relaxation in solid ethanol.

Electron spin-lattice relaxation (SLR) of TEMPO radical was measured in the crystalline and glassy states of deuterated ethanol in the temperature range 5-80K using X-band electron paramagnetic resonance (EPR). The measured SLR rates are higher in the glassy than in crystalline state and the excess SLR rate in glassy state is much lower than in ethanol. This result suggests that extra modes in glassy state, i.

A multi-frequency EPR spectroscopy approach in the detection of boson peak excitations.

The influence of boson peak (BP) excitations on low-temperature spin-lattice relaxation rate of a paramagnetic center embedded in a glassy matrix is investigated in the context of multi-frequency electron paramagnetic resonance (EPR) detection. In the theoretical analysis, the transition rate of spin one-half in the presence of a phonon field is calculated within the approximation of Fermi's golden rule. Several phonon densities of states are compared, among which one originating from a model of quasi-localized vibrations has been introduced into electron spin relaxation formalism for the first time.

Symmetry-breaking defect in coupled dipole-proton system of hydrogen-bonded ferroelectric.

We consider the coupled dipole-proton system of hydrogen-bonded ferroelectric crystal with one defect dipole that is coupled more strongly with the protons than the original dipole. This defect dipole simulates the symmetry-breaking defects that are often introduced in the crystals as probes in the electron paramagnetic resonance (EPR) experiments. A particular attention is paid to the explanation of strong isotope effects that have been detected in the measurements of static and dynamic properties of the pure crystals and the symmetry-breaking defects.