Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

The hydrogen bond plays a key role in weak directional intermolecular interactions. It is operative in determining molecular conformation and aggregation, and controls the function of many chemical systems, ranging from inorganic, organic to biological molecules. Although an enormous amount of spectroscopic information has been collected about hydrogen-bond formation between molecules with closed-shell electronic configuration, the details of such interactions between open-shell radicals and closed-shell molecules are still rare. Here we report on an investigation of hydrogen-bonded complexes between pyrroline-type as well as piperidine-type neutral nitroxide radicals and an alcohol, i.e., 2-propanol. These nitroxide radicals are commonly used as EPR spin labels and probes. To obtain information on the geometry of the complexes and their electronic structure, multi-resonance EPR techniques at various microwave frequencies (X-, Q-, W-band, 244 GHz) have been employed in conjunction with DFT calculations. The planar five-membered ring system of the pyrroline-type nitroxide radical was found to form exclusively well-defined in-plane σ-type hydrogen-bonded complexes with one 2-propanol molecule in the first solvation shell in frozen solution. The measured hyperfine parameters of the hydrogen-bridge proton and the internal magnetic parameters describing the electron Zeeman and the electron-nuclear hyperfine and nuclear quadrupole interactions are in good agreement with values predicted by state-of-the-art DFT calculations. In contrast, multi-resonance EPR on the non-planar six-membered ring system of the piperidine-type nitroxide radical (TEMPOL) reveals a more complex situation, i.e., a mixture of a σ-type with, presumably, an out-of-plane π-type complex, both present in comparable fraction in frozen solution. For TEMPOL, the DFT calculations failed to predict magnetic interaction parameters that are in good agreement with experiment, apparently due to the considerable flexibility of the nitroxide and hydrogen-bonded complex. The detailed information about nitroxide/solvent complexes is of particular importance for Dynamic Nuclear Polarization (DNP) and site-directed spin-labeling EPR studies that employ nitroxides as polarizing agents or spin labels, respectively.

Melting and Heat Transfer Characteristics of Urea Water Solution According to a Heating Module’s Operating Conditions in a Frozen Urea Tank

The urea-selective catalytic reduction (SCR) system, a nitrogen oxide reduction device for diesel vehicles, is a catalytic system that uses urea water solution (UWS) as a reducing agent. This system has a relatively wide range of operating temperatures. However, the freezing point of the reducing urea solution used in this system is −11 °C. When the ambient temperature dips below this freezing point in winter, the solution may freeze. Therefore, it is important to understand the melting characteristics of frozen UWS in relation to the operating conditions of the heating device to supply the minimum amount of aqueous solution required by the system in the initial stage of normal operation and startup of the urea–SCR system. In this study, we artificially froze a liquid solution by placing it along with a heating module in an acrylic chamber to simulate a urea solution tank. Two types of heating modules (P120 and P160) consisting of two heating elements and heat transfer bodies were used to melt the frozen solution. The melting characteristics of the frozen solution were observed, for example, changes in the temperature distribution around the heating module and the cross-sectional melting shape with the passage of time since the start of the power supply to the heating module. The shape of melting around the heating module differed depending on the level of UWS relative to the heater inside the urea tank. In case 1, it melted in a wide shape with an open top, and in case 2, it melted in a closed shape. This shape change was attributed to the formation of internal gaseous space due to volume reduction during melting and the heat transfer characteristics of the fluid and solid substances.

Nanomolar Pulse Dipolar EPR Spectroscopy in Proteins; the Copper(II)-Copper(II) and Nitroxide-Nitroxide Cases

The study of ever more complex biomolecular assemblies implicated in human health and disease is facilitated by a suite of complementary biophysical methods. Pulse Dipolar Electron Paramagnetic Resonance (PDEPR) spectroscopy is a powerful tool that provides highly precise geometric constraints in frozen solution, however the drive towards PDEPR at physiologically relevant sub-μM concentrations is limited by the currently achievable concentration sensitivity. Recently, PDEPR using a combination of nitroxide and Cu^II based spin labels allowed measuring 500 nM concentration of a model protein. Using commercial instrumentation and spin labels we demonstrate Cu^II-Cu^II and nitroxide-nitroxide PDEPR measurements at protein concentrations more than an order of magnitude below previous examples reaching 500 and 100 nM, respectively. These results demonstrate the general feasibility of sub-μM PDEPR measurements at short to intermediate distances (~1.5 - 3.5 nm), and are of particular relevance for applications where the achievable concentration is limiting.

Supramolecular Assemblies of Trinuclear Copper(II)-Pyrazolato Units: A Structural, Magnetic and EPR Study

Two anionic complexes, {[Cu3(µ3-OH)(µ-4-Ph-pz)3Cl3]2[Cu(4-Ph-pzH)4](µ-Cl)2}2− (1) and [Cu3(µ3-OH)(µ-pz)3(µ1,1-N3)2(N3)]− (2), crystallize as one-dimensional polymers, held together by weak Cu-(µ-Cl) and Cu-(µ-N3) interactions, respectively. Variable temperature magnetic susceptibility analyses determined the dominant antiferromagnetic intra-Cu3 exchange parameters in the solid state for both complexes, whereas the weak ferromagnetic inter-Cu3 interactions manifested also in the solid state EPR spectra, are absent in the corresponding frozen solution spectra. DFT calculations were employed to support the results of the magnetic susceptibility analyses.

Low-Temperature Luminescent Studies of Emissive Guanine Substitute for the Detection of Biopolymers

The optical absorption at 300 K and the fluorescence and phosphorescence at 78 K of the emissive guanine substitute, deoxythienoguanosine, (dthG) were investigated in aqueous and TRIS-HCl-buffer solutions. Two optical absorption and fluorescence centers at room temperature were attributed to two keto-enol tautomers of dthG, which confirms previously obtained results. In contrast to room temperature, only one emission band was observed at 78 K in fluorescence spectra that was close to the long-wave fluorescence band at room temperature and could be associated with the tautomer with long-wave absorption. This phenomenon can be explained by the energy transfer by excitations in a frozen solution between two types of the optical centers mentioned above. The similar conclusion is drawn for the phosphorescence: only one tautomer phosphorescence band is observed. The spectral positions of this band maximum are essentially different for aqueous and buffer solutions (∼50 nm).

FORMATION OF ICE MICROPARTICLES IN THE HOMOGENATE OF NATIVE EGGS OF STURGEON FISH DURING CRYOPRESERVATION

In this work, we studied the formation of ice microparticles in a thin layer (0.2 mm) of the protoplasm of Russian sturgeon caviar upon cooling to a temperature of -196 ° C. Upon gradual cooling from room temperature + 20 ° C to -196 ° C, the process of freezing, formation and changes of ice microparticles were observed. The shape and size of the particles depended on the composition of the frozen solution. The freezing temperature for all layers of protoplasm was different, which is due to the chemical composition.