Double Electron−Electron Resonance Measured Between Gd³⁺ Ions and Nitroxide Radicals

Double electron−electron resonance has attracted growing attention as a technique to study structure and conformational changes of biomacromolecules. Here, a new combination of paramagnetic labels is experimentally tested, one being a commonly used nitroxide radical, and the other being a Gd³⁺ ion. The Gd³⁺−nitroxide spin pair can serve as a good substitute for the nitroxide−nitroxide pair of spin labels and potentially provides a link to other experimental approaches dealing with structural information

Homogeneous Photochemical Water Oxidation with Cobalt Chloride in Acidic Media

The precise mechanisms of four-electron-transfer water oxidation processes remain to be further understood. Oxide-based precipitation from molecular catalysts as a frequent observation during water oxidation has raised extensive debates on the differentiation between homogeneous and heterogeneous catalysis. Although soluble cobalt salts are known to be active in water oxidation, CoO_<i>x</i> species formed in situ were generally considered to be the true catalyst. Here we report on the possibility homogeneous water oxidation with cobalt chloride in acidic conditions, which prevent CoO_<i>x</i> precipitation. Interestingly, both the buffer media and counteranions were found to significantly influence the oxygen evolution activity, and their roles in the water oxidation process were analyzed with various techniques. This study sheds new light on Co²⁺ ions in key transformation processes of homogeneous water oxidation catalysts

Multiple Pathway Relaxation Enhancement in the System Composed of Three Paramagnetic Species: Nitroxide Radical–Ln³⁺–O₂

Longitudinal relaxation of nitroxide spin-labels has been measured for a membrane-incorporated α-helical polypeptide in the presence and absence of residual amounts of membrane-dissolved O₂ and paramagnetic Dy³⁺ ions. Such a model system, containing three different types of paramagnetic species, provides an important example of nonadditivity of two different relaxation channels for the nitroxide spins

Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K

A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (ε) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger ε. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in ε between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on ε is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g·mol^–1, is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large ε may be obtained at higher temperatures

Solid-Phase Polarization Matrixes for Dynamic Nuclear Polarization from Homogeneously Distributed Radicals in Mesostructured Hybrid Silica Materials

Mesoporous hybrid silica–organic materials containing homogeneously distributed stable mono- or dinitroxide radicals covalently bound to the silica surface were developed as polarization matrixes for solid-state dynamic nuclear polarization (DNP) NMR experiments. For TEMPO-containing materials impregnated with water or 1,1,2,2-tetrachloroethane, enhancement factors of up to 36 were obtained at ∼100 K and 9.4 T without the need for a glass-forming additive. We show that the homogeneous radical distribution and the subtle balance between the concentration of radical in the material and the fraction of radicals at a sufficient inter-radical distance to promote the cross-effect are the main determinants for the DNP enhancements we obtain. The material, as well as an analogue containing the poorly soluble biradical bTUrea, is used as a polarizing matrix for DNP NMR experiments of solutions containing alanine and pyruvic acid. The analyte is separated from the polarization matrix by simple filtration