Photo-CIDNP MAS NMR in intact cells of Rhodobacter sphaeroides R26: molecular and atomic resolution at nanomolar concentration

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in photosynthetic reaction centers of the carotenoid-less strain R26 of the purple bacterium Rhodobacter sphaeroides by (13)C solid-state NMR at three different magnetic fields (4.7, 9.4, and 17.6 T). The signals of the donor appear enhanced absorptive (positive) and of the acceptor emissive (negative). This spectral feature is in contrast to photo-CIDNP data of reactions centers of Rhodobacter sphaeroides wildtype reported previously (Prakash, S.; Alia; Gast, P.; de Groot, H. J. M.; Jeschke, G.; Matysik, J. J. Am. Chem. Soc. 2005, 127, 14290-14298) in which all signals appear emissive. The difference is due to an additional mechanism occurring in RCs of R26 in the long-living triplet state of the donor, allowing for spectral editing by different enhancement mechanisms. The overall shape of the spectra remains independent of the magnetic field. The strongest enhancement is observed at 4.7 T, enabling the observation of photo-CIDNP enhanced NMR signals from reaction center cofactors in entire bacterial cells allowing for detection of subtle changes in the electronic structure at nanomolar concentration of the donor cofactor. Therefore, we establish in this paper photo-CIDNP MAS NMR as a method to study the electronic structure of photosynthetic cofactors at the molecular and atomic resolution as well as at cellular concentrations.     

Spatial distribution of stabilizer-derived nitroxide radicals during thermal degradation of poly(acrylonitrile-butadiene-styrene) copolymers: a unified picture from pulsed ELDOR and ESR imaging

Double Electron-Electron Resonance (DEER) provides information on the spatial distribution of radicals on the length scale of a few nanometres, while Electron Spin Resonance Imaging (ESRI) provides information on a length scale of millimetres with a resolution of about 100 micrometres. Despite the gap between these length scales, results from the two techniques are found to complement and support each other in the characterization of the identity and distribution of nitroxide radicals derived from the Hindered Amine Stabilizer (HAS) Tinuvin 770 in poly(acrylonitrile-butadiene-styrene) (ABS) copolymers. DEER measurements demonstrate that there is no significant formation of biradicals from the bifunctional HAS, and provide the distributions of local radical concentrations. These distributions are poorly resolved for model-free analysis of the DEER data by the Tikhonov regularization; the resolution was significantly improved by utilizing information obtained by ESRI. DEER data can be fitted with only one adjustable parameter, namely the average radical concentration, if 1D and 2D spectral--spatial ESRI results on both the spatial distribution of nitroxides and their distribution between the acrylonitrile--styrene-rich (SAN) and butadiene-rich (B) microphases are considered.     

Size Selectivity of a Copper Metal–Organic Framework and Origin of Catalytic Activity in Epoxide Alcoholysis

{Cu(bpy)(H₂O)₂(BF₄)₂(bpy)} (Cu‐MOF; MOF=metal–organic framework; bpy=4,4′‐bipyridine), with a 3D‐interpenetrated structure and saturated Cu coordination sites in the framework, possesses unexpectedly high activity in the ring‐opening reaction of epoxides with MeOH, although the reaction rate drops remarkably with more bulky alcohols. This (apparent) size selection and the single Cu²⁺ sites in an identical environment of the crystalline matrix resemble zeolites. The real nature of active sites was investigated by attenuated total reflection infrared (ATR‐IR), Raman, EPR, and UV/Vis spectroscopies. Cu‐MOF has highly dynamic structural properties that respond to MeOH; its framework dimensions change from 3D to 2D by restructuring to a symmetric coordination of four bpy units to Cu. This interaction is accompanied by the partial dissolution of Cu‐MOF as multi‐Cu clusters, in which Cu²⁺ ions are connected with bpy ligands. Although both molecular and surface catalysis contribute to the high rate of alcoholysis, the soluble oligomeric species (Cu_mbpy_n) are far more active. Finally, addition of diethyl ether to the reaction mixture induces the reconstruction of dissolved and solid Cu‐MOF to the original framework structure, thereby allowing excellent recyclability of Cu‐MOF as an apparent heterogeneous catalyst. In contrast, the original Cu‐MOF structure is maintained upon contact with larger alcohols, such as iPrOH and tBuOH, thus leading to poor activity in epoxide ring opening.     

Surface extenders and an optimal pore size promote high dynamic binding capacities of antibodies on cation exchange resins

Increased recombinant protein expression yields and a large installed base of manufacturing facilities designed for smaller bulk sizes has led to the need for high capacity chromatographic resins. This work explores the impact of three pore sizes (with dextran distribution coefficients of 0.4, 0.53, and 0.64), dextran surface extender concentration (11–20 mg/mL), and ligand density (77–138 μmol H+/mL resin) of cation exchange resins on the dynamic binding capacity of a therapeutic antibody. An intermediate optimal pore size was identified from three pore sizes examined. Increasing ligand density was shown to increase the critical ionic strength, while increasing dextran content increased dynamic binding capacity mainly at the optimal pore size and lower conductivities. Dynamic binding capacity as high as 200 mg/mL was obtained at the optimum pore size and dextran content.     

Mechanisms of air oxidation of ethoxylated surfactants--computational estimations of energies and reaction behaviors

Pathways for formation of previously observed autoxidation products of ethoxylated surfactants have been studied by DFT (B3LYP). In addition to the established radical-chain reaction, several mechanistic possibilities for intramolecular fragmentation of the intermediate radicals have been characterized concerning reaction barriers and energies of transition states. The results can rationalize the formation of previously observed autoxidation products, including several, which have been implicated as strongly allergenic.     

Quantum correlations in dual quantum measurements

 We analyze the quantum measurement properties of dual non-degenerate parametric amplifers in the twin-beam configuration, in the cascaded back-action-evasion configuration, and in Kerr-type photon-number quantum non-demolition measurements. It is found that Einstein-Podolsky-Rosen correlations can be obtained between the quadrature components of an idler mode and the sum of the readout of two signal modes. Furthermore, we discuss dual-mode quantum non-demolition measurements on the combination of two light modes, and the generation of number-state entanglement. Received: 12 April 1996/Revised version: 2 July 1996     

Interaction of C60 with carbon nanotubes and graphite

The interaction of C60 with single-wall carbon nanotubes (SWNTs) and graphite is studied experimentally by thermal desorption spectroscopy and theoretically by molecular-mechanics and molecular-dynamics calculations. The van der Waals parameters and force field for C60-graphene and C60-SWNT interactions are derived from the low-coverage C60 binding energy to the graphite surface. We use these to compare the efficiency of different mechanisms by which C60 can be encapsulated into SWNTs.