Pendent metals bound to heterocubanes are key components of well-known active sites in enzymes that mediate difficult chemical transformations. Investigations into the specific role of these metal ions, sometimes referred to as "danglers," have been hindered by a paucity of rational synthetic routes to appropriate model structures. To generate pendent metal ions bonded to an oxo cubane through a carboxylate bridge, the cubane Co4(μ3-O)4(OAc)4(t-Bupy)4 (OAc = acetate, t-Bupy = 4-tert-butylpyridine) was exposed to various metal acetate complexes. Reaction with Cu(OAc)2 gave the structur-ally characterized (by X-ray diffraction) dicopper dangler Cu2Co4(μ4-O)2(μ3-O)2(OAc)6(Cl)2(t-Bupy)4. In contrast, the anal-ogous reaction with Mn(OAc)2 produced the MnIV-containing cubane cation [MnCo3(μ3-O)4(OAc)4(t-Bupy)4]+ by way of a metal-metal exchange that gives Co(OAc)2 and [CoIII(μ-OH)(OAc)]n oligomers as byproducts. Additionally, reaction of the formally CoIV cubane complex [Co4(μ3-O)4(OAc)4(t-Bupy)4][PF6] with Mn(OAc)2 gave the corresponding Mn-containing cubane in 80% yield. A kinetic and mechanistic examination of the related metal-metal exchange reaction between Co4(μ3-O)4(OBz)4(py)4 (OBz = benzoate) and [Mn(acac)2(py)2][PF6] by UV-vis spectroscopy provided support for a pro-cess involving rate-determining association of the reactants and electron transfer through a μ-oxo bridge in the adduct intermediate. The rates of exchange correlate with the donor strength of the cubane pyridine and benzoate ligand sub-stituents; more electron-donating pyridine ligands accelerate metal-metal exchange, while both electron donating and withdrawing benzoate ligands can accelerate exchange. These experiments suggest that the basicity of the cubane oxo ligands promotes metal-metal exchange reactivity. The redox potentials of the Mn and cubane starting materials, and isotopic labeling studies, suggest an inner-sphere electron transfer mechanism in a dangler intermediate.
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Abstract We have characterized two new cytotoxic, monoclonal IgM antibodies specific for the murine T lymphocyte differentiation antigens Lyt-2.2 and Lyt-3.1. Initially, we determined the fractions of thymocytes and lymph node T cells that react with the antibodies; then we studied their effect on a functional subpopulation of T cells, namely cytotoxic T lymphocytes (CTL) specific for major and minor histocompatibility antigens. The antibodies lyse 45% of lymph node T cells and 90% of thymocytes of C57BL/6 series mice, in agreement with the original findings of Cantor and Boyse. However, in BALB/c series mice, only about 30% of lymph node T cells are sensitive to the antibodies. This difference probably reflects a genetic difference between C57BL/6 and BALB/ c mice that affects the relative proportions of T cell subsets. Thymocytes from an Lyt-3.1 congenic strain of BALB/ c were fractionated into peanut lectin agglutinable and nonagglutinable populations and tested with anti-Lyt-3.1 monoclonal antibodies. Twenty percent of peanut lectin nonagglutinable thymocytes, 70% of peanut lectin agglutinable thymocytes, and 70% of unseparated thymocytes were sensitive to monoclonal anti-Lyt-3.1 antibodies. Therefore, the immature peanut lectin agglutinable fraction of thymocytes contains Lyt-3 negative cells. Both hybridoma antibodies eliminate >95% of anti-H-2 and anti-minor H specific CTL activity generated in 5-day mixed lymphocyte cultures (MLC). Furthermore, pretreatment of spleen cells with the monoclonal antibodies and complement almost completely eliminated the generation of CTL, but left most of the T cells that proliferate in MLC. However, a small fraction (<5%) of CTL activity from primary MLC was resistant to elimination by monoclonal anti-Lyt-3.1 antibodies. By pre-treating spleen cells with monoclonal anti-Lyt-3.1 and complement and stimulating the surviving cells in MLC a number of times, we isolated a stable subpopulation of CTL that were completely resistant to any concentration of anti-Lyt-3.1 monoclonal antibodies, but were sensitive to anti-Thy-1.2 monoclonal antibodies. The specificity of this minor subpopulation of CTL, in terms of H-2 restriction and target antigen specificity, did not seem to be different from that of the bulk of CTL. Furthermore, the CTL resistant to lysis by monoclonal anti-Lyt-3.1 antibodies were sensitive to lysis by high concentrations of conventional anti-Lyt-3.1 serum. On this basis, we postulated that lysis by the monoclonal anti-Lyt-3.1 antibodies is very sensitive to determinant density, and that the resistant fraction of CTL have a sub-threshold density of the determinant. To investigate the effect of lowering the determinant density, we compared the lytic behavior of monoclonal and conventional antibodies on target cells homozygous or heterozygous at the Lyt-3 locus. Unlike the case with conventional serum, the titer of the monoclonal anti-Lyt-3.1 ascites dropped drastically on heterozygous target cells. Thus, it seems likely that the CTL resistant to monoclonal anti-Lyt-3.1 antibodies are a stable subpopulation with low Lyt-3.1 density.
Hydrated and anhydrous rhodium oxides, Rh/sub 2/O/sub 3/ 5H/sub 2/O and Rh/sub 2/O/sub 3/ crystallites, were used as catalysts for the hydrogenation of CO at 6 atm and in the range of 250 to 350/sup 0/C. The anhydrous oxide reduced to metallic rhodium rapidly, while the hydrated oxide was quite stable under the reaction conditions. The hydrated oxide produces a high concentration of oxygenated hydrocarbons, mostly acetaldehyde in addition to C/sub 2/ to C/sub 5/ alkenes and methane, in contrast to the unsupported metal which is a mediocre methanation catalyst. The activation energy for the formation of all of the products is 26 +- 2 kcal/mole indicating that they are likely to be produced from a common precursor intermediate, C/sub x/H/sub y/. The addition of ethylene to CO and H/sub 2/ results in the conversion of the olefin to propionaldehyde. This carbonylation reaction was not observed on the rhodium metal. The slower rates of hydrogenation on the oxide and its ability to insert CO into the C/sub t/H/sub y/ intermediates appear to be responsible for the changed product distribution in the CO/H/sub 2/ reaction. Electron spectroscopy studies indicate the presence of patches of oxide and metal both participate in themore » reaction and control the production distribution.« less
A series of 14-electron, coordinatively unsaturated Fe(II) silyl complexes featuring the anionic, tripodal phosphine ligand [PhB(CH2PiPr2)3]- have been prepared and characterized; preliminary reactivity studies indicate that at least one such complex can undergo redox processes to generate isolable Fe(I) species with concomitant loss of the silyl ligand.
To assist in the development of machine learning methods for automated classification of spectroscopic data, we have generated a universal synthetic dataset that can be used for model validation. This dataset contains artificial spectra designed to represent experimental measurements from techniques including X-ray diffraction, nuclear magnetic resonance, and Raman spectroscopy. The dataset generation process features customizable parameters, such as scan length and peak count, which can be adjusted to fit the problem at hand. As an initial benchmark, we simulated a dataset containing 35,000 spectra based on 500 unique classes. To automate the classification of this data, eight different machine learning architectures were evaluated. From the results, we shed light on which factors are most critical to achieve optimal performance for the classification task. The scripts used to generate synthetic spectra, as well as our benchmark dataset and evaluation routines, are made publicly available to aid in the development of improved machine learning models for spectroscopic analysis.
