Mechanism of Reductive Elimination of Methyl Iodide from a Novel Gold(III)−Monomethyl Complex

Oxidation of (Idipp)AuMe (Idipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with I_2 gives a monomethyl Au(III) complex, (Idipp)AuI_2Me, which decomposes cleanly to MeI and (Idipp)AuI. Kinetics experiments show that this transformation occurs primarily via three-coordinate, cationic [(Idipp)AuIMe]^+, which undergoes intramolecular reductive elimination rather than nucleophilic attack by external I^−

The nucleotide sequence of a human immnnoglobulin C-gamma-1 gene

We report the nucleotide sequence of a gene encoding the constant region of a human immnnoglobulin γ1 heavy chain (Cγ1). A comparison of this sequence with those of the Cγ2 and Cγ4 genes reveals that these three human Cγ genes share considerable homology in both coding and noncoding regions. The nucleotide sequence differences indicate that these genes diverged from one another approximately 6–8 million years ago. An examination of hinge exons shows that these coding regions have evolved more rapidly than any other areas of the Cγ genes in terms of both base substitution and deletion–insertion events. Coding sequence diversity also is observed in areas of CH domains which border the hinge

Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide

Structure-function studies on the role of pendent Lewis acids in the reductive coupling of CO are reported. Cationic rhenium carbonyl complexes containing zero, one, or two phosphinoborane ligands (Ph_2P(CH_2)_nB(C_8H_(14)), n=1-3) react with the nucleophilic hydride [HPt(dmpe)_2]^+ to reduce [M-CO]^+ to M-CHO; this step is relatively insensitive to the Lewis acid, as both pendent (internal) and external boranes of appropriate acid strength can be used. In contrast, whether a second hydride transfer and C-C bond forming steps occur depends strongly on the number of carbon atoms between P and B in the phosphinoborane ligands, as well as the number of pendent acids in the complex: shorter linker chain lengths favor such reductive coupling, whereas longer chains and external boranes are ineffective. A number of different species containing partially reduced CO groups, whose exact structures vary considerably with the nature and number of phosphinoborane ligands, have been crystallographically characterized. The reaction of [(Ph -2P(CH_2)_2B(C_8H_(14)))_2Re(CO)4]^+ with [HPt(dmpe)_2]^+ takes place via a “hydride shuttle” mechanism, in which hydride is transferred from Pt to a pendent borane and thence to CO, rather than by direct hydride attack at CO. Addition of a second hydride in C_6D_5Cl at -40 ºC affords an unusual anionic bis(carbene) complex, which converts to a C-C bonded product on warming. These results support a working model for Lewis acid-assisted reductive coupling of CO, in which B (pendent or external) shuttles hydride from Pt to coordinated CO, followed by formation of an intramolecular B-O bond, which facilitates reductive coupling

Trialkylborane-Assisted CO_2 Reduction by Late Transition Metal Hydrides

Trialkylborane additives promote reduction of CO_2 to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO_2 to give a formateborane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO_2 reduction. The mechanism likely involves a pre-equilibrium hydride transfer followed by formation of a stabilizing formateborane adduct

Homogeneous CO Hydrogenation: Dihydrogen Activation Involves a Frustrated Lewis Pair Instead of a Platinum Complex

During a search for conditions appropriate for Pt-catalyzed CO reduction using dihydrogen directly, metal-free conditions were discovered instead. A bulky, strong phosphazene base forms a “frustrated” Lewis pair (FLP) with a trialkylborane in the secondary coordination sphere of a rhenium carbonyl. Treatment of the FLP with dihydrogen cleanly affords multiple hydride transfers and C−C bond formation

An Attempt to Detect the Galactic Bulge at 12 microns with IRAS

Surface brightness maps at 12 microns, derived from observations with the Infrared Astronomical Satellite (IRAS), are used to estimate the integrated flux at this wavelength from the Galactic bulge as a function of galactic latitude along the minor axis. A simple model was used to remove Galactic disk emission (e.g. unresolved stars and dust) from the IRAS measurements. The resulting estimates are compared with predictions for the 12 micron bulge surface brightness based on observations of complete samples of optically identified M giants in several minor axis bulge fields. No evidence is found for any significant component of 12m emission in the bulge other than that expected from the optically identified M star sample plus normal, lower luminosity stars. Known large amplitude variables and point sources from the IRAS catalogue contribute only a small fraction to the total 12 micron flux.Comment: Accepted for publication in ApJ; 13 pages of text including tables in MS WORD97 generated postscript; 3 figures in postscript by Sigma Plo

Disentangling Dark Matter Dynamics with Directional Detection

Inelastic dark matter reconciles the DAMA anomaly with other null direct detection experiments and points to a non-minimal structure in the dark matter sector. In addition to the dominant inelastic interaction, dark matter scattering may have a subdominant elastic component. If these elastic interactions are suppressed at low momentum transfer, they will have similar nuclear recoil spectra to inelastic scattering events. While upcoming direct detection experiments will see strong signals from such models, they may not be able to unambiguously determine the presence of the subdominant elastic scattering from the recoil spectra alone. We show that directional detection experiments can separate elastic and inelastic scattering events and discover the underlying dynamics of dark matter models.Comment: 7 pages, 5 figures, references and figures update

Protoemics of integral membrane proteins from developing Brassica napus

Abstract only availableAs plant seeds develop the accumulation of natural products, starch, oil, and protein undergo dramatic changes. At the early stages of seed filling in oilseeds starch is the principal component. Oil (triacylglycerol) and protein concentrations do not reach a maximum until the later stages of seed development. This metabolic shift within the seed, from production of starch to production of oil and protein, indicates that seed metabolism is regulated temporally. To better understand these metabolic changes it is important to examine the cognate changes in protein expression. Integral membrane proteins represent one class of proteins which are important for inter-organellar metabolic flow. Current two-dimensional electrophoresis techniques are unsuitable for the profiling of hydrophobic membrane proteins. To specifically characterize this class of proteins, a reproducible protocol for membrane protein isolation that can be used with standard sodium dodecyl sulfate polyacrylamide gel electrophoresis needed to be developed. Alkaline sodium carbonate washing of membranes followed by ultracentrifugation appeared to yield washed membrane fractions distinct from total protein fractions. To quantify relative volume and molecular weights of individual bands, Coomassie stained gels were analyzed with ImageQuant software. Identification of these bands was performed by trypsin digesting each protein (in-gel) and obtaining accurate peptide mass 'tags' using Matrix Assisted Laser Desorption Ionization-Time of Flight (MALDI-TOF) mass spectrometry. Peptide mass fingerprinting resulted in twelve conclusive identifications (out of 25 analyzed). Of these, six proteins were involved in the glucosinolate-myrosinase defense pathway. These proteins are suspected to be membrane associated, and are involved in a defense system that protects plant tissues from herbivory and fungal, viral, and bacterial infections. Other proteins were identified as: the RuBisCO large subunit, histone H3, NADH dehydrogenase subunit, pyruvate dehydrogenase E1 alpha subunit, and two types of cruciferins which are seed storage proteins. Of these, only NADH dehydrogenase is an integral membrane protein. Based on this data, the alkaline sodium carbonate wash method did not effectively enrich for integral membrane proteins. This may be due largely to the fact that certain proteins, especially cruciferin seed storage proteins, RuBisCO and myrosinases, are expressed at much higher levels than integral membrane proteins and are not quantitatively removed from membrane fractions by salt washing alone. Future work will include alternative approaches to membrane protein isolation including organic extraction.MU Monsanto Undergraduate Research Fellowshi

Pervasive subduction zone devolatilization recycles CO₂ into the forearc

The fate of subducted CO₂ remains the subject of widespread disagreement, with different models predicting either wholesale (up to 99%) decarbonation of the subducting slab or extremely limited carbon loss and, consequently, massive deep subduction of CO₂. The fluid history of subducted rocks lies at the heart of this debate: rocks that experience significant infiltration by a water-bearing fluid may release orders of magnitude more CO₂ than rocks that are metamorphosed in a closed chemical system. Numerical models make a wide range of predictions regarding water mobility, and further progress has been limited by a lack of direct observations. Here we present a comprehensive field-based study of decarbonation efficiency in a subducting slab (Cyclades, Greece), and show that ~40% to ~65% of the CO₂ in subducting crust is released via metamorphic decarbonation reactions at forearc depths. This result precludes extensive deep subduction of most CO₂ and suggests that the mantle has become more depleted in carbon over geologic time