Binding, activation, and transformation of carbon dioxide mediated by anionic metal complexes

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2011.Pages 180 and 181 blank. Cataloged from PDF version of thesis.Includes bibliographical references.The vanadium nitride complex [Na][NV(N['Bu]Ar) 3] reacts with CO to produce the vanadium tris-anilide complex V(N['Bu]Ar)3 and NaNCO. This is the first example of complete denitrogenation of a termainal nitride complex with generation of a lower coordinate metal complex. This reactivity contrasts sharply with the reactivity of the niobium analogue, where the nitride anion complex [Na][NNb(N['Bu]Ar) 3] is synthesized from the reductive decarbonylation of the niobium(IV) isocyanate complex (OCN)Nb(N['Bu]Ar) 3. Electrochemical studies of the niobium(IV) and vanadium(IV) isocyanate (OCN)V(N['Bu]Ar) 3 complexes are presented. The reactivity of the vanadium carbamate complex [(THF) 2Na][O2CNV(N['Bu]Ar) 3] with electrophilic reagents is presented. The carbamate complex reacts readily with silylation and alkylation reagents to form the carbamate ester complexes of the type ROC(O)NV(N['Bu]Ar) 3. The vanadium carbamate complex reacts with SO2 via a decarboxylation pathway to produce the sulforyl imido complex [Na][O 2SNV(N['Bu]Ar)3], the solid-state structure of which is presented. The reactivity of the vanadium carbamate complex with typical dehydrating reagents, e.g organic acid anhydrides, is shown to proceed cleanly when cobaltocene, acting as an in situ reductant, is present to form the vanadium(IV) isocyanate complex (OCN)V(N['Bu]Ar) 3. The synthesis and structure of the bimetallic complex (TPP)MnOC(O)NV(N['Bu]Ar) 3 (TPP = tetraphenylporphyrin) is presented. Although thermally stable, the complex undergoes a photochemical transformation that forms the vanadium isocyanate complex and putative OMn(TPP), which reacts with triphenylphosphine in the reaction mixture to produce triphenylphosphine oxide. The synthesis the niobium carbamate complex [Na][O 2CNNb(N['Bu]Ar) 3] from the reaction of [Na][NNb(N[Bu]Ar) 3] with CO2 is presented. Its solid-state structure in the form of the ionpair [(12-crown-4) 2Na][O2CNNb(N['Bu]Ar) 3] has been determined. Reaction of the niobium carbamate complex with organic acid anhydrides results in the production of five-coordinate carboxylate, acetate complexes (RC(O)O)(OCN)Nb(N['Bu]Ar) 3. The reduction of these complexes by two electrons results in the regeneration of the niobium nitride complex (60-80% yield) with concomitant release of CO (30-60% yield). This three-step process represents a highly controlled conversion of CO2 to CO via a ligand-based strategy. The reactivity of CO2 with anionic complexes featuring terminal multiply bonded ligands is extended to the oxo anion complex [(Et 2O)2Li][OTi(N['Bu]Ar) 3] resulting in the formation of the carbonate complex ([Li][O 2COTi(N['Bu]Ar) 3]) 6. The binding of CO2 to the oxo complex is reversible when 12-crown-4 is bound to the lithium countercation or if the complex is dissolved in THF. The thermodynamic parameters for the CO2 binding equilibrium have been measured. Exchanging the lithium countercation for sodium or potassium results in a significant weakening of the CO2 binding ability of the oxo complex.by Jared S. Silvia.Ph.D

Experimental and computational studies on the formation of cyanate from early metal terminal nitrido ligands and carbon monoxide

An important challenge in the artificial fixation of N[subscript 2] is to find atom efficient transformations that yield value-added products. Here we explore the coordination complex mediated conversion of ubiquitous species, CO and N[subscript 2], into isocyanate. We have conceptually split the process into three steps: (1) the six-electron splitting of dinitrogen into terminal metal nitrido ligands, (2) the reduction of the complex by two electrons with CO to form an isocyanate linkage, and (3) the one electron reduction of the metal isocyanate complex to regenerate the starting metal complex and release the product. These steps are explored separately in an attempt to understand the limitations of each step and what is required of a coordination complex in order to facilitate a catalytic cycle. The possibility of this cyanate cycle was explored with both Mo and V complexes which have previously been shown to perform select steps in the sequence. Experimental results demonstrate the feasibility of some of the steps and DFT calculations suggest that, although the reduction of the terminal metal nitride complex by carbon monoxide should be thermodynamically favorable, there is a large kinetic barrier associated with the change in spin state which can be avoided in the case of the V complexes by an initial binding of the CO to the metal center followed by rearrangement. This mandates certain minimal design principles for the metal complex: the metal center should be sterically accessible for CO binding and the ligands should not readily succumb to CO insertion reactions.National Science Foundation (U.S.) (CHE-1111357

Squalamine and Its Derivatives Modulate the Aggregation of Amyloid-β and α-Synuclein and Suppress the Toxicity of Their Oligomers.

The aberrant aggregation of proteins is a key molecular event in the development and progression of a wide range of neurodegenerative disorders. We have shown previously that squalamine and trodusquemine, two natural products in the aminosterol class, can modulate the aggregation of the amyloid-β peptide (Aβ) and of α-synuclein (αS), which are associated with Alzheimer's and Parkinson's diseases. In this work, we expand our previous analyses to two squalamine derivatives, des-squalamine and α-squalamine, obtaining further insights into the mechanism by which aminosterols modulate Aβ and αS aggregation. We then characterize the ability of these small molecules to alter the physicochemical properties of stabilized oligomeric species in vitro and to suppress the toxicity of these aggregates to varying degrees toward human neuroblastoma cells. We found that, despite the fact that these aminosterols exert opposing effects on Aβ and αS aggregation under the conditions that we tested, the modifications that they induced to the toxicity of oligomers were similar. Our results indicate that the suppression of toxicity is mediated by the displacement of toxic oligomeric species from cellular membranes by the aminosterols. This study, thus, provides evidence that aminosterols could be rationally optimized in drug discovery programs to target oligomer toxicity in Alzheimer's and Parkinson's diseases

Identification of Replication Competent Murine Gammaretroviruses in Commonly Used Prostate Cancer Cell Lines

A newly discovered gammaretrovirus, termed XMRV, was recently reported to be present in the prostate cancer cell line CWR22Rv1. Using a combination of both immunohistochemistry with broadly-reactive murine leukemia virus (MLV) anti-sera and PCR, we determined if additional prostate cancer or other cell lines contain XMRV or MLV-related viruses. Our study included a total of 72 cell lines, which included 58 of the 60 human cancer cell lines used in anticancer drug screens and maintained at the NCI-Frederick (NCI-60). We have identified gammaretroviruses in two additional prostate cancer cell lines: LAPC4 and VCaP, and show that these viruses are replication competent. Viral genome sequencing identified the virus in LAPC4 and VCaP as nearly identical to another known xenotropic MLV, Bxv-1. We also identified a gammaretrovirus in the non-small-cell lung carcinoma cell line EKVX. Prostate cancer cell lines appear to have a propensity for infection with murine gammaretroviruses, and we propose that this may be in part due to cell line establishment by xenograft passage in immunocompromised mice. It is unclear if infection with these viruses is necessary for cell line establishment, or what confounding role they may play in experiments performed with these commonly used lines. Importantly, our results suggest a need for regular screening of cancer cell lines for retroviral “contamination”, much like routine mycoplasma testing

52 Genetic Loci Influencing Myocardial Mass.

BACKGROUND: Myocardial mass is a key determinant of cardiac muscle function and hypertrophy. Myocardial depolarization leading to cardiac muscle contraction is reflected by the amplitude and duration of the QRS complex on the electrocardiogram (ECG). Abnormal QRS amplitude or duration reflect changes in myocardial mass and conduction, and are associated with increased risk of heart failure and death. OBJECTIVES: This meta-analysis sought to gain insights into the genetic determinants of myocardial mass. METHODS: We carried out a genome-wide association meta-analysis of 4 QRS traits in up to 73,518 individuals of European ancestry, followed by extensive biological and functional assessment. RESULTS: We identified 52 genomic loci, of which 32 are novel, that are reliably associated with 1 or more QRS phenotypes at p < 1 × 10(-8). These loci are enriched in regions of open chromatin, histone modifications, and transcription factor binding, suggesting that they represent regions of the genome that are actively transcribed in the human heart. Pathway analyses provided evidence that these loci play a role in cardiac hypertrophy. We further highlighted 67 candidate genes at the identified loci that are preferentially expressed in cardiac tissue and associated with cardiac abnormalities in Drosophila melanogaster and Mus musculus. We validated the regulatory function of a novel variant in the SCN5A/SCN10A locus in vitro and in vivo. CONCLUSIONS: Taken together, our findings provide new insights into genes and biological pathways controlling myocardial mass and may help identify novel therapeutic targets

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time,1,2 and attempts to address it require a clear un derstanding of how ecological communities respond to environmental change across time and space.3,4 While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes,5–7 vast areas of the tropics remain understudied.8–11 In the American tropics, Amazonia stands out as the world’s most diverse rainforest and the primary source of Neotropical biodiversity,12 but it remains among the least known forests in America and is often underrepre sented in biodiversity databases.13–15 To worsen this situation, human-induced modifications16,17 may elim inate pieces of the Amazon’s biodiversity puzzle before we can use them to understand how ecological com munities are responding. To increase generalization and applicability of biodiversity knowledge,18,19 it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple or ganism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region’s vulnerability to environmental change. 15%–18% of the most ne glected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lostinfo:eu-repo/semantics/publishedVersio

Search for stop and higgsino production using diphoton Higgs boson decays

Results are presented of a search for a "natural" supersymmetry scenario with gauge mediated symmetry breaking. It is assumed that only the supersymmetric partners of the top-quark (stop) and the Higgs boson (higgsino) are accessible. Events are examined in which there are two photons forming a Higgs boson candidate, and at least two b-quark jets. In 19.7 inverse femtobarns of proton-proton collision data at sqrt(s) = 8 TeV, recorded in the CMS experiment, no evidence of a signal is found and lower limits at the 95% confidence level are set, excluding the stop mass below 360 to 410 GeV, depending on the higgsino mass