Iridium(I) and Iridium(III) Complexes Supported by a Diphenolate Imidazolyl-Carbene Ligand

Deprotonation of 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride (1a) followed by reaction with chloro-1,5-cyclooctadiene Ir(I) dimer affords the anionic Ir(I) complex [K][{OCO}Ir(cod)] (2: OCO = 1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl; cod = 1,5-cyclooctadiene), the first Ir complex stabilized by a diphenolate imidazolyl-carbene ligand. In the solid state 2 exhibits square-planar geometry, with only one of the phenoxides bound to the metal center. Oxidation of 2 with 2 equiv of [FeCp_2][PF_6] generates the Ir(III) complex [{OCO}Ir(cod)(MeCN)][PF_6] (3). Reaction of 3 with H_2 results in the liberation of cyclooctane and a species capable of catalyzing the hydrogenation of cyclohexene to cyclohexane. Displacement of cyclooctadiene from 3 can be achieved by heating in acetonitrile to form [{OCO}Ir(MeCN)3][PF_6] (4) or by reaction with either PMe_3 or PCy_3 to generate [{OCO}Ir(PMe_3)_3][PF_6] (5) or [{OCO}Ir(PCy_3)_2(MeCN)][PF_6] (6), respectively. 6 reacts with CO in acetonitrile to give an equilibrium mixture of 6 and [{OCO}Ir(PCy_3)_2(CO)][PF_6] (7) and with chloride to generate [{OCO}Ir(PCy_3)_(2)Cl] (8). The solid-state structure of 8 shows that the diphenolate imidazolyl-carbene ligand is distorted from planarity; DFT calculations suggest this is due to an antibonding interaction between the phenolates and the metal center in the highest occupied molecular orbital (HOMO) of the complex. 8 undergoes two successive reversible one-electron oxidations in CH_(2)Cl_2 at −0.22 and at 0.58 V (vs ferrocene/ferrocenium); EPR spectra, mass spectroscopy, and DFT calculations suggest that the product of the first oxidation is [{OCO}Ir(PCy_3)_(2)Cl]+ (8+), with the unpaired electron occupying a molecular orbital that is delocalized over both the metal center and the diphenolate imidazolyl-carbene ligand

Comparing Simulations and Observations of the Lyman-Alpha Forest I. Methodology

We describe techniques for comparing spectra extracted from cosmological simulations and observational data, using the same methodology to link Lyman-alpha properties derived from the simulations with properties derived from observational data. The eventual goal is to measure the coherence or clustering properties of Lyman-alpha absorbers using observations of quasar pairs and groups. We quantify the systematic underestimate in opacity that is inherent in the continuum fitting process of observed spectra over a range of resolution and signal-to-noise ratio. We present an automated process for detecting and selecting absorption features over the range of resolution and signal-to-noise of typical observational data on the Lyman-alpha "forest". Using these techniques, we detect coherence over transverse scales out to 500 h^{-1}_{50} kpc in spectra extracted from a cosmological simulation at z = 2.Comment: 52 pages, includes 14 figures, to appear in ApJ v566 Feb 200

InP homojunction solar cell performance on the LIPS 3 flight experiment

Performance data for the NASA Lewis Research Center indium phosphide n+p homojunction solar cell module on the LIPS 3 Flight Experiment is presented. The objective of the experiment is to measure the performance of InP cells in the natural radiation environment of the 1100 km altitude, 60+ deg inclination orbit. Analysis of flight data indicates that the performance of the four cells throughout the first year is near expected values. No degradation in short-circuit current was seen, as was expected from radiation tolerance studies of similar cells. Details of the cell structure and flight module design are discussed. The results of the temperature dependency and radiation tolerance studies necessary for normalization and analysis of the data are included

Gravitational waves from an early matter era

We investigate the generation of gravitational waves due to the gravitational instability of primordial density perturbations in an early matter-dominated era which could be detectable by experiments such as LIGO and LISA. We use relativistic perturbation theory to give analytic estimates of the tensor perturbations generated at second order by linear density perturbations. We find that large enhancement factors with respect to the naive second-order estimate are possible due to the growth of density perturbations on sub-Hubble scales. However very large enhancement factors coincide with a breakdown of linear theory for density perturbations on small scales. To produce a primordial gravitational wave background that would be detectable with LIGO or LISA from density perturbations in the linear regime requires primordial comoving curvature perturbations on small scales of order 0.02 for Advanced LIGO or 0.005 for LISA, otherwise numerical calculations of the non-linear evolution on sub-Hubble scales are required.Comment: 23 pages, 2 figure

Structure of yeast Argonaute with guide RNA

The RNA-induced silencing complex, comprising Argonaute and guide RNA, mediates RNA interference. Here we report the 3.2 Å crystal structure of Kluyveromyces polysporus Argonaute (KpAGO) fortuitously complexed with guide RNA originating from small-RNA duplexes autonomously loaded and processed by recombinant KpAGO. Despite their diverse sequences, guide-RNA nucleotides 1–8 are positioned similarly, with sequence-independent contacts to bases, phosphates and 2′-hydroxyl groups pre-organizing the backbone of nucleotides 2–8 in a near-A-form conformation. Compared with prokaryotic Argonautes, KpAGO has numerous surface-exposed insertion segments, with a cluster of conserved insertions repositioning the N domain to enable full propagation of guide–target pairing. Compared with Argonautes in inactive conformations, KpAGO has a hydrogen-bond network that stabilizes an expanded and repositioned loop, which inserts an invariant glutamate into the catalytic pocket. Mutation analyses and analogies to ribonuclease H indicate that insertion of this glutamate finger completes a universally conserved catalytic tetrad, thereby activating Argonaute for RNA cleavage.National Institutes of Health (U.S.) (grant AI068776)National Institutes of Health (U.S.) (grant GM61835)Human Frontier Science Program (Strasbourg, France) (Long-term Fellowship)Japan Society for the Promotion of ScienceHoward Hughes Medical Institute (Investigator)National Science Foundation (U.S.) (Graduate Research Fellowship

Competitive Oxidation and Protonation of Aqueous Monomethylplatinum(II) Complexes: A Comparison of Oxidants

[Pt^(II)(CH_3)Cl_3]^(2-) (1), generated at 95 °C in situ from Cs_2[Pt^(IV)(CH_3)_2Cl_4] in an aqueous solution of high chloride concentration and [H^+] = 0.2 M, undergoes competitive oxidation versus protonation (k_(ox)/k_(H+)) with several oxidants. A first-order dependence on oxidant concentration was determined for both CuCl_2 and FeCl_3 oxidations of 1, and k_(ox)/k_(H+) was determined to be 191 ± 24 and 14 ± 3. CuCl_2 was shown to catalyze the oxidation of 1 by dioxygen; however, [Pt^(II)Cl_4]^(2-) was also oxidized under these conditions. Anion 1, generated in a mixture of platinum(II) salts, [Cp_2Co^(III)]_2{[Pt^(II)Cl_4] + 1 + [Pt^(II)(CH_3)_2Cl_2] (4)}·xNaCl (5), also undergoes competitive oxidation and protonation at room temperature in D_2O when in the presence of oxidants. Increasing chloride decreases the ratio k_(ox)/k_(H+) for 1 when Na_2[Pt^(IV)Cl_6] is used as the oxidant, but when CuCl_2 is used as the oxidant, added chloride increases k_(ox)/k_(H+). The one-electron oxidants, Na_2[IrCl_6] and (NH_4)_2[Ce(NO_3)_6], were also shown to oxidize 1

Discovery and biochemical characterization of RNA interference in budding yeast

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis. Vita.Includes bibliographical references.RNA interference (RNAi) is a eukaryotic pathway for the post-transcriptional regulation of gene expression. In the simplest form of RNAi, a double-stranded RNA (dsRNA) trigger is converted into small-RNA duplexes by the Dicer enzyme. These duplexes are then loaded into the effector protein Argonaute to guide the cleavage of target transcripts. RNAi and related RNA-silencing pathways are found in plants, animals, fungi, and protists, suggesting origins in an early eukaryotic ancestor and selective pressures to maintain the pathway. A prominent exception to this widespread conservation of RNAi is the budding yeast Saccharomyces cerevisiae, which lacks homologs of Dicer and Argonaute. Indeed, RNAi had been presumed lost in all budding yeasts. Motivated by the presence of Argonaute homologs in some budding-yeast species, we examined whether these species contain a functional RNAi pathway. High-throughput sequencing led to the identification of endogenous small RNAs that are generated by a novel Dicer enzyme. In Saccharomyces castellii, these Argonaute-bound small RNAs serve as guides to repress mRNA targets, which are predominantly repetitive elements. RNAi can be restored to S. cerevisiae by introducing the genes encoding S. castellii Dicer and Argonaute, and the reconstituted pathway silences endogenous transposons. Budding-yeast Dicer has a different domain architecture than canonical Dicer yet generates siRNAs of a similar length. In contrast to canonical Dicer, which successively removes small-RNA duplexes from the dsRNA termini, budding-yeast Dicer molecules bind cooperatively to the interior of dsRNA substrates, with the distance between adjacent active sites determining product length. These distinct mechanisms impart corresponding substrate preferences and product characteristics that are important for Dicer function. Structural studies of budding-yeast Argonaute yielded a crystal structure of the functional Argonaute-guide complex. Eukaryotic Argonaute makes extensive sequence-independent interactions with the guide RNA to maintain the seed region in a helical conformation with the base edges accessible for target binding. An invariant glutamate residue, which is only positioned in the catalytic pocket after guide-RNA binding, constitutes the previously missing component of a ribonuclease H-like active site.by David E. Weinberg.Ph.D