Summer Lecture Series 2007: Chris Somerville, Director of the Energy Biosciences Institute and an award-winning plant biochemist with Berkeley Lab's Physical Biosciences Division, is a leading authority on the structure and function of plant cell walls. He discusses an overview of some of the technical challenges associated with the production of cellulosic biofuels, which will require an improved understanding of a diverse range of topics in fields such as agronomy, chemical engineering, microbiology, structural biology, genomics, environmental sciences, and socioeconomics.
Abstract not Available.
We report a systematic study of the stoichiometric reactions of isolated arylpalladium hydroxo and halide complexes with arylboronic acids and aryltrihydroxyborates to evaluate the relative rates of the two reaction pathways commonly proposed to account for transmetalation in the Suzuki-Miyaura reaction. On the basis of the relative populations of the palladium and organoboron species generated under conditions common for the catalytic process and the observed rate constants for the stoichiometric reactions between the two classes of reaction components, we conclude that the reaction of a palladium hydroxo complex with boronic acid, not the reaction of a palladium halide complex with trihydroxyborate, accounts for transmetalation in catalytic Suzuki-Miyaura reactions conducted with weak base and aqueous solvent mixtures.
We have isolated single, nuclear, dominant mutations in whole plants of the small crucifer Arabidopsis thaliana which confer a high level of resistance to a sulfonylurea herbicide. The ease and rapidity with which such mutations can be isolated using A.thaliana suggests that the approach may be of broad utility for studies of herbicide resistance and mode of action.
Thermal demagnetization of samples from five sites in mid‐Cretaceous volcaniclastic and carbonate rocks of south‐central Cuba reveals two or more components of magnetization. A pre‐folding magnetite borne component is recognized in all, but was difficult to isolate from secondary components in many specimens. Remagnetization circle analysis, using data for 42 specimens from all five sites, yields a pole at 30.7°N Lat., 193.3°E Long. This pole is discordant with respect to the North American APWP, indicating 43°±16° of anticlockwise rotation and 8°±6° of northward displacement since the mid‐Cretaceous. This result suggests that part or all of Cuba was transported on the Caribbean plate before accretion to North America in the Eocene.
Abstract The collapse of the Bretton Woods System in 1971–3 led to ambitious efforts to reform the international monetary system, mainly through the deliberations of the Committee of Twenty (C-20). Many of the issues considered by the C-20 will be familiar to aficionados of twenty-first century discussions of international monetary reform. Ultimately, attempts to reach a consensus in the C-20 were unsuccessful. This paper seeks to identify explanations for that failure in an effort to limit the likelihood that it will be repeated.
The task of visual object recognition benefits from feature selection as it reduces the amount of computation in recognizing a new instance of an object, and the selected features give insights into the classification process. We focus on a class of current feature selection methods known as embedded methods: due to the nature of multi-way classification in object recognition, we derive an extension of the Relevance Vector Machine technique to multi-class. In experiments, we apply Relevance Vector Machine on the problem of digit classification and study its effects. Experimental results show that our classifier enhances accuracy, yields good interpretation for the selected subset of features and costs only a constant factor of the baseline classifier
Different cluster models of isolated MoVI molybdate species anchored on silica surface were investigated using density functional theory (DFT). Isolated molybdate centers were modeled as either a penta-coordinated mono-oxo species or a tetra-coordinated di-oxo species. Standard free energy changes for interconversion between mono-oxo and di-oxo species indicate that these two species can coexist in equilibrium on the surface of amorphous silica with di-oxo species being the favored species at high temperature and low partial pressures of water. Comparison of Raman-spectra from experiment and DFT suggests that di-oxo species might be the prominent species responsible for the peak at 988 cm-1. This conclusion is strongly supported by the similarity in the EXAFS spectra obtained from experiment and theory. The thermodynamics for H2 reduction of isolated molybdate species were determined and found to be in reasonable agreement with experimental observation. DFT calculations of the structure and properties of the reduced MoIV centers and comparison of these with experimental results suggest that the reduced centers are present as mono-oxo species.
When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (/spl sim/20 /spl mu/m thick) single crystal silicon specimens were tested to failure in a controlled air environment (30/spl plusmn/0.1/spl deg/C, 50/spl plusmn/2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1/spl times/106 to 1/spl times/1011 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation.
A ruthenium-catalyzed intermolecular, anti-Markovnikov hydroamination of vinylarenes with secondary aliphatic and benzylic amines is reported. The combination of Ru(cod)(2-methylallyl)2, 1,5-bis(diphenylphosphino)pentane, and triflic acid was the most effective catalyst of those tested. Control reactions conducted without ligand or acid did not form the amine. The reaction of morpholine, piperidine, 4-phenylpiperazine, 4-BOC-piperazine, 4-piperidone ethylene ketal, and tetrahydroisoquinoline with styrene in the presence of 5 mol % of this catalyst formed the corresponding beta-phenethylamine products in 64-96% yield, with 99% regioselectivity, and without enamine side products. Acyclic amines such as n-hexylmethylamine and N-benzylmethylamine reacted with styrene in 63 and 50% yields, respectively. Alkyl-, methoxy-, and trifluoromethyl-substituted styrenes reacted with morpholine in the presence of this catalyst or a related one containing 1,1'-bis(diisopropylphosphino)ferrocene as ligand to give the products in 51-91%. Further, the hydroamination of alpha-methyl styrene was observed for the first time with a homogeneous transition metal catalyst. Preliminary mechanistic studies showed that the reaction occurred by direct, irreversible, anti-Markovnikov hydroamination and that the mechanism of the ruthenium-catalyzed hydroamination is likely to be distinct from that of the recently reported rhodium-catalyzed reaction.
We have discovered a simple chemical preparation method for silicon which virtually eliminates surface recombination. This has enabled us to rapidly survey wafers from many different boules in order to identify those in which bulk (defect assisted) Shockley–Read–Hall recombination happens to be unusually low. Thus, having minimized the two major sources of electron-hole recombination, we find that Auger recombination becomes dominant, even at carrier densities as low as 3×1015/cm3. The Auger coefficient we observe is significantly larger than the value extrapolated down from high density.
