First steps in the oxidation of a /110/ nickel surface

Initial steps in oxidation of nickel surface studied using low energy back-reflection electron diffractio

Ammonia adsorption and decomposition on a tungsten /211/ surface

Ammonia adsorption and decomposition on tungsten 211 surface studied by mass spectrometry, electron diffraction, and changes in work functio

Isolation of novel ribozymes that ligate AMP-activated RNA substrates

AbstractBackground: The protein enzymes RNA ligase and DNA ligase catalyze the ligation of nucleic acids via an adenosine-5′-5′-pyrophosphate ‘capped’ RNA or DNA intermediate. The activation of nucleic acid substrates by adenosine 5′-monophosphate (AMP) may be a vestige of ‘RNA world’ catalysis. AMP-activated ligation seems ideally suited for catalysis by ribozymes (RNA enzymes), because an RNA motif capable of tightly and specifically binding AMP has previously been isolated.Results: We used in vitro selection and directed evolution to explore the ability of ribozymes to catalyze the template-directed ligation of AMP-activated RNAs. We subjected a pool of 1015 RNA molecules, each consisting of long random sequences flanking a mutagenized adenosine triphosphate (ATP) aptamer, to ten rounds of in vitro selection, including three rounds involving mutagenic polymerase chain reaction. Selection was for the ligation of an oligonucleotide to the 5′-capped active pool RNA species. Many different ligase ribozymes were isolated; these ribozymes had rates of reaction up to 0.4 ligations per hour, corresponding to rate accelerations of ∼ 5 × 105 over the templated, but otherwise uncatalyzed, background reaction rate. Three characterized ribozymes catalyzed the formation of 3′-5′-phosphodiester bonds and were highly specific for activation by AMP at the ligation site.Conclusions: The existence of a new class of ligase ribozymes is consistent with the hypothesis that the unusual mechanism of the biological ligases resulted from a conservation of mechanism during an evolutionary replacement of a primordial ribozyme ligase by a more modern protein enzyme. The newly isolated ligase ribozymes may also provide a starting point for the isolation of ribozymes that catalyze the polymerization of AMP-activated oligonucleotides or mononucleotides, which might have been the prebiotic analogs of nucleoside triphosphates

UV-light-driven prebiotic synthesis of iron–sulfur clusters

Iron–sulfur clusters are ancient cofactors that play a fundamental role in metabolism and may have impacted the prebiotic chemistry that led to life. However, it is unclear whether iron–sulfur clusters could have been synthesized on prebiotic Earth. Dissolved iron on early Earth was predominantly in the reduced ferrous state, but ferrous ions alone cannot form polynuclear iron–sulfur clusters. Similarly, free sulfide may not have been readily available. Here we show that UV light drives the synthesis of [2Fe–2S] and [4Fe–4S] clusters through the photooxidation of ferrous ions and the photolysis of organic thiols. Iron–sulfur clusters coordinate to and are stabilized by a wide range of cysteine-containing peptides and the assembly of iron–sulfur cluster-peptide complexes can take place within model protocells in a process that parallels extant pathways. Our experiments suggest that iron–sulfur clusters may have formed easily on early Earth, facilitating the emergence of an iron–sulfur-cluster-dependent metabolism

Synthesis, structural analysis, and reactivity of bridged orthoamides by intramolecular schmidt reaction

Intramolecular Schmidt reactions can be reliably steered toward bridged heterocycles containing orthoamides in high yields. The ketal tether enhances the control of regioselectivity in the migration of the bond distal to the reactive azide nucleophile, thus providing the first examples of the intramolecular Schmidt reaction proceeding with a complete regioselectivity en route to bridged products. The method is broad in scope and allows for systematic study of compounds that are analogous to elusive tetrahedral intermediates of amide addition reactions. Some initial reactivity and structural profiling of these compounds are also reported

Corey-Chaykovsky epoxidation of twisted amides: Synthesis and reactivity of bridged spiro-epoxyamines

The first example of a Corey-Chaykovsky epoxidation employing amides as substrates is described. Medium-bridged twisted amides serve as precursors to a family of isolable aminoepoxides. The reactivity of bridged spiro-epoxyamines is also investigated

Proximity effects in nucleophilic addition reactions to medium-bridged twisted lactams: Remarkably stable tetrahedral intermediates

The reactions of a series of strained bicyclic and tricyclic one-carbon bridged lactams with organometallic reagents have been investigated. These amides permit isolation of a number of remarkably stable hemiaminals upon nucleophilic addition to the twisted amide bonds present in the lactam precursors. The factors that affect the stability of the resulting bridged hemiaminals are presented. In some cases, the hemiaminals were found to collapse to the open-form amino ketones in a manner expected for traditional carboxylic acid derivatives. Transannular N · · · C=O interactions were also observed in some nine-membered amino ketones. Additionally, tricyclic bridged lactams were found to react with some nucleophiles that typically react with ketones but not with planar amides. The effect of geometry on the reactivity of amide bonds and the amide bond distortion range that marks the boundary of amide-like and ketone-like carbonyl reactivity of lactams are also discussed

Real Time Global Tests of the ALICE High Level Trigger Data Transport Framework

The High Level Trigger (HLT) system of the ALICE experiment is an online event filter and trigger system designed for input bandwidths of up to 25 GB/s at event rates of up to 1 kHz. The system is designed as a scalable PC cluster, implementing several hundred nodes. The transport of data in the system is handled by an object-oriented data flow framework operating on the basis of the publisher-subscriber principle, being designed fully pipelined with lowest processing overhead and communication latency in the cluster. In this paper, we report the latest measurements where this framework has been operated on five different sites over a global north-south link extending more than 10,000 km, processing a ``real-time'' data flow.Comment: 8 pages 4 figure

Improved Method of Springback Compensation in Metal Forming Analysis

This paper presents an innovative compensation method for both forming and trimming die for the construction of vehicle parts manufactured by a transfer press. This method allows one to optimize the accuracy of compensation, consequently reducing springback in a more exact way than currently used methods, which account only for the influence of trimming on springback without generating compensated surfaces for the trimming die and the next forming operation. Moreover, there is a possibility to include the positioning effect during multioperation forming because of the huge impact of results on separate operations. When the positioning is not considered between operations there are problems with a proper shape even if the final part has the correct geometry. These problems generate some additional costs during the die production, which can be avoided by using the multioperation compensation