Rapid ligand exchange in the MCRred1 form of methyl-coenzyme M reductase

Methyl-coenzyme M reductase (MCR) from Methanothermobacter marburgensis (Mtm), catalyses the final step in methane synthesis in all methanogenic organisms. Methane is produced by coenzyme B-dependent two-electron reduction of methyl-coenzyme M. At the active site of MCR is the corphin cofactor F(430), which provides four-coordination through the pyrrole nitrogens to a central Ni ion in all states of the enzyme. The important MCRox1 ("ready") and MCRred1 ("active") states contain six-coordinate Ni(I) and differ in their upper axial ligands; furthermore, red1 appears to be two-electrons more reduced than in ox1 and other Ni(II) states that have been studied. On the basis of the reactivity of MCRred1 and MCRox1 with a substrate analogue and inhibitor (3-bromopropanesulfonate) and other small molecules (chloroform, dichloromethane, mercaptoethanol, and nitric oxide), we present evidence that the six-coordinate Ni(I) centers in the MCRred1 and MCRox1 states exhibit markedly different inherent reactivities. MCRred1 reacts faster with chloroform (2100-fold or 35000-fold when corrected for temperature effects), nitric oxide (90-fold), and 3-bromopropanesulfonate (10(6)-fold) than MCRox1. MCRred1 reacts with chloroform and dichloromethane and, like F(430), can catalyze dehalogenation reactions and produce lower halogenated products. We conclude that the enhanced reactivity of MCRred1 is due to the replacement of a relatively exchange-inert thiol ligand in MCRox1 with a weakly coordinating upper axial ligand in red1 that can be easily replaced by incoming ligands.     

Metabolites with Cytotoxic Activity from the Formosan Soft Coral Cladiella Australis

The metabolites, (24S)‐3β‐acetoxyergost‐5‐en‐21‐oic acid ( 2 ), 5′‐O‐acetylthymidine ( 3 ), 3′‐O‐acetylthymidine ( 4 ), and p‐vinylbenzyl alcohol ( 5 ), along with a known steroid ( 1 ) were isolated from the EtOAc extract of the Formosan soft coral Caldiella australis. The structures of new metabolites were determined on the basis of spectroscopic (including 1D and 2D NMR) analyses and by comparison of their NMR spectral data with those of related compounds. Except for 3 , all compounds exhibited cytotoxic activity of various degrees of potency against a limited panel of human liver and breast cancer cell lines.     

Low temperature synthesis of bilayer hydrated cesium cobalt oxide

Bilayer hydrated Na_0.35CoO₂·1.3H₂O structure has re-directed superconductivity research in recent years. Here, we develop a low temperature synthesis method to prepare a novel hydrous Cs_0.2CoO₂·0.63H₂O compound in one step. The bilayer-hydrate of Cs_0.2CoO₂·0.63H₂O with a greatest interlayer spacing d=10.0(2) Å among alkali cobalt oxides has been grown in crystal form. Magnetic susceptibility measurement of Cs_0.2CoO₂·0.63H₂O displays a paramagnetic behavior down to 1.9 K. With the assistance of low temperature molten CsOH solvent, crystals of Rb_0.30CoO₂·0.36H₂O and K_0.35CoO₂·0.4H₂O can be grown. The results provide the capability for preparing a novel hydrous structure and the systematic investigation of interlayer coupling effect of alkali ion insertion compounds.     

E.S.R. of a one-dimensional Heisenberg antiferromagnet which undergoes a crystalline to discotic phase transition

Abstract

Single crystals and polycrystalline samples of the compounds bis(1,3-di(p-n-octylphenyl)propane-l,3-dionato)copper(II) [I], and bis(l,3-di(p-n-octyloxy-phenyl)propane-l,3-dionato)copper(II) [II] have been studied by E.S.R. The E.S.R. spectra of single crystals of [I] at 298 K have the features associated with a spin 1/2 one-dimensional Heisenberg antiferromagnet while the spectra of single crystals of [II] do not. E.S.R. of single crystals of [I] and [II] provides a reliable means of detecting previously reported crystalline to discotic phase transitions. Single crystals of [I] heated beyond the temperature of the first discotic phase transition yield single line E.S.R. spectra which have an angular dependence and an asymmetric lineshape; these observations suggest that, in the discotic phase, exchange interactions are still significant and a degree of long range order is maintained.     

Melting of mesoscopic lattices of magnetic fluid thin film subjected to perpendicular fields

Applying a magnetic field on the magnetic fluid thin film perpendicularly, leads a phase separation that is concentrated in particles separating from a dilute phase. The concentrated phase forms cylindrical columns that construct two-dimensional lattices. This kind of artificial lattice is a novel mesoscopic system and has been explored with optical microscope, CCD, and digital imaging analysis. We explore the ordering evolution of the two-dimensional extraordinary lattice by varying the applied field. The ordering of these lattices is analyzed in terms of translational and bond-orientation correlation functions to address the two-dimensional melting.     

Study of Anion Effects on Separation Phenomenon for the Vinyloctylimidazolium Based Ionic Liquid Polymer Stationary Phases in GC

Poly-vinyloctylimidazolium ionic liquid polymers with different counter ions (bromide, hexafluorophosphate and bis-trifluoromethanesulfonylimide) were directly coated in capillary fused silica tubing as the stationary phases for gas chromatography. The anion effects on the polymers were examined and discussed. The results suggest that the poly-vinyloctylimidazolium, bis-trifluoromethanesulfonylimide capillary column has the highest thermal stability and separation efficiency. Column-to-column reproducibility was also studied. The results indicate that anions significantly influence ionic liquid polymers which give them potential to extend the range of options for the stationary phases in gas chromatography.     

Ptychantins from the Liverwort Ptychanthus Striatus

Five new ptychantins, J‐N, along with two known ones, G and I, were isolated from the liverwort Ptychanthus striatus. Their structures were established by NMR spectroscopic analyses.     

Two Novel 15(10→1)Abeomuurolane Sesquiterpenes from Cosmos sulphureus

Two novel 15(10→1)abeomuurolane sesquiterpenes, cosmosoic acid ( 1 ) and cosmosaldehyde ( 2 ), were isolated from the whole plant of Cosmos sulfurous. Their structures were established by a combination of 1D‐ and 2D‐NMR spectroscopic techniques. Additionally, a chemical correlation between 1 and 2 was also established.     

A nanoengineering approach to regulate the lateral heterogeneity of self-assembled monolayers

Using a scanning probe lithography method known as nanografting in conjunction with knowledge of self-assembly chemistry, regulation of the heterogeneity of self-assembled monolayers (SAMs) is demonstrated. While nanografting in single-component thiols produces areas of SAMs with designed geometry and size, nanofabrication in mixed thiol solution yields segregated domains. The reaction mechanism in nanografting differs significantly from self-assembly in mix-and-grow methods, as proven in systematic studies reported in this article and a companion paper of theoretical calculations of the nanografting process. Knowledge of the reaction pathways enables development of methods for shifting the interplay between the kinetics and thermodynamics in SAM formation, and thus the heterogeneity of mixed SAMs. By varying fabrication parameters, such as shaving speed, and reaction conditions, such as concentration and ratio of the components, the lateral heterogeneity can be adjusted ranging from near molecular mixing to segregated domains of several to tens of nanometers.