Orientational order and finite strain in nematic elastomers

Nematic elastomers exhibit large, spontaneous shape changes at the transition from the high-temperature isotropic phase to the low-temperature nematic phase. These finite deformations are studied here in the context of a nonlinear, properly invariant, variational theory that couples the orientational order and elastic deformation. The theory is based on the minimization of a free-energy functional that consists of two contributions: a nematic one due to the interaction of the mesogenic units and an elastic one arising from the stretching of the cross-linked polymer chains. Suitable choices for these two contributions allow for large, reversible, spontaneous shape changes in which the elastic deformation can affect the isotropic-nematic transition temperature. The change in transition temperature as well as the magnitude of the resulting spontaneous deformation is illustrated for various parameter values. The theory includes soft elasticity as a special case but is not restricted to it.     

Single‐Molecule Nanocatalysis Shows In Situ Deactivation of Pt/C Electrocatalysts during the Hydrogen‐Oxidation Reaction

By coupling a Pt‐catalyzed fluorogenic reaction with the Pt‐electrocatalyzed hydrogen‐oxidation reaction (HOR), we combine single‐molecule fluorescence microscopy with traditional electrochemical methods to study the real‐time deactivation kinetics of a Pt/C electrocatalyst at single‐particle level during electrocatalytic hydrogen‐oxidation reaction. The decay of the catalytic performance of Pt/C could be mainly attributed to the electrocatalysis‐induced etching or dissolution of Pt nanoparticles. Spontaneous regeneration of activity and incubation period of the Pt electrocatalyst were also observed at single‐particle level. All these new insights are practically useful for the understanding and rational design of highly efficient electrocatalysts for application in fuel cells.     

Photoprotection in Cyanobacteria: Regulation of Light Harvesting†

To cope with a rapidly fluctuating light environment, vascular plants and algae have evolved a photoprotective mechanism that serves to downregulate the transfer of excitation energy in the light‐harvesting complexes to the photosynthetic reaction centers. This process dissipates excess excitation energy in the chlorophyll pigment bed by a nonradiative pathway. Since this pathway competes with and therefore quenches chlorophyll fluoresence in a nonphotochemical manner, it has been termed Non‐photochemical Quenching (NPQ). For many years, cyanobacteria were not considered capable of performing NPQ as a photoprotective mechanism. Instead, the redistribution of the phycobilisome (PBS) light‐harvesting antenna between reaction centers by a process called state transitions was considered the major means of regulating the utilization of harvested light energy. Recently, it was demonstrated that cyanobacteria are able to use NPQ as one component of their photoprotective strategies. Cyanobacteria exhibit significant NPQ during nutrient‐replete growth, but it becomes a more prominent means of managing absorbed excitation energy when the cells experience iron starvation. Rapid progress in understanding the molecular mechanism of cyanobacterial NPQ has revealed a process that is very distinct from the functionally analogous process in plants and algae. Cyanobacterial NPQ involves the absorption of blue light by a carotenoid binding protein, termed the Orange Carotenoid Protein, and most likely involves quenching in the PBS core. In this study, we summarize work leading to the discovery of NPQ in cyanobacteria and the elucidation of molecular mechanisms associated with this important photoprotective process.     

Aspergillazines A-E: novel heterocyclic dipeptides from an Australian strain of Aspergillus unilateralis

Biological and chemical profiling of an Australian strain of the fungus Aspergillus unilateralis(MST-F8675), isolated from a soil sample collected near Mount Isa, Queensland, revealed a complex array of metabolites displaying broad chemotherapeutic properties. Noteworthy among these metabolites were a unique series of highly modified dipeptides aspergillazines A-E, incorporating a selection of unprecedented and yet biosynthetically related heterocyclic systems. Co-occurring with the aspergillazines was the recently described marine-derived fungal metabolite trichodermamide A (cf. penicillazine), whereas re-fermentation of A. unilateralis in NaCl (1%) enriched media resulted in co-production of the only other known example of this structure class, the marine-derived fungal metabolite trichodermamide B. Further investigation of A. unilateralis returned the known terrestrial fungal metabolite viridicatumtoxin as the cytotoxic and antibacterial principle, together with E-2-decenedioic acid, ferulic acid, (7E,7'E)-5,5'-diferulic acid and (7E,7'E)-8,5'-diferulic acid. The aromatic diacids have previously been reported from the chemical and enzymatic (esterase) treatment of plant cell wall material, with their isolation from A. unilateralis being their first apparent reported occurrence as natural products. Structures for all metabolites were determined by detailed spectroscopic analysis and, where appropriate, comparison to literature data and/or authentic samples.     

Supramolecular assemblies of DNA with neutral nucleoside amphiphiles

A neutral uridine-based amphiphile is described which condenses plasmid DNA. AFM studies show that the three distinct structural components of the amphiphile (i.e, nucleobase, alkyl chains, and poly(ethylene glycol)) are required for the formation of DNA-amphiphile supramolecular assemblies on a mica surface.     

Anatomical-Molecular Distribution of EphrinA1 in Infarcted Mouse Heart Using MALDI Mass Spectrometry Imaging

EphrinA1 is a tyrosine kinase receptor localized in the cellular membrane of healthy cardiomyocytes, the expression of which is lost upon myocardial infarction (MI). Intra-cardiac injection of the recombinant form of ephrinA1 (ephrinA1-Fc) at the time of ligation in mice has shown beneficial effects by reducing infarct size and myocardial necrosis post-MI. To date, immunohistochemistry and Western blotting comprise the only experimental approaches utilized to localize and quantify relative changes of ephrinA1 in sections and homogenates of whole left ventricle, respectively. Herein, we used matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) coupled with a time-of-flight mass spectrometer (MALDI/TOF MS) to identify intact as well as tryptic fragments of ephrinA1 in healthy controls and acutely infarcted murine hearts. The purpose of the present study was 3-fold: (1) to spatially resolve the molecular distribution of endogenous ephrinA1, (2) to determine the anatomical expression profile of endogenous ephrinA1 after acute MI, and (3) to identify molecular targets of ephrinA1-Fc action post-MI. The tryptic fragments detected were identified as the ephrinA1-isoform with 38% and 34% sequence coverage and Mascot scores of 25 for the control and MI hearts, respectively. By using MALDI-MSI, we have been able to simultaneously measure the distribution and spatial localization of ephrinA1, as well as additional cardiac proteins, thus offering valuable information for the elucidation of molecular partners, mediators, and targets of ephrinA1 action in cardiac muscle.

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Molecular mechanics analysis of the conformations of thymidine and implications for the design of anti-AIDS drugs

The steric energies of a number of the conformations of thymidine, derived from appropriate crystal structures, have been determined with the MM2 molecular mechanics force field. With one exception, these have very similar values which are close to 120 kJ mol^–1. The C2-exo/C3-endo conformation has a far higher value of 240 kJ mol^–1. The C3-endo conformation has previously been associated with thymidine derivatives which do not exhibit anti-HIV activity, and it may be that this is due to the relatively low predicted stability.     

On acyclic and unicyclic graphs whose minimum rank equals the diameter

The minimum rank of a graph G is defined as the smallest possible rank over all symmetric matrices governed by G. It is well known that the minimum rank of a connected graph is at least the diameter of that graph. In this paper, we investigate the graphs for which equality holds between minimum rank and diameter, and completely describe the acyclic and unicyclic graphs for which this equality holds.     

Chiral Thiazolidine-2-Thione Derived Iminium Salts as Reagents for Kinetic Resolution

Abstract

There are currently few simple reagent systems available which can effect efficient kinetic resolution of racemic compounds. We report here a series of chiral bicyclic iminium salts, which can resolve racemic secondary alcohols to give both the unreacted alcohol and the sulphide resulting from reaction in moderate enantiomeric excess.