Peptide synthesis in aqueous environments: the role of extreme conditions and pyrite mineral surfaces on formation and hydrolysis of peptides

A comprehensive study of free energy landscapes and mechanisms of COS-mediated polymerization of glycine via N-carboxy anhydrides (NCAs, "Leuchs anhydrides") and peptide hydrolysis at the water-pyrite interface at extreme thermodynamic conditions is presented. Particular emphasis is set on the catalytic effects of the mineral surface including the putative role of the ubiquitous sulfur vacancy defects. It is found that the mere presence of a surface is able to change the free energetics of the elementary reaction steps. This effect can be understood in terms of a reduction of entropic contributions to the reactant state by immobilizing the reactants and/or screening them from bulk water in a purely geometric ("steric") sense. Additionally, the pyrite directly participates chemically in some of the reaction steps, thus changing the reaction mechanism qualitatively compared to the situation in bulk water. First, the adsorption of reactants on the surface can preform a product-like structure due to immobilizing and scaffolding them appropriately. Second, pyrite can act as a proton acceptor, thus replacing water in this role. Third, sulfur vacancies are found to increase the reactivity of the surface. The finding that the presence of pyrite speeds up the rate-determining step in the formation of peptides with respect to the situation in bulk solvent while stabilizing the produced peptide against hydrolysis is of particular interest to the hypothesis of prebiotic peptide formation at hydrothermal aqueous conditions. Apart from these implications, the generality of the studied organic reactions are of immediate relevance to many fields such as (bio)geochemistry, biomineralization, and environmental chemistry.     

A class of deformational flow test cases for linear transport problems on the sphere

A class of new benchmark deformational flow test cases for the two-dimensional horizontal linear transport problems on the sphere is proposed. The scalar field follows complex trajectories and undergoes severe deformation during the simulation; however, the flow reverses its course at half-time and the scalar field returns to its initial position and shape. This process makes the exact solution available at the end of the simulation, and facilitates assessment of the accuracy of the underlying transport scheme. A procedure to eliminate possible cancellations of errors when the flow reverses is proposed.     

Studies on the thermal properties of silicone polymer based thermal protection systems for space applications

Journal of Thermal Analysis and Calorimetry - Thermal decomposition, thermal conductivity, specific heat and flammability of polydimethyl siloxane (PDMS) resin and three PDMS-based systems with...     

Use of chemical etching of CR-39 foils at elevated temperature for fast neutron personnel monitoring in India

CR-39 Solid State Nuclear Track Detecting foils (SSNTD), along with 1 mm thick polyethylene radiator, sealed in triple laminated pouches, are used for country wide Fast Neutron Personnel Monitoring in India. With the present system of processing by elevated temperature electrochemical etching (ETECE) and evaluation using automatic image analysis, only 16 foils are processed at a time and it is useful over the dose equivalent range 0.2 mSv to 10 mSv. It has been reported that, by processing CR-39 of good detection efficiency by chemical etching at elevated temperature, more numbers of foils can be processed simultaneously. In the present study, CR-39 foils from Pershore Moulding (UK) have been chemically etched using 7 N KOH under various conditions of temperature and etching durations and evaluated using high magnification microscope. The duration of chemical etching, has been optimized at a constant temperature of 60°C for chemical etching process. The characteristics of the chemically etched CR-39 foils are compared with the characteristics of the CR-39 foils processed by the existing system of ETECE and the detailed results are presented in the full text of the paper. It has been observed that by chemical etching process, the dose equivalent range of CR-39 foils can be extended above 60 mSv.      

Engineered-membranes: A novel concept for clustering of native lipid bilayers

A strategy for clustering of native lipid membranes is presented. It relies on the formation of complexes between hydrophobic chelators embedded within the lipid bilayer and metal cations in the aqueous phase, capable of binding two (or more) chelators simultaneously Fig. 1. We used this approach with purple membranes containing the light driven proton pump protein bacteriorhodopsin (bR) and showed that patches of purple membranes cluster into mm sized aggregates and that these are stable for months when incubated at 19 °C in the dark. The strategy may be general since four different hydrophobic chelators (1,10-phenanthroline, bathophenanthroline, Phen-C10, and 8-hydroxyquinoline) and various divalent cations (Ni²⁺, Zn²⁺, Cd²⁺, Mn²⁺, and Cu²⁺) induced formation of membrane clusters. Moreover, the absolute requirement for a hydrophobic chelator and the appropriate metal cations was demonstrated with light and atomic force microscopy (AFM); the presence of the metal does not appear to affect the functional state of the protein. The potential utility of the approach as an alternative to assembled lipid bilayers is suggested.     

Bis‐tert‐Alcohol‐Functionalized Crown‐6‐Calix[4]arene: An Organic Promoter for Nucleophilic Fluorination

A bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene (BACCA) was designed and prepared as a multifunctional organic promoter for nucleophilic fluorinations with CsF. By formation of a CsF/BACCA complex, BACCA could release a significantly active and selective fluoride source for S_N2 fluorination reactions. The origin of the promoting effects of BACCA was studied by quantum chemical methods. The role of BACCA was revealed to be separation of the metal fluoride to a large distance (>8 Å), thereby producing an essentially “free” F^?. The synergistic actions of the crown‐6‐calix[4]arene subunit (whose O atoms coordinate the counter‐cation Cs⁺) and the terminal tert‐alcohol OH groups (forming controlled hydrogen bonds with F^?) of BACCA led to tremendous efficiency in S_N2 fluorination of base‐sensitive substrates.     

A CAN-induced cyclodimerization-Ritter trapping strategy for the one-pot synthesis of 1-amino-4-aryltetralins from styrenes

[reaction: see text]. A facile one-pot synthesis of 1-amino-4-aryl-tetralin derivatives by the CAN-induced cyclodimerization of various styrenes in acetonitrile and acrylonitrile is described.     

On Variation Functions for Subsequence Ergodic Averages

 Suppose denote the ergodic averages for the natural numbers . Let denote the corresponding maximal function and let for . We show that for if there exists such that then there exists such that . Similar weak (1,1) inequalities follow for V _q when you know them for M too also with q > 1. We also show this fails completely if q= 1. We also show that for certain polynomial like and random sequences , if

A number theoretic estimate of Davenport from a functional analytic viewpoint

We consider the problem of the identification of continuous functionsf∶[0, 1]→R, by means of the sums . This is not possible, in general, but we prove that it may be the case under auxiliary conditions. We also study the behaviour of a well known exceptional function.

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Sunto Consideriamo il problema dell’identificazione delle funzioni continuef:[0,1]→ →R, mediante le somme . Ciò non è, in generale, possibile: dimostriamo però tale possibilità sotto condizioni ausiliarie. Studiamo inoltre il comportamento di una ben nota funzione eccezionale.