Modeling the induction of lipid membrane electropermeabilization

Experiments show significant effects of an electric field on lipid membrane, leading to a pore formation when a high intensity field is applied. The phenomenon of electroporation is preceded by the induction and expansion of defects, responsible for the pre-pore excitation. We examine the mechanism of the induction of the field-driven defects by Monte Carlo simulations. The study is based on the improved Pink's model, which includes explicit interactions between the polar heads and energy of interactions between the heads and the field. No anomalous deformation of the molecules is considered. The study, provided for bilayer dipalmitoyl-phosphatidylcholine (DPPC) membrane in the gel (300 K) and fluid (330 K) phases, shows dependence of the membrane conformational and energetical state on the value of the electric field. We observe that the electric field affects the number of molecules in the gel and in the fluid states. In the layer at the negative potential, when the transmembrane voltage is above U(c) approximately 280 mV, lipid heads abruptly reorient and the number of local spots with fluid conformation increases. The other layer slightly tends to tighten its structure, producing additional mechanical stress between layers. Lipids showed complete insensitivity to the electric field within physiological limits, U<70 mV.     

Similarities and differences between azomethines and ketimines: synthesis, materials characterization and structure of novel imines compounds

Imines (ketimines and azomethines) derived from p-dibenzoylbenzene (DB) and terephthalic aldehyde (TA) and two aromatic amines: aniline and 2,6-dimethylaniline have been investigated. Compounds were synthesized via condensation of amines with carbonyl monomers in DMA or amine solution. When using DMA as a solvent, azomethines with high yields were obtained. On the other hand, the amines used as a monomers served also as an effective solvent for the synthesis of the ketanils. This different reactivity of the aldehyde and ketone groups in DMA and in amine depends on the dehydration mechanism being dominated by a kinetic process or thermodynamic one. On the basis of FTIR, 13C and 1H NMR, UV-vis spectra, thermal characteristic and theoretical calculations conclusions are drawn regarding the similarities and differences between azomethines and ketimines.     

Highly asymmetric synthesis of (+)-corsifuran A. Elucidation of the electronic requirements in the Ruthenium–NHC catalyzed hydrogenation of benzofurans

A short and efficient synthesis of ent-corsifuran A by a highly asymmetric hydrogenation of a benzofuran precursor is reported. In addition, the electronic influence of the substituents on the asymmetric hydrogenation of benzofurans is provided. Whereas the hydrogenation of electron-deficient benzofurans was achieved under very mild conditions, the presence of electron-donating groups in the benzofuran required harsher reaction conditions for achieving full conversion to the 2,3-dihydrobenzofuran.     

Extended Taylor series and interpolation of physically meaningful functions

An extension of the concept of the Taylor series to arbitrary functions that are physically meaningful is presented. The series is obtained using a matrix to describe the behaviour of the function at any position of discontinuity while performing the Taylor series expansion in the rest of the considered domain. The matrix is derived from the continuity conditions that are inherent to a particular physical problem. This allows an introduction of function classes that are relevant from the physical problem point of view. Several illustrative examples of physically meaningful function classes are derived. It is then demonstrated that using the derived function classes and the concept of the extended Taylor series one can obtain the interpolation formulae fitted for a particular class of physically meaningful functions. The application of the interpolation formulae based on these novel concepts shows clear advantages over the standard approach. Finally, it is also shown that the developed concept of the extended Taylor series and classes of physically meaningful functions can be used for the extrapolation and an elegant derivation of finite difference approximations for physical problems.     

Ligand Recognition Processes in the Formation of Homochiral C3‐Symmetric LnL3 Complexes of a Chiral Alkoxide

The reaction of a chiral racemic bidentate ligand HL¹ (tBu₂P(O)CH₂CH(tBu)OH) with mid to late trivalent lanthanide cations affords predominantly homochiral lanthanide complexes (RRR)‐[Ln(L¹)₃] and (SSS)‐[Ln(L¹)₃]. A series of reactions are reported that demonstrate that the syntheses are under thermodynamic control, and driven by a ligand ‘self‐recognition’ process, in which the large asymmetric bidentate L¹ ligands pack most favourably in a C₃ geometry around the lanthanide cation. The synthesis of bis(L¹) adducts [Ln(L¹)₂X] (X=N(SiMe₃)₂, OC₆H₃tBu‐2,6) is also reported. Analysis of the diastereomer mixtures shows that homochiral (L¹)₂ complexes are favoured but to a lesser extent. The complexes Ln(L¹)₃ and [Ln(L¹)₂(OC₆H₃tBu‐2,6)] have been studied as initiators for the polymerization of ε‐caprolactone and its copolymer with lactide, glycolide and its copolymer with lactide, and ε‐caprolactam.     

Influence of wood mercerization on the crystallization of polypropylene in wood/PP composites

Mercerization process is very significant because the alkali treatment facilitates reactivity of lignocellulosic fillers, thus allowing better response to chemical modification. In the present study, the effect of mercerization of pine wood on the nucleation ability of polypropylene was investigated by means of differential scanning calorimetry. We discovered that for the composites with wood containing cellulose II, the decrease in the crystal conversion of the polymer matrix and increase in the half-time of crystallization values are significant. It can be concluded that the amount of cellulose II formed upon alkalization of lignocellulosic fillers determines their nucleation ability. To evaluate the transcrystalline effects caused by various woods, which were untreated or treated with sodium hydroxide, the polarized optical microscopy was also performed. The nucleation of polypropylene on the surface of wood was investigated by induction time measurement. It was found that surfaces of the unmodified wood generate epitaxial nucleation, whereas the mercerized wood generates nonepitaxial nucleation. The differences in the type of nucleation suggest that the effectiveness of formation of transcrystalline structures depends on the contribution of cellulose I and cellulose II. Moreover, the presence of epitaxy is not necessary for the appearance of transcrystalline structures. The results showed that the transcrystalline structures appeared in each system, even with wood containing significant contribution of cellulose II. The only difference noted was the change in the nucleation abilities of the wood surface. Results of this study imply the necessity of quantitative determination of the contributions of cellulose I and cellulose II, whose presence determine the type of nucleation and nucleation ability of the filler surface.     

Intermolecular interactions in electron transfer through stretched helical peptides

The helical peptide Cys-Ala-Lys-(Glu-Ala-Ala-Ala-Lys)(2)-Ala-NH-(CH(2))(2)-SH has been organized forming a self-assembled monolayer on gold (0.602 peptides per nm(2)), its conductance behavior under stretching conditions being studied using scanning tunnelling microscopy and current sensing atomic force microscopy. The helical conformation of the peptide has been found to play a fundamental role in the conductance. Moreover, variation of the current upon molecular stretching indicates that peptides can be significantly elongated before the conductance drops to zero, the critical elongation being 1.22 ± 0.47 nm. Molecular dynamics simulations of a single peptide in the free state and of a variable number of peptides tethered to a gold surface (i.e. densities ranging from 0.026 to 1.295 peptides per nm(2)) have indicated that the helical conformation is intrinsically favored in solvated environments while in desolvated environments it is retained because of the fundamental role played by peptide-peptide intermolecular interactions. The structure obtained for the system with 24 tethered peptides, with a density of 0.634 peptides per nm(2) closest to the experimental one, is in excellent agreement with experimental observations. On the other hand, simulations in which a single molecule is submitted to different compression and stretching processes while the rest remain in the equilibrium have been used to mimic the variation of the tip-substrate distance in experimental measures. Results allowed us to identify the existence, and in some cases coexistence, of intermolecular and intramolecular ionic ladders, suggesting that peptide-mediated electron transfer occurs through the hopping mechanism. Finally, quantum mechanical calculations have been used to investigate the variation of the electronic structure upon compression and stretching deformations.     

Rapid screening of antibiotic toxicity in an automated microdroplet system

We report an automated microfluidic platform for 'digitally' screening the composition space of droplets containing cocktails of small molecules and demonstrate the features of this system by studying epistatic interactions between antibiotics and Escherichia coli ATCC 25922. This system has several key characteristics: (i) it uses small (<100 μL) samples of liquids and suspensions of bacteria that are introduced directly into the chip; (ii) it generates a sequence of droplets with compositions, including reagents and bacterial cell suspensions that are programmed by the user; (iii) it exports the sequence of droplets to an external segment of tubing that is subsequently disconnected for incubation and storage; and (iv) after incubation of bacteria in droplets, the droplets are injected into a second device equipped with an in-line fiber optic spectrophotometer that measures cell growth. The system generates and fuses droplets with precise (<1% in standard deviation) control of liquid volumes and of the concentrations of input substrates. We demonstrate the application of this technology in determining the minimum inhibitory concentration and pair-wise interactions of ampicillin, tetracycline, and chloramphenicol against E. coli. The experiments consumed small volumes of reagents and required minutes to create the droplets and several hours for their incubation and analysis.