Excess Thermodynamics of Mixtures Involving Xenon and Light Linear Alkanes by Computer Simulation

Excess molar enthalpies and excess molar volumes as a function of composition for liquid mixtures of xenon + ethane (at 161.40 K), xenon + propane (at 161.40 K) and xenon + n-butane (at 182.34 K) have been obtained by Monte Carlo computer simulations and compared with available experimental data. Simulation conditions were chosen to closely match those of the corresponding experimental results. The TraPPE-UA force field was selected among other force fields to model all the alkanes studied, whereas the one-center Lennard−Jones potential from Bohn et al. was used for xenon. The calculated and for all systems are negative, increasing in magnitude as the alkane chain length increases. The results for these systems were compared with experimental data and with other theoretical calculations using the SAFT approach. An excellent agreement between simulation and experimental results was found for xenon + ethane system, whereas for the remaining two systems, some deviations that become progressively more significant as the alkane chain length increases were observed

Influence of the sterol aliphatic side chain on membrane properties: a molecular dynamics study

Following a recent experimental investigation of the effect of the length of the alkyl side chain in a series of cholesterol analogues (Angew. Chem., Int. Ed., 2013, 52, 12848–12851), we report here an atomistic molecular dynamics characterization of the behaviour of methyl-branched side chain sterols (iso series) in POPC bilayers. The studied sterols included androstenol (i-C0-sterol) and cholesterol (i-C8-sterol), as well as four other derivatives (i-C5, i-C10, i-C12 and i-C14-sterol). For each sterol, both subtle local effects and more substantial differential alterations of membrane properties along the iso series were investigated. The location and orientation of the tetracyclic ring system is almost identical in all compounds. Among all the studied sterols, cholesterol is the sterol that presents the best matching with the hydrophobic length of POPC acyl chains, whereas longer-chained sterols interdigitate into the opposing membrane leaflet. In accordance with the experimental observations, a maximal ordering effect is observed for intermediate sterol chain length (i-C5, cholesterol, i-C10). Only for these sterols a preferential interaction with the saturated sn-1 chain of POPC (compared to the unsaturated sn-2 chain) was observed, but not for either shorter or longer-chained derivatives. This work highlights the importance of the sterol alkyl chain in the modulation of membrane properties and lateral organization in biological membranes

Partial purification of penicillin acylase from Escherichia Coli in poly(ethylene glycol)-sodium citrate aqueous two-phase systems

Studies on the partition and purification of penicillin acylase from osmotic shock extract Escherichia coli were performed in poly (ethylene glycol)- citrate systems. Both partition behavior of the enzyme and total protein are similar to those described in other reports increasing ,with pH and tie-line length and decreasing with PEG molecular weight . However, some selectivity could be attained with PEG 1000 systems and long tie-line at pH 6.9. In these conditions 2.6 fold purification with 83% yield were achieved. Influence of pH on partition shows that is the composition of the system and not the net charge of the enzyme that determine the behaviour in these conditions. Addition of NaCl to PEG 3350 systems significantly increases the partition of the enzyme. Althought protein partition also increased, purification conditions were possible with 1.5 M NaCl where 5.7 fold purification and 85% yield was obtained. This was possible due to the higher hydrophobicity of the enzyme compared to that of most of contaminants proteins

Testicular aging involves mitochondrial dysfunction as well as an increase in UCP2 levels and proton leak

http://www.sciencedirect.com/science/article/B6T36-4V1DJMK-4/2/e5e215dfcd6f54b36d458bf36b793db

Solubilities of oxygen and carbon dioxide in PDMS oils by computer simulation

Bioconversion reactions involving whole cells are usually characterized by low volumetric productivities in aqueous media, due to the low solubility of both the substrates and the reaction products in water. In order to overcome this problem, organic solvents have been used as substrate/product pools, including some environmentally friendly substances in view of their low volatility and toxicity [1]. Silicone oils have successfully been used as production medium in conversion of sterols to steroids with whole cells in monophasic and biphasic systems [1]. For the process scale-up, a number of physical properties of the system are needed, such as viscosity of the production medium and solubility of metabolic gases (oxygen and carbon dioxide) in the organic solvent. Polydimethylsiloxane (PDMS) and polymethylphenylsiloxane have been the most used silicone oils in production media for bioconversion reactions. In the case of polydimethylsiloxane, oils with kinematic viscosities of around 20 cSt are common (molecular weight of 2000). Solubilities of oxygen and carbon dioxide in such silicone oils are very scarce as well as their prediction by molecular simulation. Makrodimitri et al [2] have developed a united atom force field for the calculation of structural and thermodynamic properties of pure PDMS. Using this force field, they have obtained, by simulation, solubilities of oxygen in PDMS (with an average molecular weight of around 6000), as well as solubilities and diffusion coefficients of other penetrant gases, such as n-alkanes, n-perfluoroalkanes and noble gases [3]. This work focused on prediction of the properties of silicone oils of short to moderate chain length (8 to 25 monomers, which corresponds to a molecular weight between around 750 and 2000) by computer simulation, in particular the solubility of oxygen and carbon dioxide in these liquids. Two different force fields were tested against experimental measurements of density for the shortest dimethylsiloxanes methyl terminated: Gromos-43 A1 and the force field developed by Makrodimitri et al specifically for PDMS. Henry’s constants for the dissolution of oxygen and carbon dioxide in PDMS oils were obtained as a function of temperature by computer simulation using both the force fields tested. The calculations were done for PDMS with three different chain lengths: 8, 16 and 25 monomers [(CH3)2SiO units] with methyl termination in all cases. The force field developed by Makrodimitri et al provides the best description of the PDMS oils studied both in terms of density and gas solubility. Liquid structure and interaction between gas solute and solvent were also investigated by simulation

Behaviour of NBD-head group labelled phosphatidylethanolamines in POPC bilayers: a molecular dynamics study

A complete homologous series of fluorescent phosphatidylethanolamines (diCnPE), labelled at the head group with a 7-nitrobenz-2-oxa-1,3-diazo-4-yl(NBD) fluorophore and inserted in 1-palmitoyl, 2-oleoyl-snglycero- 3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics simulations. The longer-chained derivatives of NBD-diCnPE, with n = 14, 16, and 18, are commercially available, and widely used as fluorescent membrane probes. Properties such as location of atomic groups and acyl chain order parameters of both POPC and NBD-diCnPE, fluorophore orientation and hydrogen bonding, membrane electrostatic potential and lateral diffusion were calculated for all derivatives in the series. Most of these probes induce local disordering of POPC acyl chains, which is on the whole counterbalanced by ordering resulting from binding of sodium ions to lipid carbonyl/glycerol oxygen atoms. An exception is found for NBD-diC16PE, which displays optimal matching with POPC acyl chain length and induces a slight local ordering of phospholipid acyl chains. Compared to previously studied fatty amines, acyl chain-labelled phosphatidylcholines, and sterols bearing the same fluorescent tag, the chromophore in NBD-diCnPE locates in a similar region of the membrane (near the glycerol backbone/carbonyl region) but adopts a different orientation (with the NO2 group facing the interior of the bilayer). This modification leads to an inverted orientation of the P–N axis in the labelled lipid, which affects the interface properties, such as the membrane electrostatic potential and hydrogen bonding to lipid head group atoms. The implications of this study for the interpretation of the photophysical properties of NBD-diCnPE (complex fluorescence emission kinetics, differences with other NBD lipid probes) are discussed

Avaliação de propriedades estruturais de membranas lipídicas após substituição do colesterol por análogos fluorescentes

A espectroscopia e a microscopia de fluorescência têm sido usadas em biofísica de membranas há décadas. Como a unidade estrutural básica das membranas biológicas é a bicamada de lípidos e estes não fluorescem, o uso de sondas extrínsecas de membrana é uma necessidade. Contudo, duas questões preocupantes se levantam quanto ao uso de sondas extrínsecas de fluorescência em estudos de membranas. Em primeiro lugar, o comportamento das moléculas de sonda na bicamada (que região da bicamada elas reportam, as suas dinâmicas translacional e rotacional) é frequentemente mal conhecido. Em segundo lugar, na interpretação de resultados de experiências de fluorescência, pode ser difícil distinguir entre propriedades legítimas da membrana e efeitos de perturbação resultantes da incorporação da sonda. Para este efeito, as simulações por dinâmica molecular (MD), ao providenciarem informação detalhada à escala atómica, representam um meio valioso para caracterizar a localização e dinâmica de sondas na bicamada, assim como a magnitude de perturbação que elas induzem na estrutura lipídica [1]. Neste contexto, optimizaram-se, com recurso ao programa Firefly, as estruturas do colesterol e de dois análogos fluorescentes (desidroergoesterol e colestatrienol) ao nível de teoria DFT/R-B3LYP/6-31G(d) e submeteram-se em seguida ao servidor de topologias ATB, inscrevendo simultaneamente as cargas parciais calculadas na topologia molecular. Estas topologias foram utilizadas na construção de modelos de membranas lipídicas constituídas por POPC, colesterol e uma das sondas fluorescentes acima identificadas. Os modelos assim obtidos foram hidratados e sujeitos a simulações de MD, donde se calculou a área por lípido, a espessura e densidade da bicamada, os coeficientes de difusão lateral para as espécies presentes e os parâmetros de ordem das cadeias acilo. As simulações foram efectuadas em ensemble NPT através do pacote de software GROMACS. Análises preliminares permitiram a comparação dos comportamentos na bicamada dos esteróis fluorescentes com o do colesterol, informação vital para validar o uso dos primeiros como análogos fluorescentes do segundo. REFERÊNCIAS [1] Loura, L.M.S.; Prates Ramalho, J.P. Biophys. Rev. 1 (2009), 141

Towards a competitive use of solar driers : a case study for the lumber industry

The aim of this paper is to contribute to the discussion of the methodology that leads to a better systematization of the knowledge on solar drying. Based on a case study for the lumber industry, the options and solutions adopted will be reported, along with their evaluation criteria and existing or developed tools. The kilns have 50 m3 interior capacity and proved the capability to dry maritime pine 27 mm thick, from green to 12% moisture content in about 33 days. The performance of the drying process has a significant seasonal and weather dependence, so an interactive control system is essential in order to profit as much as possible from the favorable exterior conditions