1,721,110 research outputs found
Ionic Peptide Aggregation: Exploration of Conformational Dynamics in Aqueous Solution by Computational Techniques
No full textThe effects of end groups on KEK peptide conformational characteristics and self-assembling properties in water solution are investigated by using long lasting all-atom molecular dynamics simulations. The analysis of the structural macroscopic and microscopic properties and the examination of intra- and intermolecular interactions suggest, in agreement with experimental observations, the role played by side chains and terminal regions in determining the characteristic features of the assemblages. Competition between intra- and interchain interactions greatly affects the diffusivity of peptide molecules and the conformational space that they can sample, ultimately controlling the shape, size, and distribution of the aggregate configurations. Different peptide end groups influence peptide flexibility and seem to play a crucial role in determining the aggregates' supramolecular architectures
Thermodynamic Study of Perfluorohexane + Ether Mixtures. VLE, LLE, and HE
No full textof perfluoro-n-hexane plus an ether (diethyl, dipropyl, dibutyl, butyl methyl, and butyl ethyl ether), have
been determined using a head-space gas-chromatographic technique, a turbidimetric apparatus, and a
heat-flow calorimeter, respectively. A recently designed titration technique and calculation procedure
have been used to obtain HE from heats of solution. The observed liquid–liquid coexistence curves have
been compared with those predicted by the activity coefficients i and their temperature dependence.
All mixtures are strongly endothermic (HE > 0) and show large positive deviations from ideality (GE > 0),
which increase with the size of the ether. Molecular interactions have been examined by calculating and
discussing solvation functions and Kirkwood–Buff (KB) integrals. Perfluorohexane proved to be an inert
molecule that interacts with ethers more weakly than hexane
Thermodynamic study of (perfluoroalkane plus alkane) mixtures: Excess and solvation enthalpies
No full textA newly designed calorimetric technique and calculation procedure have been used to obtain partial molar enthalpies, Hi, and excess
enthalpies, HE, for binary mixture of hexane + perfluoro-n-alkanes (C5–C8) and perfluorohexane + n-alkanes (C5–C8), + cyclohexane,
and + 2-methylheptane. All mixtures are endothermic, and the heat effects increase with the size of the second component. The HE and
Hi values found are the largest ever observed for mixtures of non-polar compounds. An estimate of the excess heat capacity for (perfluorohexane
+ 2-methylheptane) has been obtained from Hi at two different temperatures. From Hi at infinite dilution and from the
known enthalpies of vaporization, the enthalpies of solvation, DsolvH, have been evaluated either for alkanes and perfluoroalkanes
in both hexane and perfluorohexane solvent. Solute–solvent interactions have been examined by describing the DsolvH with an additive
scheme of surface interactions and by applying the Scaled Particle Theory. The effects of chain lengthening, branching, and cyclization
have been discussed. Perfluoroalkanes proved to be inert molecules that interact weakly with themselves as well as with alkanes
VLE and LLE of perfluoroalkane + alkane mixtures
No full textVapour-liquid equilibria (VLE) of binary mixtures of n-perfluorohexane plus a n-alkane (C-5-C-8) and of n-hexane plus a n-perfluoroalkane (C-5-C-8) were determined using a head-space gas-chromatographic technique. Excess molar Gibbs energies, G(E), for the systems investigated have been obtained by a least-square treatment of equilibrium data. Liquid-liquid equilibria (LLE) were determined by turbidimetry for n-perfluorohexane + n-CnH2n+2 (n = 6-8) mixtures and the observed T-x curves compared with those predicted by the temperature dependence of activity coefficients. All mixtures show strong positive deviations from ideality, which increase with the size of the second component. Solute-solvent and solute-solute interactions have been examined by calculating and discussing solvation Gibbs energies and Kirkwood-Buff integrals. Perfluoroalkanes proved to be inert molecules that interact weakly with themselves as well as with alkanes
Analytical pyrolysis of ovalbumin
No full textIn this study the thermal degradation of ovalbumin (OVA) under nitrogen atmosphere was investigated. For this scope, a multi instrumental approach based on thermogravimetry (TG), thermogravimetry coupled with infrared spectroscopy (TG/FTIR) and pyrolysis coupled with mass spectrometric detection, i.e. flash pyrolysis-coupled with gas chromatography-mass spectrometry (Py/GC/MS), evolved gas analysis coupled with mass spectrometry (EGA/MS) and double shot pyrolysis-coupled with gas chromatography-mass spectrometry (DSP/GC/MS), was used. The pyrolysis of a protein involves a combination of several complex mechanisms resulting in a very high number of products. The study highlighted that pyrolysis of OVA produces low-molecular weight gasses, such as CO2, H2O, HCNO, NH3and CO, as main compounds. In addition, a series of organic compounds containing heteroatoms and unsaturations were also identified, whose formation occurred at different temperatures over the pyrolytic process. Among these, cyclic pyrolysis products were identified: dialkyl substituted 2,5-diketopiperazines (DKPs) and, for the first time, unsaturated-DKPs (un-DKPs), 3,5-alkyl-3,4-dihydro-2H-pyrrole-2,4-diones (ADPDs) and 3-alkenyl-5-alkyl-pyrrolidine-2,4-diones (AAPDs). These compounds are formed below 350° and are produced by cyclisation reactions of two neighbouring amino acids. Pyroglutamic acid was also found among the main pyrolysis products of OVA, obtained as pyrolytic product of Glu, which is the most abundant amino acid in OVA. Aromatic compounds, such as pyridine, pyrrole, toluene, alkyl-benzenes and alkyl-pyrroles, phenol and alkyl-phenols, benzeneacetonitrile, benzenepropanenitrile, indole and alkyl-indoles, were detected, produced over a wide range of temperatures. This study highlighted for the first time that aromatic compounds produced below 320 °C are associated to the pyrolysis of specific amino acid side chains, while at higher temperatures, they are the pyrolysis products of the residual material remaining after condensation reactions, pyrolytic scissions and cyclization reactions
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