Search results for "theoretical"
showing 10 items of 11439 documents
Thermodynamic Behavior of Non-Ionic Tri-block Copolymers in Water at Three Temperatures
2006
Apparent molar volumes (V Φ) of aqueous solutions of some copolymers, based on ethylene oxide (EO) and propylene oxide (PO) units, were determined as functions of concentration at three temperatures. Viscosity measurements were also carried out on some of these systems. The effects studied include how the molecular architecture and the molecular weight affect the aggregation of the copolymer, keeping constant the EO/PO ratio. Modeling of the volumetric data yielded the partial molar volume of the copolymer in the standard (V°) and the aggregated (V M) states, as well as the equilibrium constant for micellization and the aggregation number. Analysis of the viscosity data supported the insigh…
Mass action model for solute distribution between water and micelles. Partial molar volumes of butanol and pentanol in dodecyl surfactant solutions
1986
The densities of 1-butanol and 1-pentanol were measured in aqueous solutions of dodecyltrimethylammonium bromide and dodecyldimethylamine oxide and the partial molar volumes at infinite dilution of the alcohols in aqueous surfactants solutions were obtained. The observed trends of this quantity as a function of the surfactant concentration were rationalized using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. At the same time, the model was revised to account for the alcohol effect on the surfactant micellization equilibrium. The partial molar volume of alcohols in the aqueous and in the micellar phases and the ratios between the binding constan…
Thermodynamic properties of alcohols in a micellar phase. Binding constants and partial molar volumes of pentanol in sodium dodecylsulfate micelles a…
1984
Densities of the ternary system water-sodium dodecylsulfate (NaDS)-pentanol and of the binary systems butanol-octane and pentanol-octane were measured at 15, 25, and 35 °C. The apparent molar volume of pentanol in the ternary system was analyzed using a mass-action model for the alcohol distribution in micellar solutions. The partial molar volume of alcohol bound to the micelles and the ratio between the binding constant and the aggregation number of the surfactant are calculated. The partial molar volume binding constant, is discussed in terms of solubilization sites of the alcohol in the micelles whereas the binding constant is compared with that derived from the Nernstian partition const…
Anti-Aging Effects of GDF11 on Skin
2020
International audience; Human skin is composed of three layers: the epidermis, the dermis, and the hypodermis. The epidermis has four major cell layers made up of keratinocytes in varying stages of progressive differentiation. Skin aging is a multi-factorial process that affects every phase of its biology and function. The expression profiles of inflammation-related genes analyzed in resident immune cells demonstrated that these cells have a strong ability to regenerate adult skin stem cells and to produce endogenous substances such as growth differentiation factor 11 (GDF11). GDF11 appears to be the key to progenitor proliferation and/or differentiation. The preservation of youthful phenot…
A simplified framework to optimize MRI contrast preparation
2018
PURPOSE This article proposes a rigorous optimal control framework for the design of preparation schemes that optimize MRI contrast based on relaxation time differences. METHODS Compared to previous optimal contrast preparation schemes, a drastic reduction of the optimization parameter number is performed. The preparation scheme is defined as a combination of several block pulses whose flip angles, phase terms and inter-pulse delays are optimized to control the magnetization evolution. RESULTS The proposed approach reduces the computation time of B 0 -robust preparation schemes to around a minute (whereas several hours were required with previous schemes), with negligible performance loss. …
Atmospheric protein chemistry influenced by anthropogenic air pollutants: nitration and oligomerization upon exposure to ozone and nitrogen dioxide
2017
The allergenic potential of airborne proteins may be enhanced via post-translational modification induced by air pollutants like ozone (O3) and nitrogen dioxide (NO2). The molecular mechanisms and kinetics of the chemical modifications that enhance the allergenicity of proteins, however, are still not fully understood. Here, protein tyrosine nitration and oligomerization upon simultaneous exposure of O3 and NO2 were studied in coated-wall flow-tube and bulk solution experiments under varying atmospherically relevant conditions (5–200 ppb O3, 5–200 ppb NO2, 45–96% RH), using bovine serum albumin as a model protein. Generally, more tyrosine residues were found to react via the nitration pathw…
Impact of histidine spacing on modified polyhistidine tag – Metal ion interactions
2018
Abstract Histidine rich sequences are chosen both by nature and by molecular biologists due to their high affinity towards metal ions. In this work, we examine the affinity and binding modes of Cu 2+ , Ni 2+ and Zn 2+ towards two histidine tags, the common His 6 -tag (Ac-HHHHHH-NH 2 ) and its modified sequence, which also contains six histidines, but separated with two alanine residues (Ac-HAAHAAHAAHAAHAAHAA-NH 2 ). The spatial separation of histidines has an important impact on its coordination properties. Cu 2+ and Ni 2+ complexes with Ac-HHHHHH-NH 2 are more stable than those with Ac-HAAHAAHAAHAAHAAHAA-NH 2 ; the contrary is observed for Zn 2+ . In a narrow range of pH, Cu 2+ -Ac-HHHHHH-…
Conformational studies into N-methylation of alanine diamide models: A quantitative approach
2006
Abstract A systematic theoretical analysis was performed on N -acetyl- l -alanine N ′-methylamide (Ac- l -Ala-NHMe) and the analogues methylated on the N-terminus (Ac- l -(Me)Ala-NHMe), C-terminus (Ac- l -Ala-NMe 2 ), and both N/C-termini (Ac- l -(Me)Ala-NMe 2 ), to evaluate the influence of methylation of the amide group on the conformational properties of the affected residues. The ϕ , ψ potential energy surfaces were calculated at the B3LYP/6-31+G**//HF/3-21G level of theory with inclusion of the solvent (water) effect (SCRF method). The conformers localised were fully optimised at the B3LYP/6-31+G** in vacuo. The accessible areas of the potential energy surfaces; the number of conformer…
Anomalous temperature dependence of the IR spectrum of polyalanine
1997
Abstract We have studied the temperature dependence of the infrared spectra of acetanilide (ACN), tryptophan–(alanine) 15 , and tyrosine–(alanine) 15 . No sidebands of the amide-I vibration were observed in the polypeptides, but two anomalous sidebands of the NH stretch with a similar temperature dependence as that of the anomalous amide-I vibrational mode at 1650 cm −1 of crystalline ACN were detected. Fermi resonance combined with the appearance of a red-shifted sideband of NH stretch through coupling to lattice modes seems to explain this band structure. Observations are indicative of excitons that may occur in polypeptides as well as in single crystals of ACN.
Aminoacid zwitterions in solution : Geometric, energetic, and vibrational analysis using density functional theory-continuum model calculations
1998
Glycine and alanine aminoacids chemistry in solution is explored using a hybrid three parameters density functional (B3PW91) together with a continuum model. Geometries, energies, and vibrational spectra of glycine and alanine zwitterions are studied at the B3PW91/6-31+G∗∗ level and the results compared with those obtained at the HF and MP2/6-31+G∗∗ levels. Solvents effects are incorporated by means of an ellipsoidal cavity model with a multipolar expansion (up to sixth order) of the solute’s electrostatic potential. Our results confirm the validity of the B3PW91 functional for studying aminoacid chemistry in solution. Taking into account the more favorable scaling behavior of density funct…