The aggregation of sodium perfluorooctanoate in water

The self-aggregation of sodium perfluorooctanoate (SPFO) has been studied by pH, ion-selective electrodes, changes in Rhodamine 6G colour and fluorescence, conductivity, surface tension and viscosity measurements. It has been determined that the aggregation of SPFO is gradual. It starts to aggregate at 0.01 mol dm^–3 with the formation of pre-micelles of perfluorooctanoate ions, which capture counterions at C=0.02 mol dm^–3 and form micelles at a critical micelle concentration (CMC) of 0.03 mol dm^–3. Micelles at the CMC are highly ionised and strongly hydrated. At C*=0.06 mol dm^–3, a low ionisation degree was found, indicating the formation of a more compact micellar structure. Micelles at the CMC show energetic advantage in comparison with the adsorbed state at the air/solution interface, and this advantage increases at C* originating desorption.     

Effect of water on the temperatures of human immunoglobulin conformation transitions

A differential thermal analysis of native and denaturated human immunoglobulin (G isotype) and mixtures of the native protein with water over the temperature range of 80–570 K is conducted. Temperatures of the protein conformational transitions and the effect of water on them are investigated. The limit of water solubility in the native protein is determined calorimetrically from the enthalpy of excess water phase melting. A physical state diagram of the immunoglobulin-water system over a wide range of temperatures and component concentrations is built and analyzed.     

Conformational changes in human serum albumin induced by sodium perfluorooctanoate in aqueous solutions

Conformational changes in the bulk solution and at the air-aqueous interface of human serum albumin (HSA) induced by changes in concentration of sodium perfluorooctanoate (C(7)F(15)COO(-)Na(+)) were studied by difference spectroscopy, zeta-potential data, and axisymmetric drop shape analysis. zeta-potential was used to monitor the formation of the HSA-C(7)F(15)COO(-)Na(+) complex and the surface charge of the complex. The conformational transition of HSA in the bulk solution was followed as a function of denaturant concentration by absorbance measurements at 280 nm. The data were analyzed to obtain values for the Gibbs energies of the transition in water (DeltaG(0)(W)) and in a hydrophobic environment (DeltaG(0)(hc)) pertaining to saturated protein-surfactant complexes. The conformational changes that surfactants induce in HSA molecules alter its absorption behavior at the air-water interface. Dynamic surface measurements were used to evaluate this behavior. At low [C(7)F(15)COO(-)Na(+)], proteins present three adsorption regimes: induction time, monolayer saturation, and interfacial gelation. When surfactant concentration increases and conformational changes in the bulk solution occur, the adsorption regimes disappear. HSA molecules in an intermediate conformational state migrate to the air-water interface and form a unique monolayer. At high [C(7)F(15)COO(-)Na(+)], the adsorption of denatured molecules exhibits a behavior analogous to that of dilute solutions.     

Simulation of conformational transitions

Conformational transitions are essential for the functioning of many proteins, and understanding this dynamical behavior is a central goal in molecular biology. Computer simulations are playing an important role towards this aim by providing insights into how the conformational changes are induced, propagated and used. Popular methods for the simulation of conformational transitions will be reviewed, with a focus on atomistic molecular dynamics techniques for the calculation of transition pathways     

Effect of cationic surfactants on the interaction between sodium perfluorooctanoate and beta-lactoglobulin

The interaction between the fluorocarbon surfactant, sodium perfluorooctanoate (SPFO), and beta-lactoglobulin (BLG) was studied. In particular, the effects of cationic surfactants, such as alkyltriethylammonium bromide (C(n)NE, n=8, 10, 12), on SPFO-BLG interaction were examined. It was shown that the anionic fluorocarbon surfactant, SPFO, was a strong denaturant of BLG. The ability of SPFO to denature BLG could be weakened by the addition of C(n)NE. The effect of C(n)NE on SPFO-BLG interaction was related to the hydrocarbon chain length of C(n)NE, and also the molar ratio of the added C(n)NE to the SPFO in SPFO-BLG solutions ([C(n)NE]/[SPFO]). Our findings might provide a way to design surfactant systems that are less denaturing to proteins or tailor the ability of surfactant to denature proteins through the appropriate mixing with other surfactants.     

Cooperative conformational transitions of linear biopolymers

Conformational transitions in proteins, nucleic acids, and other biopolymers evidently play a decisive role in many biological processes, particularly in control processes. They often proceed cooperatively, i.e. the elementary process of the transition of an individual segment of these macromolecules in influenced by the state of other segments via intramolecular interactions. In general, the segments favor the same state as their neighbours. The resulting equilibrium properties of cooperative systems, e.g. the sharpness of the transitions and their dependence on the chain length, can be quantitatively explained for linear systems by the linear Ising model. The molecular causes of the cooperativity can be explained for simple model polymers.     

Molecular structure and conformational transitions of dichloroacetylchloride

RHF and MP2 techniques in 6–31G(d) basis set have been used to determine the structure of the isolated molecule CHCl₂COCl in two stable conformations (cis-and gosh-), as well as in transition states arising due to the rotary motion of CHCl₂ group around the C—C bond. The energy gap between the conformers and the relevant potential barriers has been calculated using the obtained potential dependence of the internal rotation. Plausible conformation of dichloroacetylchloride is discussed on the basis of ³⁵Cl NQR.     

Multistep mechanism of conformational transitions in metallatranes

The results of an investigation of the spin-spin coupling constants of the protons in the OCH₂CH₂N fragments of asymmetric derivatives of metallatranes and the two-dimensional ¹H NMR (NOESY) spectra of 1-substituted 3,6,10-trimethylsilatranes provide evidence in support of a multistep exchange mechanism for conformational transitions, rather than the synchronous conversion of metallatranes in solution.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 129–133, January, 1987.     

Kinetics of cooperative conformational transitions of lineal biopolymers

Cooperative conformational transitions of proteins and nucleic acids are of decisive importance to many processes of molecular biology, and particularly to their regulation. They proceed via numerous interdependent elementary processes, and their kinetics are therefore often complicated. They are frequently also very fast. However, kinetic analyses can be carried out by chemical relaxation methods. The theoretical interpretation is comparatively simple in the case of linear biopolymers. When the linear Ising model extended for kinetics was applied to model peptides and polynucleotides, it provided an insight into the fundamental principles of cooperative transformations.     

Thermodynamic aspects of conformational transitions in synthetic polypeptides

The helix-coil transition of organic solvent soluble homopolypeptides has been discussed from a largely thermodynamic view-point. The present state and the future prospects are considered.

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Zusammenfassung Der Helix-Ungeordnete Übergang von in organischen Lösungsmitteln löslichen Homopolypeptiden wurde aus einem breiten Gesichtspunkt diskutiert, gegenwärtige Lage und zukünftige Entwicklungsaussichten erörtert.

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R'esumé On a discuté d'une pointe de vue thermodynamique large la transition helix—non helix des homopolypeptides solubles dans des solvants organiques et considéré la situation actuelle ainsi que les perspectives futures.

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The present paper was supported by NSF Grant GB 33484.     

The influence of sulphur atoms on the electronic transitions in some chemical compounds

From literature there is clear evidence of the distinct influence of sulphur atoms on electronic transitions in various chemical compounds. However, so far, no attempt has been made to discuss this problem in more detail. Based on many examples concerning inorganic as well as organic compounds, the author has tried to show some connection between the shapes and intensities of absorption bands with the properties and structure of the corresponding compounds. The problem of d orbitals which are often assumed to participate in bonding in sulphur compounds is thoroughly discussed.     

The influence of sodium chloride on the crystallization rate of gypsum

The growth kinetics of gypsum was studied in supersaturated solutions both in pure water and in the presence of NaCl, using a seeded growth technique. Radioactive tracer techniques were employed to follow the growth process.The mean linear growth rate R was plotted against the relative supersaturation σ. For low σ values the relation between R and σ is given by a linear law, for higher σ values by a parabolic law and for the highest σ values by a growth “order” higher than two.It was shown that the addition of sodium chloride increased the crystallization rate remarkably. The higher the NaCl concentration the higher the growth rate.     

Ultrafast solvation dynamics of human serum albumin: correlations with conformational transitions and site-selected recognition

Human serum albumin, the most abundant protein found in blood plasma, transports a great variety of ligands in the circulatory system and undergoes reversible conformational transitions over a wide range of pH values. We report here our systematic studies of solvation dynamics and local rigidity in these conformations using a single intrinsic tryptophan (W214) residue as a local molecular probe. With femtosecond resolution, we observed a robust bimodal distribution of time scales for all conformational isomers. The initial solvation occurs in several picoseconds, representing the local librational/rotational motions, followed by the dynamics, in the tens to hundreds of picoseconds, which result from the more bonded water in the tryptophan crevice. Under the physiological condition of neutral pH, we measured approximately 100 ps for the decay of the solvation correlation function and observed a large wobbling motion at the binding site that is deeply buried in a crevice, revealing the softness of the binding pocket and the large plasticity of the native structure. At acidic pH, the albumin molecule transforms to an extended conformation with a large charge distribution at the surface, and a similar temporal behavior was observed. However, at the basic pH, the protein opens the crevice and tightens its globular structure, and we observed significantly faster dynamics, 25-45 ps. These changes in the solvation dynamics are correlated with the conformational transitions and related to their structural integrity.     

The influence of annealing on thermal transitions in a nematic copolyester

Thermal transitions of a glassy, main chain, liquid crystalline, random copolyester, HIQ‐40, have been characterized. HIQ‐40 is made from 40 mol percent p‐hydroxybenzoic acid (HBA) and 30 mol % each of p‐hydroquinone (HQ) and isophthalic acid (IA). This polymer is soluble in organic solvents, permitting the preparation of thin, solution‐cast films that are in a glassy, metastable, optically isotropic state. On first heating of an isotropic HIQ‐40 film in a calorimeter, one glass transition is observed at low temperature (approximately 42°C), and is ascribed to the glass/rubber transition of the isotropic polymer. A cold crystallization exotherm centered near 150°C is observed. This is associated with the development of low levels of crystalline order. A broad melting endotherm is centered at about 310°C; this endotherm marks the melting of crystallites and the transformation to a nematic fluid. A nematic to isotropic transition was not observed by calorimetry. After quenching from the nematic melt, a T_g is observed in the range of 110–115°C and is associated with the glass/rubber transition of the nematically ordered polymer. Annealing optically isotropic films at temperatures above the isotropic glass transition results in the systematic development of axial order. In these annealed samples, T_g increases rapidly until it is near the annealing temperature, then T_g increases more slowly at longer annealing times. In as‐cast films annealed at 120–135°C, the light intensity transmitted through a sample held between crossed polarizers in an optical microscope (a qualitative measure of birefringence and, in turn, axial order) initially increases rapidly and uniformly throughout the sample and, at longer annealing times, approaches asymptotic values that are higher at higher annealing temperatures. The increase in transmitted intensity is ascribed to the development of axial order. The uniform increase in transmitted intensity suggests that ordering occurs by a rather global process and not via a nucleation and growth mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 505–522, 1999     

The influence of the solvent on the conformational energy differences due to the anomeric effect

The solvation free energy ΔG^sol of molecules exhibiting the anomeric effect is computed in an approach that considers a continuous distribution for the solvent. A partition of ΔG^sol into separately evaluated contributions confirms that changes in the energy of the systems due to changes in conformation of the solute are ruled by the electrostatic contribution. A comparison with the “exact” values indicates that the approximate expression for the electrostatic contribution to ΔG^sol are not accurate enough to permit a proper modeling of the solvent influence on the anomeric effect. The systems are composed of methanediol, methoxymethanol, dimethoxymethane, and 2-methoxytetrahydropyran in carbon tetrachloride, chloroform, acetone, and water. The calculations have been performed at the SCF level with the STO -3G and 4–31G basis sets.     

Simulation of conformational transitions by the restricted perturbation-targeted molecular dynamics method

A method for the simulation of conformational transitions is presented. The method, based on targeted molecular dynamics (TMD), limits the conformational change at each molecular dynamics step to a fixed size, that minimizes the root mean square deviation from the target. The method is more efficient than standard TMD and yields lower energy pathways, but, like the TMD method, requires only a single molecular dynamics simulation. Test calculations and comparisons with standard TMD calculations for the alanine dipeptide with the analytic continuum electrostatics implicit solvent model are presented.     

Effects of hydrophobic and dipole-dipole interactions on the conformational transitions of a model polypeptide

We studied the effects of hydrophobicity and dipole-dipole interactions between the nearest-neighbor amide planes on the secondary structures of a model polypeptide by calculating the free energy differences between different peptide structures. The free energy calculations were performed with low computational costs using the accelerated Monte Carlo simulation (umbrella sampling) method, with a bias-potential method used earlier in our accelerated molecular dynamics simulations. It was found that the hydrophobic interaction enhances the stability of alpha helices at both low and high temperatures but stabilizes beta structures only at high temperatures at which alpha helices are not stable. The nearest-neighbor dipole-dipole interaction stabilizes beta structures under all conditions, especially in the low temperature region where alpha helices are the stable structures. Our results indicate clearly that the dipole-dipole interaction between the nearest neighboring amide planes plays an important role in determining the peptide structures. Current research provides a more unified and quantitative picture for understanding the effects of different forms of interactions on polypeptide structures. In addition, the present model can be extended to describe DNA/RNA, polymer, copolymer, and other chain systems.     

Mixed micelle formation in aqueous solutions of nonyl-N-methylglucamine with sodium perfluorooctanoate at different pressures

For the mixed system of nonyl-N-methylglucamine (MEGA9) with sodium perfluorooctanoate (SPFO), the critical micelle concentrations (CMC) at atmosphreic pressure and 30°C were determined from measurement of surface tension, and those at high pressures were determined by the electroconductivity method at mole fractions of MEGA9 up to 0.6. All of MEGA9-SPFO mixed systems have been found to have a surface activity much greater than the respective pure systems, i.e., a synergism of surface activity caused by mixing MEGA9 and SPFO. The mixing reduces the pressure dependence of the CMC. This suggests that this combination is useful when it is desirable for a surfactant solution to be independent of pressure. The composition of the mixed micellar phase has been estimated by applying the Motomura equation. The Gibbs energy of the mixed micelle formation has also been calculated as a function of mole fraction of a surfactant in the surfactant mixture.To whom correspondence should be addressed.