Adding Salt to an Aqueous Solution of t-Butanol: Is Hydrophobic Association Enhanced or Reduced?

Recent neutron scattering experiments on aqueous salt solutions of amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. {\bf 89}, 215508 (2002); J. Chem. Phys. {\bf 118}, 8357 (2003)] suggest the formation of t-butanol pairs, bridged by a chloride ion via ${O}-{H}...{Cl}^-$ hydrogen-bonds, and leading to a reduced number of intermolecular hydrophobic butanol-butanol contacts. Here we present a joint experimental/theoretical study on the same system, using a combination of molecular dynamics simulations and nuclear magnetic relaxation measurements. Both theory and experiment clearly support the more intuitive scenario of an enhanced number of hydrophobic contacts in the presence of the salt, as it would be expected for purely hydrophobic solutes [J. Phys. Chem. B {\bf 107}, 612 (2003)]. Although our conclusions arrive at a structurally completely distinct scenario, the molecular dynamics simulation results are within the experimental errorbars of the Bowron and Finney work.Comment: 15 pages twocolumn revtex, 11 figure

Low-Temperature and High-Pressure Induced Swelling of a Hydrophobic Polymer-Chain in Aqueous Solution

We report molecular dynamics simulations of a hydrophobic polymer-chain in aqueous solution between $260 {K}$ and $420 {K}$ at pressures of $1 {bar}$, $3000 {bar}$, and $4500 {bar}$. The simulations reveal a hydrophobically collapsed state at low pressures and high temperatures. At $3000 {bar}$ and about $260 {K}$ and at $4500 {bar}$ and about $260 {K}$, however, a transition to a swelled state is observed. The transition is driven by a smaller volume and a remarkably strong lower enthalpy of the swelled state, indicating a steep positive slope of the corresponding transition line. The swelling is stabilized almost completely by the energetically favorable state of water in the polymers hydrophobic first hydration shell at low temperatures. Although surprising, this finding is consistent with the observation of a positive heat capacity of hydrophobic solvation. Moreover, the slope and location of the observed swelling transition for the collapsed hydrophobic chain coincides remarkably well with the cold denaturation transition of proteins.Comment: 9 pages, 9 figures, twocolumn revte

Full-Length L1CAM and Not Its Δ2Δ27 Splice Variant Promotes Metastasis through Induction of Gelatinase Expression

Tumour-specific splicing is known to contribute to cancer progression. In the case of the L1 cell adhesion molecule (L1CAM), which is expressed in many human tumours and often linked to bad prognosis, alternative splicing results in a full-length form (FL-L1CAM) and a splice variant lacking exons 2 and 27 (SV-L1CAM). It has not been elucidated so far whether SV-L1CAM, classically considered as tumour-associated, or whether FL-L1CAM is the metastasis-promoting isoform. Here, we show that both variants were expressed in human ovarian carcinoma and that exposure of tumour cells to pro-metastatic factors led to an exclusive increase of FL-L1CAM expression. Selective overexpression of one isoform in different tumour cells revealed that only FL-L1CAM promoted experimental lung and/or liver metastasis in mice. In addition, metastasis formation upon up-regulation of FL-L1CAM correlated with increased invasive potential and elevated Matrix metalloproteinase (MMP)-2 and -9 expression and activity in vitro as well as enhanced gelatinolytic activity in vivo. In conclusion, we identified FL-L1CAM as the metastasis-promoting isoform, thereby exemplifying that high expression of a so-called tumour-associated variant, here SV-L1CAM, is not per se equivalent to a decisive role of this isoform in tumour progression

A Method to Calculate the g-Coefficients of the Molecular Pair Correlation Function from Molecular Dynamics Simulations

International audienceIt is known that the rotational equation of motion of rigid molecules in MD simulations can be solved in a singularity-free form if quaternions are used for the description of the rotational motion. We show that these quaternions are also suited for the calculation of the so-called ‘g-coefficients’, which are the expansion coefficients of the molecular pair correlation function (MPCF) in terms of Wigner functions. This is due to the fact that quaternions arc themselves a representation of the rotation group and can be referred to an arbitrary coordinate frame in a particularly simple way. As an application for the quaternion formalism we calculate some g-coefficients of the MPCF of methylene chloride (CH2 Cl2 )

Molecular dynamics studies and neutron scattering experiments on methylene chloride. I. Structure

International audienceMolecular dynamics (MD) simulations of methylene chloride (CH2Cl2) with two different potentials are used to calculate the intermolecular static structure factor inter(q) for a series of isotopic substituted mixtures CX 2Cl2. The intermolecular structure factors are compared with corresponding experimental data obtained from neutron diffraction. We discuss the validity of the rigid molecule approximation in the MD simulation and show that the quantitative comparison of the static structure factors of molecular liquids obtained from neutron diffraction experiments and MD simulation must be considered with some care for molecules containing hydrogen atoms with low effective masses

Molecular-dynamics studies and neutron-scattering experiments on methylene chloride. II. Dynamics

International audienceWe calculate inelastic neutron-scattering intensities of methylene chloride for cold neutrons from molecular-dynamics (MD) simulations and compare them with experimental data. To obtain realistic scattering intensities, the effect of multiple scattering is taken into account by a Monte Carlo (MC) simulation, using the dynamic structure factor calculated from our MD simulations as input. The MD simulations of methylene chloride are performed with the same potentials as in Part I of this work. The dynamic structure factor is calculated using fast correlation algorithm (FCA), which is based on the fast-Fourier-transform (FFT) algorithm and the Wiener-Khinchin theorem for discrete functions