Gain In Translational Entropy Research Articles

On the Stabilizing Effect of Aspartate and Glutamate and Its Counteraction by Common Denaturants.

By performing differential scanning calorimetry(DSC) measurements on RNase A, we studied the stabilization provided by the addition of potassium aspartate(KAsp) or potassium glutamate (KGlu) and found that it leads to a significant increase in the denaturation temperature of the protein. The stabilization proves to be mainly entropic in origin. A counteraction of the stabilization provided by KAsp or KGlu is obtained by adding common denaturants such as urea, guanidinium chloride, or guanidinium thiocyanate. A rationalization of the experimental data is devised on the basis of a theoretical approach developed by one of the authors. The main contribution to the conformational stability of globular proteins comes from the gain in translational entropy of water and co-solute ions and/or molecules for the decrease in solvent-excluded volume associated with polypeptide folding (i.e., there is a large decrease in solvent-accessible surface area). The magnitude of this entropic contribution increases with the number density and volume packing density of the solution. The two destabilizing contributions come from the conformational entropy of the chain, which should not depend significantly on the presence of co-solutes, and from the direct energetic interactions between co-solutes and the protein surface in both the native and denatured states. It is the magnitude of the latter that discriminates between stabilizing and destabilizing agents.

Dual Transient Networks of Polymer and Micellar Chains: Structure and Viscoelastic Synergy.

Dual transient networks were prepared by mixing highly charged long wormlike micelles of surfactants with polysaccharide chains of hydroxypropyl guar above the entanglement concentration for each of the components. The wormlike micelles were composed of two oppositely charged surfactants potassium oleate and n-octyltrimethylammonium bromide with a large excess of anionic surfactant. The system is macroscopically homogeneous over a wide range of polymer and surfactant concentrations, which is attributed to a stabilizing effect of surfactants counterions that try to occupy as much volume as possible in order to gain in translational entropy. At the same time, by small-angle neutron scattering (SANS) combined with ultrasmall-angle neutron scattering (USANS), a microphase separation with the formation of polymer-rich and surfactant-rich domains was detected. Rheological studies in the linear viscoelastic regime revealed a synergistic 180-fold enhancement of viscosity and 65-fold increase of the longest relaxation time in comparison with the individual components. This effect was attributed to the local increase in concentration of both components trying to avoid contact with each other, which makes the micelles longer and increases the number of intermicellar and interpolymer entanglements. The enhanced rheological properties of this novel system based on industrially important polymer hold great potential for applications in personal care products, oil recovery and many other fields.

On-Off of Co-non-solvency for Poly(N-vinylcaprolactam) in Alcohol-Water Mixtures: A Molecular Dynamics Study.

Poly(N-vinylcaprolactam) (PVCL) exhibits co-non-solvency in aqueous solutions of 2-propanol but not in methanol. What distinguishes the impact of these two cosolvents on the polymer conformational stability? We report a molecular dynamics simulation study on PVCL 50-mer and monomers dissolved in methanol-water and 2-propanol-water mixtures. We show that the alcohol-concentration dependence of the effective attraction between a pair of PVCL monomers closely resembles the conformational changes in a single PVCL 50-mer as well as the experimentally observed behavior for PVCL chains. We also found that, at the co-non-solvency maximum, the monomer-monomer attraction works over a long-range beyond the solvent-separated distance. Then, we correlate the long-range attraction to the appearance of a dense alcohol concentration accumulated between the monomers. Furthermore, we distinctly demonstrate that the co-non-solvency of PVCL monomers can be switched on/off by artificially tuning the alcohol size while keeping the energetic parameters. Thus, we conclude that the magnitude of the excluded volume effect in alcohol accompanying the gain in translational entropy of the solvent is crucial to the occurrence of PVCL polymer co-non-solvency.

Why does urea have a different effect on the collapse temperature of PDEAM and PNIPAM?

The two polymers poly(N,N-diethylacrylamide), PDEAM, and poly(N-isopropylacrylamide), PNIPAM, undergo a coil-to-globule collapse transition in water on increasing the temperature. The process is endothermic and it is driven by the gain in translational entropy of water molecules due to the decrease in solvent-excluded volume associated with the transition from swollen to globular conformations of the polymer. Curiously, in the presence of urea, the two polymers have a different behaviour: globular conformations are stabilized for PNIPAM, while swollen conformations are stabilized for PDEAM [Macromolecules 49 (2016) 234]. Here, we present an application of the theoretical approach already used to rationalize several aspects of PNIPAM collapse [Phys. Chem. Chem. Phys. 17 (2015) 27750; Phys. Chem. Chem. Phys. 18 (2016) 14426]. This model is able to provide a reliable explanation for the different behaviour of urea towards PDEAM and PNIPAM. The difference might be in fact due to the number of urea-polymer contacts that is larger in the case of PDEAM with respect to PNIPAM for the presence of two extra methylene groups in each side chain of the former polymer.

On the opposite effect of guanidinium chloride and guanidinium sulphate on the kinetics of the Diels-Alder reaction

The transition state of Diels-Alder reactions resembles the contact-minimum configuration of pairwise hydrophobic interaction. On this ground, it has been possible to find a robust correlation between the experimental k2 values for the reaction between cyclopentadiene and methyl acrylate and the Gibbs energy estimates for the formation of this contact-minimum configuration. The enhancement of rate constant caused by Gdm2SO4 comes from the large density increase caused by the addition of this salt to water (i.e., the strong electrostrictive power of sulphate ions), that renders very large the Gibbs energy cost of cavity creation. The WASA decrease associated with the formation of the contact-minimum configuration leads to a decrease in the Gibbs energy cost of cavity creation with respect to the situation of the two molecules far apart from each other. This corresponds to a gain in translational entropy of ions and water molecules that is larger the greater the density or better the number density of the aqueous solution. This entropy gain in the case of Gdm2SO4 overwhelms the energy decrease associated with the loss of several attractive interactions between the Gdm+ions and the nonpolar solute molecules.

Energetics of the contact minimum configuration of two hard spheres in water

Changes in thermodynamic functions for the formation of the contact minimum, cm, configuration of two hard spheres (i.e., two cavities) are calculated by means of a physically-based geometric approach over a large temperature range. The decrease in water accessible surface area due to cm formation causes a gain in translational entropy of water molecules, driving the process. This produces a negative Gibbs energy change, whose magnitude slightly increases with temperature. The process is exothermic due to the decrease in hydration shell size, but this enthalpy change is entirely compensated by a corresponding entropy contribution.

Temperature Dependence of the Pairwise Association of Hard Spheres in Water

It is shown that the Gibbs energy change associated with the formation of the contact-minimum configuration of two hard spheres in water becomes more negative on increasing the temperature, at 1 atm, by extending the geometric approach previously developed [G. Graziano, Chem. Phys. Lett. 499, 79 (2010)]. This is because the decrease in water accessible surface area accompanying the association leads to a gain in translational entropy of water molecules. The process is exothermic, due to the release of some water molecules from the hydration shell to the bulk. This water reorganization is characterized by a complete enthalpy–entropy compensation and does not affect the Gibbs energy change.

On the effect of hydrostatic pressure on the conformational stability of globular proteins.

The model developed for cold denaturation (Graziano, PCCP 2010, 12, 14245-14252) is extended to rationalize the dependence of protein conformational stability upon hydrostatic pressure, at room temperature. A pressure- volume work is associated with the process of cavity creation for the need to enlarge the liquid volume against hydrostatic pressure. This contribution destabilizes the native state that has a molecular volume slightly larger than the denatured state due to voids existing in the protein core. Therefore, there is a hydrostatic pressure value at which the pressure-volume contribution plus the conformational entropy loss of the polypeptide chain are able to overwhelm the stabilizing gain in translational entropy of water molecules, due to the decrease in water accessible surface area upon folding, causing denaturation.

Translational-Entropy Gain of Solvent upon Protein Folding

We show that even in the complete absence of potential energies among the atoms in a protein-aqueous solution system, there is a physical factor that favors the folded state of the protein. It is a gain in the translational entropy (TE) of water originating from the translational movement of water molecules. An elaborate statistical-mechanical theory is employed to analyze the TE of water in which a protein or peptide with a prescribed conformation is immersed. It is shown that if the number of residues is sufficiently large, the TE gain is powerful enough to compete with the conformational-entropy loss upon folding. For protein G we have tested over 100 compact conformations generated by a computer simulation with the all-atom potentials as well as the native structure. A significant finding is that the largest TE is attained in the native structure. The translational movement of water molecules is quite effective in achieving the tight packing in the interior of a natural protein. These results are true only when the solvent is water whose molecular size is the smallest among the ordinary liquids in nature.

Large gain in translational entropy of water is a major driving force in protein folding

A view lacking in earlier studies on protein folding is that the process is critically influenced by the thermal motion of water molecules. Here it is pointed out that there is a powerful driving force, a large gain in the translational entropy (TE) of water, which competes with the large loss of the conformational entropy in the folding. We analyze the TE of water in which a protein molecule with a prescribed conformation is immersed. We consider a number of conformations and show that the largest TE is attained only in the native structure.

3P079 タンパク質の折れ畳みに伴う水の並進エントロピーの増加(蛋白質 C) 物性 : 安定性、折れたたみなど)

3P079 タンパク質の折れ畳みに伴う水の並進エントロピーの増加(蛋白質 C) 物性 : 安定性、折れたたみなど)

Entropy Explained: The Origin of Some Simple Trends

Density functional theory computational methods were used to calculate the entropies of various molecules; computed entropies correlated closely with measured values. For organic systems, an average of 8.4 kcal/mol for the reaction entropy (one particle to two at 298.15 K) was observed; this value is largely determined by translational entropy gain. The average reaction entropy is slightly lower for reactions that produce two linear molecules and up to 4 kcal/mol higher when no linear molecules are produced, due to differences in rotational entropy of the reactants and products. Translational and rotational entropy are generally independent of molecular identity except for increases in mass and generation of additional moments of inertia; vibrational entropy, which is more dependent on the molecule itself, is a small contributor to the nearly constant entropy of reaction. A variety of inorganic and non-hydrocarbon main group reaction entropies were also calculated; there is an increased contribution of vi...

Interaction of polyelectrolyte networks with oppositely charged micelle-forming surfactants

The conformational properties of polyelectrolyte networks swelling in a solvent containing micelle-forming SAs have been studied. The charge of the SA molecules is opposite to that of the network chains. It is shown that in this case there is heavy sorption of the SA molecules by the gel accompanied by the formation of SA aggregates within the gel and considerable fall in its size. Increase in the ionic strength of the solution leads to fracture of the network-SA complex. These phenomena are due to the fact that firstly on entry of the SA molecules into the gel the low molecular mass counter-ions are released, i.e. heavy gain in translational entropy is reached, and, secondly, aggregation in the gel is more advantageous than micelle formation in the solvent since it leads to immobilization of a smaller number of counter-ions.