arXiv subjects

Susanne Moelbert

Publications and source records attributed to Susanne Moelbert.

4 recordsLinked to original sources

arXiv2004

Kosmotropes and chaotropes: modelling preferential exclusion, binding and aggregate stability

Kosmotropic cosolvents added to an aqueous solution promote the aggregation of hydrophobic solute particles, while chaotropic cosolvents act to destabilise such aggregates. We discuss the mechanism for these phenomena within an adapted version of the two-state Muller-Lee-Graziano model for water, which provides a complete description of the ternary water/cosolvent/solute system for small solute particles. This model contains the dominant effect of a kosmotropic substance, which is to enhance the formation of water structure. The consequent preferential exclusion both of cosolvent molecules from the solvation shell of hydrophobic particles and of these particles from the solution leads to a stabilisation of aggregates. By contrast, chaotropic substances disrupt the formation of water structure, are themselves preferentially excluded from the solution, and thereby contribute to solvation of hydrophobic particles. We use Monte Carlo simulations to demonstrate at the molecular level the preferential exclusion or binding of cosolvent molecules in the solvation shell of hydrophobic particles, and the consequent enhancement or suppression of aggregate formation. We illustrate the influence of structure-changing cosolvents on effective hydrophobic interactions by modelling qualitatively the kosmotropic effect of sodium chloride and the chaotropic effect of urea.

cond-mat.soft↗

arXiv2003

Correlation between sequence hydrophobicity and surface-exposure pattern of database proteins

Hydrophobicity is thought to be one of the primary forces driving the folding of proteins. On average, hydrophobic residues occur preferentially in the core, whereas polar residues tends to occur at the surface of a folded protein. By analyzing the known protein structures, we quantify the degree to which the hydrophobicity sequence of a protein correlates with its pattern of surface exposure. We have assessed the statistical significance of this correlation for several hydrophobicity scales in the literature, and find that the computed correlations are significant but far from optimal. We show that this less than optimal correlation arises primarily from the large degree of mutations that naturally occurring proteins can tolerate. Lesser effects are due in part to forces other than hydrophobicity and we quantify this by analyzing the surface exposure distributions of all amino acids. Lastly we show that our database findings are consistent with those found from an off-lattice hydrophobic-polar model of protein folding.

q-bio.BM↗

arXiv2003

Hydrophobic Interaction Model for Upper and Lower Critical Solution Temperatures

Hydration of hydrophobic solutes in water is the cause of different phenomena, including the hydrophobic heat-capacity anomaly, which are not yet fully understood. Because of its topicality, there has recently been growing interest in the mechanism of hydrophobic aggregation, and in the physics on which it is based. In this study we use a simple yet powerful mixture model for water, an adapted two-state Muller-Lee-Graziano model, to describe the energy levels of water molecules as a function of their proximity to non-polar solute molecules. The model is shown to provide an appropriate description of many-body interactions between the hydrophobic solute particles. The solubility and aggregation of hydrophobic substances is studied by evaluating detailed Monte Carlo simulations in the vicinity of the first-order aggregation phase transition. A closed-loop coexistence curve is found, which is consistent with a mean-field calculation carried out for the same system. In addition, the destabilizing effect of a chaotropic substance in the solution is studied by suitable modification of the MLG model. These findings suggest that a simple model for the hydrophobic interaction may contain the primary physical processes involved in hydrophobic aggregation and in the chaotropic effect.

cond-mat.soft↗

arXiv2001

The Local Minority Game

Ecologists and economists try to explain collective behavior in terms of competitive systems of selfish individuals with the ability to learn from the past. Statistical physicists have been investigating models which might contribute to the understanding of the underlying mechanisms of these systems. During the last three years one intuitive model, commonly referred to as the Minority Game, has attracted broad attention. Powerful yet simple, the minority game has produced encouraging results which can explain the temporal behaviour of competitive systems. Here we switch the interest to phenomena due to a distribution of the individuals in space. For analyzing these effects we modify the Minority Game and the Local Minority Game is introduced. We study the system both numerically and analytically, using the customary techniques already developped for the ordinary Minority Game.

cond-mat.stat-mech↗