Magnitude Of Hydrophobic Interactions Research Articles

Efectos Isotópicos del Agua Ligera (H2O) y del Agua Pesada (D2O) en las Interacciones Hidrofóbicas

Las propiedades fisicoquímicas del agua ligera (H2O) y del agua pesada (D2O) difieren en cierta medida debido a las diferencias que surgen de los efectos de los isótopos. Para comprender mejor los efectos de los isótopos en las interacciones hidrófobas, informamos la comparación experimental directa de las interacciones hidrófobas en agua ligera versus agua pesada entre superficies de mica hidrofobizadas molecularmente extendidas y lisas utilizando un Aparato de Fuerzas de Superficie (SFA). En este estudio sintetizamos superficies hidrófobas estables, lisas y fácilmente reproducibles mediante la deposición de monocapas de perfluorodeciltriclorosilano (FDTS) sobre superficies de mica activadas por plasma utilizando la técnica de deposición de vapor molecular (MVD). La humectabilidad y la morfología de la superficie de las muestras de mica recubiertas con FDTS se controlaron directamente utilizando una célula de ángulo de contacto y un microscopio de fuerza atómica (AFM). La inestabilidad en la curva de fuerza se observa durante el acercamiento y la retracción de las superficies en los experimentos SFA y nuestros experimentos preliminares de espectroscopia de fuerza demostraron que la magnitud de la interacción hidrofóbica entre las superficies de mica recubiertas con FDTS es 20% más fuerte en agua pesada que en agua ligera. Nuestros resultados indican que los perfiles de fuerza-distancia obtenidos para los casos tanto de H2O como de D2O no pudieron ser descritos razonablemente por la teoría clásica de Derjaguin − Landau − Verwey − Overbeek (DLVO) y la fuerte adhesión medida entre las capas FDTS-FDTS que interactúan es dominado por las interacciones hidrófobas.

Direct and quantitative AFM measurements of the concentration and temperature dependence of the hydrophobic force law at nanoscopic contacts

By virtue of its importance for self-organization of biological matter the hydrophobic force law and the range of hydrophobic interactions (HI) have been debated extensively over the last 40years. Here, we directly measure and quantify the hydrophobic force–distance law over large temperature and concentration ranges. In particular, we study the HI between molecularly smooth hydrophobic self-assembled monolayers, and similarly modified gold-coated AFM tips (radii∼8–50nm). We present quantitative and direct evidence that the hydrophobic force is both long-ranged and exponential down to distances of about 1–2nm. Therefore, we introduce a self-consistent radius-normalization for atomic force microscopy data. This approach allows quantitative data fitting of AFM-based experimental data to the recently proposed Hydra-model. With a statistical significance of r2⩾0.96 our fitting and data directly reveal an exponential HI decay length of 7.2±1.2Å that is independent of the salt concentration up to 750mM. As such, electrostatic screening does not have a significant influence on the HI in electrolyte concentrations ranging from 1mM to 750mM. In 1M solutions the observed instability during approach shifts to longer distances, indicating ion correlation/adsorption effects at high salt concentrations. With increasing temperature the magnitude of HI decreases monotonically, while the range increases slightly. We compare our results to the large body of available literature, and shed new light into range and magnitude of hydrophobic interactions at very close distances and over wide temperature and concentration regimes.

Cooperative Ligand Binding to Globular Protein: A Statistical Mechanical Theory Based on a Simple Geometrical Model and Its Application to Lysozyme Systems

A simple geometrical model for statistical mechanical analysis of cooperative binding of monoionic ligands to globular proteins was developed. It is assumed that the ligand has the charged group, which is able to bind the oppositely charged residues on protein surface by electrostatic interactions, and the hydrophobic region, which causes cooperativity by the interactions between bound ligands. The bound ligand together with the side chain of the ionic residue is assumed to be mobile over the protein surface within the tangential half sphere with the center at the β-carbon of the side chain and the radius of the effective ligand length. The potential around a bound ligand was approximated by the cylindrical square-well potential with an exclusion core along the center line. The shape ofprotein was assumed to be spherical. The distance between charged residues on the protein surface was taken from crystallographic data. Grand partition function was calculated as the sum of the terms of all possible bound states. In each term, the electrostatic free energy was taken into account as the Debye-Huckel type electrostatic potential energy for the distribution of isolated charges, and the partition function due to hydrophobic and/or stacking interactions was calculated as the product of pairwise interactions. The model was applied to the binding of an anionic azo dye or anionic surfactants to hen egg white lysozyme at several experimental conditions. The calculated binding isotherms well-reproduced the experimental data. The values ofthe model parameters estimated were consistent with the ligand size and the magnitude ofhydrophobic interaction. Other detailed information such as species fraction, variance, and bound fraction of each site was obtained. The results show the wide applicability of the present model theory.