Attractive Dispersion Interactions Research Articles

Van der Waals interactions involving proteins

Van der Waals (dispersion) forces contribute to interactions of proteins with other molecules or with surfaces, but because of the structural complexity of protein molecules, the magnitude of these effects is usually estimated based on idealized models of the molecular geometry, e.g., spheres or spheroids. The calculations reported here seek to account for both the geometric irregularity of protein molecules and the material properties of the interacting media. Whereas the latter are found to fall in the generally accepted range, the molecular shape is shown to cause the magnitudes of the interactions to differ significantly from those calculated using idealized models, with important consequences. First, the roughness of the molecular surface leads to much lower average interaction energies for both protein-protein and protein-surface cases relative to calculations in which the protein molecule is approximated as a sphere. These results indicate that a form of steric stabilization may be an important effect in protein solutions. Underlying this behavior is appreciable orientational dependence, one reflection of which is that molecules of complementary shape are found to exhibit very strong attractive dispersion interactions. Although this has been widely discussed previously in the context of molecular recognition processes, the broader implications of these phenomena may also be important at larger molecular separations, e.g., in the dynamics of aggregation, precipitation, and crystal growth.

Solvent interaction with the (2p3s) Rydberg state of hexamethylenetetramine: Energetics and relaxation dynamics

The (1+1) mass resolved excitation spectra are reported for the (2p3s)←(2p)2 Rydberg transition of the tricyclic, high symmetry molecule hexamethylenetetramine [HMT (C6H12N4)] and its van der Waals clusters. The solvent molecules employed include both nonpolar (Ar, CH4) and polar (NH3, CH3OCH3) species. HMT and its clusters are generated and cooled in a supersonic expansion. The observed electronic transition is assigned as T2←A1 within the Td print group. A transition blue shift of 52 cm−1 for each Ar atom and 65 cm−1 for each methane molecule in the HMT van der Waals cluster is characterized. These shifts are caused by an excited state repulsive interaction between the excited Rydberg electron and the closed shell solvent which reduces the attractive dispersion interaction between the HMT and nonpolar solvent species in the van der Waals cluster. A transition red shift of more than 600 cm−1 for NH3 and CH3OCH3 solvent/HMT clusters is observed; this large increase in interaction energy for the excited Rydberg state of HMT with respect to the ground state of HMT is associated with the delocalization of the excited electron into available (virtual) Rydberg orbitals of the solvent molecules. The interaction is characterized as an electron transfer interaction. These results and assignments are consistent with previously reported ones for DABCO/solvent clusters. Relaxation dynamics of excited HMT and its clusters are investigated via a pump/probe (ionization) technique. Relaxation of the clusters is dominated by an intersystem crossing mechanism resulting in an excited state singlet lifetime of ∼5 ns compared to a bare molecule HMT excited state lifetime of ∼1.0 μs. A triplet state of HMT lies 255 cm−1 below the first excited singlet Rydberg state as determined by two-color threshold ionization studies.

Role of dispersion potential anisotropy in the theory of nematic liquid crystals. I. The mean-field potential

We reexamine the effect of the anisotropy in the intermolecular attractive (dispersion) interaction on the attractive mean-field potential used in the generalized van der Waals theory of the nematic–isotropic transition. Contrary to results in the literature, we show (1) that the anisotropic part of the dispersion potential reduces the anisotropy of the mean field contributed by the isotropic part of the dispersion potential; (2) that the form of anisotropy in the attractive intermolecular interaction usually chosen for simplicity in thermodynamic calculations corresponds to a preaveraged form of the dispersion interaction. The anisotropy of both the dispersion and of the mean included angle potential (which corresponds to the mean-field potential in a perfectly ordered nematic) are shown graphically, and the expansion coefficients for the latter in terms of even-ordered Legendre polynomials are tabulated.

Role of anisotropic dispersion potential in the theory of nematic liquid crystals. II. Thermodynamic calculations

We reexamine the effect of the anisotropy in the intermolecular attractive (dispersion) interaction on the thermodynamic quantities characterizing the nematic–isotropic phase transition, as estimated from generalized van der Waals theory. Contrary to statements in the literature, this anisotropy significantly affects the transition temperature and the slope of the coexistence curve. The results obtained here, as in generalized van der Waals calculations in the literature, in general do not well reproduce the experimental data. Sources of these discrepancies are discussed briefly.

Molecular recognition by cyclophane hosts

Abstract

The dispersion damping functions and interaction energy curves for Xe-Xe

Ab initio potential energy curves have been calculated for the ground state of Xe 2 including dispersion interactions. The total interaction energy has been approximated by a sum of the repulsive Hartree-Fock interaction plus the attractive dispersion interaction which is appropriately damped to take account of charge overlap.