Electrostatic interaction, theoretical study

Theoretical studies aimed at rationalizing the interaction between the chiral modifier and the pyruvate have been undertaken using quantum chemistry techniques, at both ab initio and semi-empirical levels, and molecular mechanics. The studies were based on the experimental observation that the quinuclidine nitrogen is the main interaction center between cinchonidine and the reactant pyruvate. This center can either act as a nucleophile or after protonation (protic solvent) as an electrophile. In a first step, NH3 and NH4 have been used as models of this reaction center, and the optimal structures and complexation energies of the pyruvate with NH3 and NHa, respectively, were calculated [40]. The pyruvate—NHa complex was found to be much more stable (by 25 kcal/mol) due to favorable electrostatic interaction, indicating that in acidic solvents the protonated cinchonidine will interact with the pyruvate. 

Theoretical calculations on the dithiazolyl radical 4 (R=CF3) have recently shown that n -n dimerisation was unfavourable but association of two such dimers via electrostatic interactions generated a thermodynamically stable tetramer consistent with single crystal X-ray studies. Thus while the value of [AE-P ] may favour (or disfavour) dimer formation, the van der Waals, dipole contributions and electrostatic interactions to the lattice enthalpy should not be underestimated in assessing the thermodynamic stability or instability of these... 

To date, D coefficients of carbohydrates established with the PFGSE approactf - " have been undertaken to (1) validate the theoretical self-diffusion coefficients calculated from MD trajectories, (2) demonstrate the complexation of lanthanide cations by sugars,(3) probe the geometry of a molecular capsule formed by electrostatic interactions between oppositely charged P-cyclodextrins, (4) study the influence of concentration and temperature dependence on the hydrodynamic properties of disaccharides, and (5) discriminate between extended and folded conformations of nucleotide-sugars. ... 

Hirshfeld and Mirsky (1979) evaluated the relative contributions to the lattice energy for the crystal structures of acetylene, carbon dioxide, and cyanogen, using theoretical charge distributions. Local charge, dipole and quadrupole moments are used in the evaluation of the electrostatic interactions. When the unit cell dimensions are allowed to vary, inclusion of the electrostatic forces causes an appreciable contraction of the cell. In this study, the contributions of the electrostatic and van der Waals interactions to the lattice energy are found to be of comparable magnitude. 

Theoretical studies of the role of polymer additives lag behind their analogs in electrostatic stability since polymer molecules have considerably more configurational freedom and since the interaction of the polymer molecules with the solvent is an inseparable part of phenomena in polymer-colloid mixtures. We begin with some of the general issues and a thermodynamic analysis of the role of polymer on stability in Section 13.5. 

Hope et al. (116) presented a combined volumetric sorption and theoretical study of the sorption of Kr in silicalite. The theoretical calculation was based on a potential model related to that of Sanders et al. (117), which includes electrostatic terms and a simple bond-bending formalism for the portion of the framework (120 atoms) that is allowed to relax during the simulations. In contrast to the potential developed by Sanders et al., these calculations employed hard, unpolarizable oxygen ions. Polarizability was, however, included in the description of the Kr atoms. Intermolecular potential terms accounting for the interaction of Kr atoms with the zeolite oxygen atoms were derived from fitting experimental results characterizing the interatomic potentials of rare gas mixtures. In contrast to the situation for hydrocarbons, there are few direct empirical data to aid parameterization, but the use of Ne-Kr potentials is reasonable, because Ne is isoelectronic with O2-. 

A series of theoretical papers have recently investigated the structure and thermodynamic stability of CL/ DNA complexes (May and Ben-Shaul, 1997 Dan, 1998 Bruinsma, 1998 Bminsma and Mashl, 1998 Harries et al., 1998 O Hem and Lubensky, 1998 Golubovic and Golubovic, 1998). Analytical and numerical studies of DNA-DNA interactions bound between membranes show the existence of a novel long-range repulsive electrostatic interaction (Bminsma, 1998 Bruinsma and Mashl, 1998 Harries et al., 1998). 

X. Song, Theoretical studies of dielectric solvation dynamics, in L. R. Pratt and G. Hummer (eds), Simulation and Theory of Electrostatic Interactions in Solution, AIP Corf. Proc., Melville, NY, 1999, Vol. 492, pp 417—428. 

How then can we account for the high degree of internal order routinely found within globular proteins We believe that combinations of the wide variety of electrostatic interactions reviewed above determine the precise three-dimensional structure of the interior of a protein. We argue that the sum of these interactions produces, at least in part, the enthalpy change on protein folding that is independent of the hydrophobic effect. Crystal structures of small organic compounds provide a useful model of protein interiors, and we now discuss some recent theoretical studies of these systems. 

Addition of polymers can both stabilize and destabilize a solution. If the polymer contains ionizable units it is usually referred to as a polyelectrolyte. In this report we will focus on the effect from polyelectrolytes on the colloidal stability. In high dielectric media like water, where the monomers are ionized, the behavior of a polyelectrolyte is mainly governed by electrostatics and the connectivity of the monomers. Therefore, in theoretical studies, many important features of the polyelectrolyte behavior in water solution can be studied by a schematic description of the polyelectrolyte as a linear chain of charged monomers connected with springs. The bonding interaction between two monomers is Ub=K(r —a)2, where K is the spring constant, a is the equilibrium value and r is the distance between the two monomers (see Fig. 11). 

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