Fields local electrostatic

The concentration of salt in physiological systems is on the order of 150 mM, which corresponds to approximately 350 water molecules for each cation-anion pair. Eor this reason, investigations of salt effects in biological systems using detailed atomic models and molecular dynamic simulations become rapidly prohibitive, and mean-field treatments based on continuum electrostatics are advantageous. Such approximations, which were pioneered by Debye and Huckel [11], are valid at moderately low ionic concentration when core-core interactions between the mobile ions can be neglected. Briefly, the spatial density throughout the solvent is assumed to depend only on the local electrostatic poten-... 

Figure 6.16. Different modes of adsorption of CHjOH on Pt under ultra-high vacuum (left) and in aqueous solutions (right) showing the effect of local electrostatic field and surface work function on the mode of adsorption.100 Reprinted with permission from the American Chemical Society.

As reviewed above, when a solute is placed in a dielectric medium, it electrically polarizes that medium. The polarized medium produces a local electrostatic field at the site of the solute, this field polarizes the solute, and the polarized solute interacts with the polarized medium. The interaction is typically too large to be treated by perturbation theory, and some sort of self-consistent treatment of polarized solute and polarized medium is more appropriate. At this point several options present themselves. It promotes orderly discussion to classify these... 

These external fields can affect the physical properties and reactivity of the molecular systems. Such effects are especially important in ordered crystalline environments such as solid oxides and biological macromolecules [25-28]. These local electrostatic fields play an important role in catalytic functions and in governing the stabilization of many biomolecular systems [29-32]. These environmental effects cause dramatic changes in the reactivity, which can be different from the gas phase. 

The replacement of Si4+ by Al3+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). 

Solute-solvent interactions in dithiole-thiones and -ones are attributed to localized electrostatic induction. The strength of the hetero-association complexes is determined by the distribution of the electric field around the carbonyl or thiocarbonyl group (72HCA213). 

The physical basis of the second type of approach rests upon the effect of the local electrostatic potential upon dynamic interactions at encounters with charged quenching molecules resulting in fluorescence (phosphorescence) (Vogel et al., 1986 Anni et al., 1994) or between a stable radical, e.g. nitroxide, and another charged paramagnetic species (Likhtenshtein et al., 1972 Likhtenshtein, 1976, 1988,1993). In such cases, the relaxation parameters, i.e. the life-time of the fluorescence (phosphorescence) chromophore or spin-spin and spin-lattice relaxation rates of paramagnetic species are dependent upon the frequency of encounters, and, therefore, on local electrostatic fields... 

While the differently charged spin-probes have only been used so far on a rather limited number of examples reported herein, the results show that these new methodologies hold great promise for the investigation of local electrostatic fields in a wide range of biologically significant molecules. 

Carlo simulation. " Several other global factors, such as hydrophobic properties and hydrogen-bonding networks have been reported to contribnte to calcinm affinity." " These factors have less defined roles compared to the electrostatic interaction. They are likely to contribnte to the dielectric environment that serves to shape the local electrostatic field." " " " " Unfortunately, the estimation of the charge contribution to calcium affinity and selectivity in natural calcium-binding proteins is complicated by the interference of cooperative binding, calcium-dependent conformational change, and electrostatic interaction between multiple calcium ions. "" ... 

Factors 1 and 2 are universally found in sorption systems, and the approximate additivity of dispersion energies for all atom pair interactions ensures the sorption, at low temperatures, of large molecules, even n-paraffins. When a sorbent is composed of positive and negative ions (as in a zeolite), there exist local electrostatic fields, F, that polarize the sorbate of polarizability a. Thus = — olF, where the negative sign implies exothermal reaction. Such effects could be visualized in terms of distortion of electron clouds in certain p- or 7r-electron systems. 

In molecular systems the polarization P results from the individual molecular dipoles and has two main contributions. One is associated with the average orientation induced by an external field in the distribution of permanent molecular dipoles. The other results from the dipoles induced in each individual molecule by the local electrostatic field. The characteristic timescale associated with the first effect is that of nuclear orientational relaxation, x , typically 10 " s for small molecule fluids at room temperature. The other effect arises mostly from the distortion of the molecular electronic charge distribution by the external field, and its typical... 

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