Interactions, apolar electrostatic

The model of a dipole in a spherical cavity can only provide qualitative insights into the behaviour of real molecules moreover, it cannot explain the effect of electrostatic interactions in the case of apolar molecules. More accurate predictions require a more detailed representation of the molecular charge distribution and of the cavity shape this is enabled by the theoretical and computational tools nowadays available. In the following, the application of these tools to anisotropic liquids will be presented. First, the theoretical background will be briefly recalled, stressing those issues which are peculiar to anisotropic fluids. Since most of the developments for liquid crystals have been worked out in the classical context, explicit reference to classical methods will be made however, translation into the quantum mechanical framework can easily be performed. Then, the main results obtained for nematics will be summarized, with some illustrative... 

These qualitative assays show that one-armed cationic guanidiniocarbonyl pyrrole receptors can indeed effectively bind tetrapeptides even in water. Molecular modeling studies suggest a complex structure as shown for one specific example, the receptor Val-Val-Val-CBS, in Figure 2.3.11. Receptor and substrate form a hydrogen bonded //-sheet which is further stabilized by additional hydrophobic interactions between the apolar groups in the side-chains. Recognition of the tetrapep-tide thus seems to be controlled by a fine balanced interplay between electrostatic and hydrophobic interactions. 

Fig. 15. A single heptad repeat of an idealized model for coiled-coil proteins (Talbot and Hodges, 1982 Hodges et al., 1981). The primary driving force for formation of the structure arises from the interdigitation of the apolar leucyl side chains. In addition, electrostatic interactions between the oppositely charged residues on neighboring helices may also contribute to the stability of the parallel form of the coiled coil (Hodges et al., 1981).

Thrombin is the pivotal trypsin-like protease for the regulation of thrombosis and hemostasis. Thrombin hydrolyzes its natural substrates by recognition of the Pro-Arg motif in the apolar S2- and the primary specificity SI pocket [42]. The molecular structure of the thrombin-CtA complex (Figure 1.11) showed that CtA was bound to the active site of the enzyme. The arginine side chain formed an electrostatic interaction with Aspl89, located at the bottom of the SI binding pocket. 

The Armstrong model (1 7) can describe the retention of apolar, polar and even ionic solutes, provided they were binding solutes. The highest IL-, values corresponded to electrostatic interactions (1600 for CTAB with SDS micelles and 2600 for benzoic acid with CTAB with SDS micelles and 2600 for benzoic acid with CTAB micelles) or to comicellization (190 for SOBS with SDS micelles and 3000 for CPC with CTAB micelles). 

Secondly, many zeolite catalyzed reactions occur in apolar media and stabilization of the charged state by hydration of solvent complexation is absent. The high energy cost of charge separation involved with the cleavage of the zeolite OH bond is partially compensated for in zeolites by the electrostatic interaction between cation and negatively charged zeolite wall. 

The copol3nners in 20% ethanol-water were more effective catalysts than PVIm with S2 where electrostatic interactions are more important than hydrophobic interactions. However, with S12 where apolar interactions are predominant, the copolymers were less efficient catalysts than PVIm.(14)... 

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