Interaction non-covalent

Non-covalent interactions between molecules often occur at separations where the van der Waals radii of the atoms are just touching and so it is often most useful to examine the electrostatic potential in this region. For this reason, the electrostatic potential is often calculated at the molecular surface (defined in Section 1.5) or the equivalent isodensity surface as shown in Figure 2.18 (colour plate section). Such pictorial representations... 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

As mentioned earlier, adhesive bond formation is governed by interfacial processes occurring between the adhering surfaces. These interfacial processes, as summarized by Brown [13] include (1) van der Waals or other non-covalent interactions that form bonds across the interface (2) interdiffusion of polymer chains across the interface and coupling of the interfacial chains with the bulk polymer and (3) formation of primary chemical bonds between chains or molecules at or across the interface. 

The van der Waals and other non-covalent interactions are universally present in any adhesive bond, and the contribution of these forces is quantified in terms of two material properties, namely, the surface and interfacial energies. The surface and interfacial energies are macroscopic intrinsic material properties. The surface energy of a material, y, is the energy required to create a unit area of the surface of a material in a thermodynamically reversible manner. As per the definition of Dupre [14], the surface and interfacial properties determine the intrinsic or thermodynamic work of adhesion, W, of an interface. For two identical surfaces in contact ... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Chapter 2, Polar Covalent Bonds Acids and Bases—A new Section 2.13 on non-covalent interactions has been added. 

Clostridial neurotoxins are bacterial protein toxins that consist of a heavy and a light chain connected by a disulfide bond and non-covalent interactions. They... 

For the purposes of this review the criterion has been refined to include only those porphyrin complexes where there is direct structural or spectroscopic evidence for a metal-carbon interaction. This interaction will not, however, be limited to covalent bonds. The last decade has seen the rise in importance of supramolecular chemistry and non-covalent interactions, and a small set of examples involving porphyrin complexes will be included as the last section in the review. 

NON-COVALENT INTERACTIONS BETWEEN METALLOPORPHYRINS AND ORGANIC MOLECULES... 

Fluorescence investigations of the partitioning of the aromatic fluorophore Prodan in water/AOT/n-heptane, water/DTAB/n-hexanol/n-heptane, and water/CTAB/n-hexanol/n-heptane microemulsions proved that this molecule, as a consequence of a variety of non-covalent interactions, is distributed in several distinct micellar domains [140]. 

Raevsky, O. A. Quantification of non-covalent interactions on the basis of the thermodynamic hydrogen bond parameters./. Phys. Org. Chem. 1997, 10, 405 13. 

According to the definition given above, the term XB comprehensively covers a vast class of non-covalent interactions, from the weak N- Cl XB [35] to the very strong I" I2 interaction, which forms the triiodide anion 10 and 200 kj mol-1 can be assumed as the energy extremes for these interactions, respectively. 

The term halogen bonding addresses exclusively the former contacts and its usefulness relies on the identification of a specific subset of the numerous and diverse non-covalent interactions that halogens can give rise to [58]. 

Although non-covalent interactions of anions are one of the most actively explored areas of supramolecular chemistry [15], the anion sensing and recognition have up to now relied primarily on electrostatic binding or hydrogen bonding to the receptor [16,54-61]. However, recent UV-Vis and NMR spectral studies clearly reveal that complex formation takes place in the solutions between halides and neutral olefinic and aromatic it-acceptors such as those in Fig. 3 [23,62],... 

Liquid Crystals Formed Through Non-covalent Interactions. 167... 

First, it is important to appreciate that all liquid crystal mesophases exist due to non-covalent interactions between molecules, namely the anisotropic dispersion forces mentioned earlier. However, this section will address more specific non-covalent interactions that have been used either to induce liquid-crystalline behaviour or to generate a new species that is liquid crystalline. 

However, by far the most common non-covalent interaction responsible for generating new liquid-crystalline species is the hydrogen bond, and this area has been well reviewed [ 16,17]. In fact, hydrogen bonding in liquid crystals is a very old concept and it has been known for some time that, for example, the... 

An alternative way for the generation of a bidentate ligand makes use of a self-assembly process of monodentate to bidentate ligands employing non-covalent interactions [87]. 

There has been considerable recent activity developing appropriate parameters to allow semi-empirical methods to describe a variety of biologically important systems, and their related properties, such as (i) enzyme reactivity, including both over- and through-barrier processes, (ii) conformations of flexible molecules such as carbohydrates, (iii) reactivity of metalloenzymes and (iv) the prediction of non-covalent interactions by addition of an empirical dispersive correction. In this review, we first outline our developing parameterisation strategy and then discuss progress that has been made in the areas outlined above. 

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