Multi-point bonding

Nonspecific multi-point bonding of inert protein... 

Nonspecific multi-point bonding of isolated enzyme in incorrect orientation (left) and in specific multipoint bonding (right)... 

A unique feature of the enzyme is multi-point bonding of reaction intermediates to the enzyme cavity and the participation of several protein substituents often belonging to different amino acids in the activation of a substrate (Fig. 7.1c). 

To be more exact, every bond is a multi-center bond with contributions of the wave functions of all atoms. However, due to the charge concentration in the region between two atoms and because of the inferior contributions %H2, Xm> and Xh4> the bond can be taken to a good approximation to be a two-center-two-electron bond (2c2e bond) between the atoms C and HI. From the mathematical point of view the hybridization is not necessary for the calculation, and in the usual molecular orbital calculations it is not performed. It is, however, a helpful mathematical trick for adapting the wave functions to a chemist s mental picture. 

The unique practical properties of adsorption have promoted its extensive use in genetic analysis. The disadvantages of adsorption with respect to covalent immobihzation are mainly that (1) nucleic acids may be readily desorbed from the substrate, and (2) base moieties may be unavailable for hybridization if they are bonded to the substrate in multiple sites [34]. However, the electrochemical detection strategy based on the intrinsic oxidation of DNA requires the DNA to be adsorbed in close contact with the electrochemical substrate by multi-point attachment. This multi-site attachment of DNA can be thus detrimental for its hybridization but is crucial for the detection based on its oxidation signals. 

It is important to point out that the covalent bond has many faces. They can range from simple s, p, d, bonds to sp- or spd- hybridization bonds which may result in a and ji bonds that form the building block for single, double, and triple bonds. In more complex cases they can be resonance bonds or multi-centered bonds that are often observed in metals and alloys. 

As stated, hydrogen bonds have been used to construct the majority of finite molecular assemblies. Thus, most synthons used to form finite assemblies in the solid state have been based on hydrogen bonds. Many such synthons have also been used to form networks.2 Examples include single-point hydrogen bonds based on phenols and imidazoles, as well as multi-point hydrogen bonds based on carboxylic acid dimers, pyridone dimers, urea dimers, cyanuric acid-melamine complexes, and pyridine-carboxylic acid complexes.2... 

In order to understand HDS reactions, it is important to define the ways in which thiophenes are bonded to metal centers on catalytic surfaces. A number of modes in which T interacts with surfaces have been proposed, the most important ones being the one-point adsorption , that is, a strong interaction between the S atom and a vacancy on the surface, and the multi-point adsorption involving the S atom plus one or both of the C=C bonds in a delocalized rr-bonding. While it is difficult to experimentally obtain detailed information on the bonding of thiophenes to surface sites, several coordination modes of thiophenes have been authenticated in metal complexes (Figure 2). 

One of the major reasons for an increased interest in boron nanostructures is that boron itself has some attractive properties it has a very low density of 2.340 g/cm it has a high melting point (2076°C) and a Mohs hardness of 9.3 (diamond = 10.0). These should be reflected in boron s nanomaterials. Chemically it is of interest in that it has more valence orbitals (4) than valence electrons (3). Therefore, it tends to form so-called electron-deficient compounds having delocalized multi-centered bonds. 

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