Charge movement, 0-0 bonding interactions

Mechanical movements in supramolecular structures rely on modulation of noncovalent-bonding interactions. Such changes occur when charge-transfer interactions between electron-donor and electron-acceptor groups are weakened. 

As noted by Kati and Wolfenden, this remarkable affinity appears to suggest that the 6-hydroxyl group, which has very limited freedom of movement, is likely to be in almost ideal alignment with the active site, and that at least one charged active site residue is also likely to be involved in its hydrogen bonding interaction. This conjecture has since been verified by the determination of the crystal structure of the inhibitory complex between adenosine deaminase and 6-hydroxy-1,6-dihydropurine ribonucleoside, which showed that the 6-hydroxyl group interacts with a zinc atom, with a protonated histidyl residue, and with an aspartic acid residue at the enzyme s active site. 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

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