Bonds as Electron Donors or Acceptors

Jensen [3.11] as well as Teeter [3.12] studied by X-ray diffraction the structure of water molecules in the vicinity, at the surface and inside of protein crystals. Jensen used rubredoxin (CEB) crystals to deduce the structure of water from the density distribution of electrons, calculated from diffraction pictures. Jensen found that water molecules which are placed within approx. 60 nm of the protein surface form a net, which is most dense in the distance of a hydrogen bond at the donor- or acceptor- molecules of a protein. In distances larger than 60 nm, the structure of water becomes increasingly blurred, ending in a structureless phase. Water molecules are also in the inside of proteins, but are more strongly bound than... 

The relationship between the local structure of the HB network and electronic properties was investigated and it was concluded that the water dipole moment and electron binding energies exhibit some dependence on the local environment of the network. Specifically, the dipole moment in liquid water increases with the number of H bonds, although it is not dependent on the role played by the water molecule as H donor or acceptor. On the other hand, the lbi EBE increases when the interaction with the surrounding water molecules involves directly the oxygen atom of the central water molecule, which then plays the role of H bond acceptor. 

The general trend for acyclic 1,4-dienes substituted with functional groups possessing electron donor or acceptor characteristics is highlighted by the examples shown in equation (46). The polarity of the intermediates plays a major role in determining the regiochemistry. From an empirical viewpoint, the more electron-rich double bond in the substrate remains as the double bond in the product. 

The unique properties of polymers such as polyacetylene, whose backbones consist of an alternating succession of single and double bonds, and most of which show extraordinary electrical, optical and magnetic properties including electrical conductivity when "doped" with electron donors or acceptors [35], are also outside the scope of this work. Sophisticated quantum mechanical treatments are required to predict these properties of such polymers. 

The addition or removal of a proton can promote electron flow, i.e., bond formation and breakage, during the reaction. As far as enzymes are concerned, only general acid or base catalysis can occur, because enzymes have no means of concentrating or OH ions. A number of amino acid side chains can act as proton donors or acceptors, and in many Zn-dependent enzymes, such as carboxypeptidase. the metal ion acts as an effective Lewis acid to enhance polarization of the carbonyl moiety of the amide bond. Among the amino acids His (which has a pKa generally close to 7) plays an especially important role in enzyme catalysis, because at neutral pH, there is a good balance between its protonated and deprotonated forais. In most cases, enzymes have suitably positioned pairs of side chains to provide push and pull of electrons. 

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