Electron transfer partners, classes

The Zintl-Klemm concept evolved from the seminal ideas of E. ZintI that explained the structural behavior of main-group (s-p) binary intermetaUics in terms of the presence of both ionic and covalent parts in their bonding description [31, 37]. Instead of using Hume-Rother/s idea of a valence electron concentration, ZintI proposed an electron transfer from the electropositive to the electronegative partner (ionic part) and related the anionic substructure to known isoelectronic elemental structures (covalent part), e.g., TK in NaTl is isoelectro-nic with C, Si and Ge, and consequenUy a diamond substructure is formed. ZintI hypothesized that the structures of this class of intermetallics would be salt-like [16b, 31 f, 37e]. 

The differences in structures and redox partners between the two classes of P450 and NOS enzymes give rise to differences in reduction potentials and electron transfer mechanisms. The fusion of the oxygenase domain to its diflavin reductase domain facilitates ET (of relevance here is that P450 BM3 has the highest mono-oxygenase activity of all P450s [56]). 

Reduction of the heme iron of cytochromes P450 to the ferrous state 3 is necessary for the binding and subsequent activation of atmospheric dioxygen. Initially, two electrons are derived from NAD(P)H by flavin adenine dinucleotide (FAD)-containing proteins and then are used sequentially via one-electron transfers. AU cytochromes P450 can be divided into two main classes with respect to the reduction mechanism and the structure of their immediate redox partner. The first class includes most soluble... 

The electrons that must be delivered to the heme iron to accomplish catalysis are transferred from the reduced cofactor NAD PH by the redox partners. Nature uses a variety of strategies to accomplish this, almost all of which rely on intermediary electron transfer proteins that themselves require nonprotdnaceous cofactors. There are currendy at least 10 different systems or classes known [15], but for the purposes of applied biocatalysis, it is important to be aware of the four most commonly encountered in the biocatalysis literature (Figure 8.4). 

In summary, there are two classes of amino acid residues one involved in critical alignment of docking for electron transfers and the other involved in crude orientation or transient complex formation. Those involved in critical docking aUgnment tend to be different for the two substrates, Fd and those bound to opposite sides of FNR. Finally, hydrophobic clusters usually line the binding cavity and are critical for electron transfer. The situation with plastocyanin and cytochrome Cg to be presented next is similar the two independent sets of data demonstrate a common design scheme for redox partners of long-distance electron transfers. 

The efficient quenching of the atomic and molecular fluorescence by collisions has been observed in early studies of the luminescence of gaseous compounds (for a review of early work see Ref. 2). In a large number of cases these processes have been explained by the electronic-to-vibrational energy transfer, charge transfer or excited-complex (excimer or exciplex) formation. There remains, however, an important class of collisional processes corresponding to the essentially intramolecular relaxation induced (or assisted) by collisions with chemically inert partners. In such... 

A further important class of materials are the donor-acceptor complex crystals. They consist of two partner compounds in a stoichiometric ratio, of which one transfers charge to the other. When the charge transfer occurs only in an electron-... 

A class of HAT reactions for which the additivity postulate appears not to hold are those with strong polar effects. In some HAT reactions, as pointed out by Tedder, ... the rate of atom transfer is very dependent on the degree of polarity in the transition state. For instance, Rong et al. showed that alkyl radicals abstract H faster from thiols than from silanes or stannanes, while the kinetic preference is reversed for perfluoroalkyl radicals. " The more electron rich R" radical preferentially abstracts the electron deficient RS -H while the electron deficient Rp" radical reacts faster with R3Sn -H . Such an inversion of reactivity cannot be accounted for by a cross relation treatment, because from the additivity postulate the reactivity of a reagent is not dependent on its partner. 

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