Electron acceptor defined

Directionality of H-bonding Due to the high donor ability of non-bonding orbitals, most H-bonds involve a suitable lone pair as an electron donor (often alternatively, and confusingly, referred to as H-bond acceptor ) in nY o 5 jj hyperconjugative interactions. Such interactions involve two stereoelectronic components. First, the symmetry of the electron acceptor defines the 180° X-H...Y angle of attack (note the ste-... 

A transfer of electrons from NADH to molecular oxygen in the absence of added electron acceptors defines an NADH oxidase activity recently identified in purified animal plasma membranes (Morre and Brightman, 1991). The oxidase is also stimulated by epidermal growth factor and diferric transferrin (Morr6 et al., 1991), ceruloplasmin (Alcam et al., 1992), and guanine nucleotides (Morre et al.,... 

Midpoint potential values are useful quantitites for defining the role of the various centers in the system. In some instances, these values have even been used to predict the location of the centers in the electron transfer chain, assuming that the potential increases along the chain from the electron donor to the electron acceptor. In several oxidoreductases, however, the measured potential of some centers was found to be clearly outside the range defined by the donor and the acceptor, which raised an intriguing question as to their function. This was observed, for instance, in the case of the [4Fe-4S] (Eni = -320 mV) center in E. coli fumarate reductase (249), the [3Fe-4S] + (Era = -30 mV) center in D. gigas hydrogenase (207), and the low-potential [4Fe-4S] + + (E, = 200 and -400 mV) centers in E. 

Horseradish peroxidase (HRP) is a member of the large class of peroxidases, which are enzymes defined as oxidoreductases using hydroperoxide as electron acceptor. HRP has been widely used for the construction of amperometric biosensor for the determination of H202 and small organic and inorganic substrates. 

The microbial transformations of the wastewater described in the concept shown in Figure 5.5 deal with the COD components defined in Section 3.2.6. The figure also depicts the major processes that include the transformations of the organic matter (the electron donors) in the two subsystems of the sewer the suspended wastewater phase and the sewer biofilm. The air-water oxygen transfer (the reaeration) provides the aerobic microbial processes with the electron acceptor (cf. Section 4.4). Sediment processes are omitted in the concept but are indirectly taken into account in terms of a biofilm at the sediment surface. Water phase/biofilm exchange of electron donors and dissolved oxygen is included in the description. 

Oxidation is defined as the reduction of electron state by addition of oxygen or removal of electrons. Thermodynamic balance requires balance. For every oxidation, there must be a corresponding reduction. Thus, for oxidation to occur, there must be a compound capable of receiving the transferred electrons. Electron acceptor compounds can include oxygen, sulfate, Fe3+, phosphate, nitrate, C02, and certain organics. 

The nitrosonium cation bears a formal relationship to the well-studied halogens (i.e. X2 = I2, Br2, and Cl2), with both classes of structurally simple diatomic electron acceptors forming an extensive series of intermolecular electron donor-acceptor (EDA) complexes that show well-defined charge-transfer absorption bands in the UV-visible spectral region. Mulliken (1952a,b 1964 Mulliken and Person, 1969) originally identified the three possible nonbonded structures of the halogen complexes as in Chart 7, and the subsequent X-ray studies established the axial form II to be extant in the crystals of the benzene complexes with Cl2 and Br2 (Hassel and Stromme, 1958, 1959). In these 1 1 molecular complexes, the closest approach of the... 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Variations in ferritin protein coats coincide with variations in iron metabolism and gene expression, suggesting an Interdependence. Iron core formation from protein coats requires Fe(Il), at least experimentally, which follows a complex path of oxidation and hydrolytic polymerization the roles of the protein and the electron acceptor are only partly understood. It is known that mononuclear and small polynuclear Fe clusters bind to the protein early in core formation. However, variability in the stoichiometry of Fe/oxidant and the apparent sequestration and stabilization of Fe(II) in the protein for long periods of time indicate a complex microenvironment maintained by the protein coats. Full understanding of the relation of the protein to core formation, particularly at intermediate stages, requires a systematic analysis using defined or engineered protein coats. 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

The picture begins to come somewhat into focus. Starting off with some basic mechanics of electrons, one was able to define the quantum mechanical condition for the tunneling of electrons from a metallic donor to electron acceptors through an electron-energy barrier. The tunneling condition could be expressed in terms of an energy barrier for ion movement, e.g., the movement of protons toward the metal in the reaction ... 

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