Acceptor control

The frontier orbital theory [7-9] assumes that the stabihzation by the electron delocalization could control chemical reactions. The stabilization comes from the interactions between the occupied molecular orbitals of one molecule and the unoccupied molecular orbitals of another (Sect. 1.4). The strong interaction occurs when the energy gap is small (Sect. 1.3). The HOMO and the LUMO are the closest in energy to each other. The HOMO-LUMO interaction, especially the interaction between the HOMO of electron donors and the LUMO of electron acceptors, controls the chemical reactions (Scheme 20). The HOMO and the LUMO are termed the frontier orbitals. ... 

This factorization of the rate of the elementary process (Eq. 1) leads (with a few approximations) to the compartmentalization of the experimental parameters in the following way the dependence of the rate upon reaction exo-thermicity and upon environmental polarity controls and is reflected in the activation energy and the temperature dependence, whereas the dependence of the rate upon distance, orientation, and electronic interactions between the donor and the acceptor controls and is reflected in Kel- We refer to this eleetronie interaction energy as A rather than the common matrix element symbol H f, since we require that A include contributions from high-order perturbations and in particular superexchange processes. Experimentally, the y-intereept of the Arrhenius plot of the eleetron transfer rate yields the prefactor [KelAcxp)- - AS /kg)], and hence the true activation entropy must be known in order to extract Kel- An interesting example of the extraction of the temperature independent prefaetor has been presented in Isied s polyproline work [35]. 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

The rate of respiration (02 consumption) in mitochondria is tightly regulated it is generally limited by the availability of ADP as a substrate for phosphorylation. Dependence of the rate of 02 consumption on the availability of the Pj acceptor ADP (Fig. 19-18b), the acceptor control of respiration, can be remarkable. In some animal tissues, the acceptor control ratio, the ratio of the maximal rate of ADP-induced 02 consumption to the basal rate in the absence of ADP, is at least ten. 

ATP synthase 704 FiATPase 708 rotational catalysis 711 P/O ratio 712 P/2e ratio 712 acceptor control 716 mass-action ratio 716 light-dependent reactions 723 light reactions 723 carbon-assimilation reactions 723 carbon-fixation reaction 723 thylakoid 724 stroma 724 exciton transfer 725 chlorophylls 725... 

The regulation of the rate of oxidative phosphorylation by the ADP level is called respiratory control or acceptor control. The level of ADP likewise affects the rate of the citric acid cycle because of its need for NAD+ and FAD. The physiological significance of this regulatory mechanism is evident. The ADP level increases when ATP is consumed, and so oxidative phosphorylation is coupled to the utilization of ATP. Electrons do not flow from fuel molecules to O ... 

How is the rate of the electron-transport chain controlled Linder most physiological conditions, electron transport is tightly coupled to phosphorylation. Electrons do not usually Jlow through the electron-transport chain let CT unless ADP is simullaneously phosphorylated to ATP. When ADP concentration rises, as would be the case in active muscle, the rate of oxidative phosphorylation increases to meet the ATP needs ot the muscle. T he regulation of the rate of oxidative phosphorylation by the ADP level is called res piratory control or acceptor control. Experiments on isolated mitochondria demonstrate the importance of ADP level (Figure 18,40). The rate of oxygen consumption by mitochondria increases markedly when ADP is added and then returns to its initial value when the added ADP has been converted... 

Acceptor control The regulation of the rate of an enzymatic reaction by the concentration of one or more of the substrates. 

Nessi et al. (Nl) succeeded in rupturing the PMN membrane by adding heparin to the cell suspension. The oxygen consumption, which was measured by a photometric method, could be stimulated by the addition of ADP. An oxygen curve in stage 3, an acceptor control index, and an oxidative phosphorylation quotient with different substrates were obtained according to the scheme of Chance and Williams (Cl). 

Photosynthetically grown (red) as well as heterotrophically in the dark developed (white) membranes from R. palustris undergo "respiratory control" (2) by influencing the electron flow rates, when substrate is present in excess, upon the addition of ADP, Pi and Mg2+ (3). This acceptor control by phosphorylating substrates resulted in a retardation of electron transport in chromatophores, whereas the flow rate was stimulated in white membranes from / . palustris (Fig.2). However, even in totally RpFi-depleted chromatophores an acceptor control by ADP, Pi and Mg2+ can be observed (Tab.2) with an acceptor control index (4) of 1.6. 

FIGURE 2. Acceptor control by phosphory-lating substrates during cytochrome c-induced 02 consumption in chromatophores from / . palustris, - (1)... 

A chemoselective synthesis of tetrasubstituted furans has been accomplished using an intramolecular Wittig reaction as the key step. The presence of an additional electron-withdrawing group in the a-position of Michael acceptors controlled the chemoselectivities of the phosphorus ylides presumed to be involved. Mechanistic studies by DFT calculations were carried out to understand the origin of the chemoselectivity observed. 

Feo, R, Canute, R.A., and Garcea, R. (1973). Acceptor control ratio of mitochondria. Factors affecting it in Morris hepatoma 5123 and Yoshida hepatoma AH-130. Eur. J. Cancer 9, 203-214. 

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