Falling acceptor

Deprived of their substrate in severe or prolonged hypoxia, some ATPase-driven systems, including ion pumps, may become impaired. Further, with the decrease in the availability of O2 as its terminal electron acceptor, the mitochondrial transport chain becomes increasingly unable to accept reducing equivalents from cellular metabolic processes. Hence the intracellular pH falls, subjecting the cell as a whole to a reductive stress and favouring those enzyme systems with acid pH optima. 

Romero-Rossi and Stone (11, 12) observed an increase in the positive effect (02 on ZnO) on addition of lithium (acceptor) to the sample and, on the contrary, a fall in it when gallium (donor) was introduced. The dependence of the magnitude and sign of the effect on the character and degree of stoichiometric disturbance in the sample has been observed by a number of authors. 

As a result of the adsorption of a donor gas, we are transferred up the curve go = go(er) in Fig. 8a. The adsorption of an acceptor gas, on the contrary, transfers one down this curve. If one remains on the acceptor branch of the curve, this will mean that the catalytic activity must increase when a donor gas is adsorbed and fall upon adsorption of an acceptor gas. This is in accord with much experimental data (see Section III.A). 

Special emphasis must be made on the experiments carried out by Voltz and Weller (35) who observed a fall in activity caused by the adsorption of water which usually acts as a donor. To understand this result, one must suppose that the authors were dealing with the donor branch of the curve in Fig. 8a. Or else that they remained on the acceptor branch but the water molecules acted as acceptors. It should be noted in this connection that the acceptor functions of water (the negative charging of the surface upon adsorption of water) had also been observed (before Voltz and Weller) in certain cases by Yelovich and Margolis (46). 

Thus, the preliminary chemisorption of a foreign donor gas causing a fall in e8 (at ev = const) must increase K (transfer from the point A to the point D in Fig. 9). The chemisorption of an acceptor gas accompanied by an increase in e8 (at tv = const) must weaken the effect (transfer from the point A to the point E). 

The dependence of g0 on the position of the Fermi level at the surface of the catalyst is presented in Fig. 11 by a heavy curve, in accordance with (80) and (7). We see that within the framework of the mechanism considered above the oxidation of CO falls into the category of acceptor reactions (i.e., it is retarded as the Fermi level is lowered). 

Although satisfactory for weak donor-acceptor Interactions, this model does not hold in the case of strong charge transfer such as happens in some disubstltuted benzene derivatives. A simple additive assumption falls short of the experimental values for the three nitroanllines in the para, ortho and meta positions (14). Discrepancies between an additivity assumption and experimental values are seen to Increase with the degree of conjugation Induced by the substituent relative locations. 

Here, Ox is the oxidant that reacts with the reductant L, kr is the pseudo-first-order rate constant causing the rise in P, the excited product that is the intermediate in the type II reaction, and ks is the rate constant for deactivation of A [fcs = kf + kd in Eq. (1)]. P will fall in concentration with rate constant ksT as it transfers electronic excitation to the acceptor A to form the excited acceptor A. ... 

Figure 3.113 Leading ct-t donor-acceptor interactions in B1211122 Note that both t orbitals appear truncated in this view (cf. Fig. 3.112) because atoms 1 and 6 fall below the chosen (4—8—12) contour plane.

Figure 5.1 Hydrogen-bonded B -HA binary complexes (left) and leading nB CTHA+ donor-acceptor interactions (right), with second-order stabilization energies in parentheses (cf. Table 5.1). (Note that the H atom falls slightly out of the contour plane in the upper-right panel, so that the cross-hairs symbol for this nucleus is absent.)...

Cyanine dyes fall within the more general category of polymethine dyes, in which a chain of methine groups is terminated with a donor group and an acceptor group respectively [82]. 

A detailed theory of energy transfer by the Coulombic mechanism was developed by Forster, so the process is often referred to as Forster resonance energy transfer (FRET). According to the Forster theory, the probability of Coulombic energy transfer falls off inversely with the sixth power of the distance between the donor and the acceptor. For... 

Triafulvene compounds (41) with acceptor groups for X and Y have been studied notably by Eicher et al. (55,56), who found C=C barriers from 14 kcal/mol upward (Table 9). The barrier-lowering effect of the acceptor groups seems to fall in the same order as for ketene mercaptals and aminals, but the capacity of the cyclopropenium ring to stabilize the transition state seems to be less than that of the (MeS)2C group in ketene mercaptals. The barrier is quite sensitive... 

The static trans effect of L on the CO-stretching frequencies in [30a-30g] does not show any correlations to the Osn/Osm redox potentials (see Table 10), which range between 0.48 and 0.56 V irrespective of L, with the exception of the species [30c], [30i], and [30k] where L contains an imidazole skeleton in the latter compounds, they are about 0.1 V lower than the mean value of the others. As this deviation has no parallel with the optical spectra, it is assumed that this low redox potential indicates an additional rr-donor effect of the imidazole moiety. As this additional 7r-donor effect is also ascribed to the oxygen donors, e.g., DMF and THF, the latter should also show low redox potentials. However, their potentials fall into the normal range. This may be due to the fact that they have a different a-rr-donor 7r-acceptor balance, the oxygen donors acting as weaker a-donors and not at all as 7r-acceptors. For this reason, their w-donor function becomes very effective in the fixation of... 

More convincing proof for a particle-enhanced energy transfer mechanism comes from a study of the concentration dependence of the transfer. Bulk Forster transfer leads to a linear dependence on acceptor concentration with constant donor-to-acceptor ratio. The resonance mechanism would be expected to saturate at (relatively) high concentrations and fall off linearly at very low concentrations. 

P has a very suggestive form in relation to Figure 8.26. For a large concentration of acceptors, the second term in the denominator can be made considerably smaller than 1 (i.e., Xt is proportional to acceptor concentration [A]), and P will be independent of concentration. On the other hand, for a small concentration of acceptors, the second term in the denominator can be made considerably larger than 1, and P will fall off linearly as the concentration is reduced. The scale factor in all of this is Q. With Q large, the transition from concentration independence to linear concentration dependence will be at low acceptor concentrations. P falls to 5 when the second term in the denominator of Eq. (8.27) is equal to 1, and so a critical concentration of acceptors [A], /2 can be defined to characterize the falloff. Expressing Xt in terms of molecular parameters (x, = em[A] ln(10)/, where n is the particle refractive index, em is the molar decadic extinction coefficient, [A] is the concentration of acceptors, and k is 2n/X) yields... 

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