Acceptors direct measurement

Said et al. [78] directly measured the acid microclimate on the surface of gastrointestinal tract (GIT) epithelial cells (intact with mucus layer) in rats. The pH on the apical (donor) side of the cells varied from 6.0 to 8.0, while the pH on the basolateral (acceptor) side was 7.4. Furthermore, the pH gradient between... 

Table 1 provides values of the prefactor valid for either Eq. 9 or Eq. 10 above in addition the rate constant is equal to this prefactor in the special case of AG = 0 (Eq. 9) or the summation = 1 in Eq. 10, which is the maximum possible rate constant. The fact that one should be able to experimentally locate the AG = 0 maximal speed point by varying the donor/acceptor reaction energetics has been successfully exploited by Miller and Gloss [50] to obtain a direct measurement of this non-adiabatic pre-factor. 

A bound divalent metal ion, usually Mn2+, is required in the transcarboxylation step. A possible function is to assist in enolization of the carboxyl acceptor. However, measurement of the effect of the bound Mn2+ on 13C relaxation times in the substrate for pyruvate carboxylase indicated a distance of 0.7 ran between the carbonyl carbon and the Mn2+, too great for direct coordination of the metal to the carbonyl oxygen.68 Another possibility is that the metal binds to the carbonyl of biotin as indicated in Eq. 14-11. Pyruvate carboxylase utilizes two divalent metal ions and at least one monovalent cation.683... 

Direct measurements of rate constants for quenching of donor triplets by various acceptors in flash experiments indicate that the rates never exceed those of diffusion-controlled reactions. Moreover, the rates are roughly inversely proportional to viscosity.143 Consequently, we infer that transfer occurs at a measurable rate only if the two partners are nearest neighbors in solution. 

Thus, the results of direct measurements of diffusion rate constants for reactions of etr with acceptors in water-alkaline matrices in the vicinity of the temperature of their devitrification corroborate the conclusion that long-range electron tunneling is the main channel of performing these reactions at low temperatures. 

SM-FRET experiments are typically performed by using a dual-channel detection scheme. More specifically, one photo-excites the donor with CW radiation or a train of pulses, while simultaneously detecting the fluorescence photons from the donor and acceptor in a selective manner. The fraction of photons detected in the acceptor channel, over a given time averaging window of length Tw, provides a direct measure of the time-averaged FRET efficiency, which we will denote by E(Tw)- One may then define a time-averaged and TV-dependent donor-acceptor distance, which will be denoted by R)tw, such that... 

Fig. 7. Redox titration of the primary acceptor Qa in ths reaction-center compiex of Rb. sphaeroides monitored by the amplitude of the g= S EPR signal at 10 K vs. the redox potential. Figure source Dutton, Leigh and Wraight (1973) Direct measurement of the midpoint potentiai of the primary electron acceptor in Rhodopseudomonas sphaeroides in situ and in the isolated state some relationships with pH and o-phenanthroline. FEES Lett 36 171.

In GaP, selected pair luminescence (SPL), whose principle is explained in Sect. 1.3.3, has been used by Street and Senske [163] to directly measure the transition energies of the MgGa, Znoa, and Cp acceptors. The advantage of this method is that a value of the ground state energy can also be obtained directly. The absorption by the classical method of a few lines of Be, Mg, Cd, and C acceptors in GaP has also been reported by Kopylov and Pikhtin [103]. 

At present, there is widespread interest in directly measuring the time-dependent processes. This is because considerably more information is av lilable from the time-dependent data. For example, the time-dependent decays of protein fluorescence can occasionally be used to recover the emission spectra of individual tryptophan residues in a protein. The time-resolved anisotropies can reveal the shape of the protein and/or the extent of residue mobility within the protein. The time-resolved energy transfer can reveal not only the distance between the donor and acceptor, but also the distribution of these distances. The acquisition of such detailed information requires high resolution instrumentation and careful data acquisition and analysis. 

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