Donor local concentrations

As in the case of most FRET imaging formalisms, Eq. (12.4) applies to the donor at a particular x, y, x coordinate j as a virtual species with an apparent kt(j) it applies for arbitrary absolute and relative local concentrations of donor and acceptor but only if population distributions are properly considered (see other chapters). For example, assuming a population of donors, a fraction a of which are in a unique DA environment (e.g., as a bound complex) and the rest free, one can generate an expression for a in terms of known parameters and experimental signals (varying from point to point), kt/kf, Qd, and Qa- Related expressions can be formulated in terms of donor and acceptor emission anisotropies and lifetimes, and for conditions of nonlinearity such as ground state depletion of the donor and/or acceptor [1] (see Section 12.3). 

RET can be used to investigate the lateral organization of phospholipids (range of 100 A) in gel and fluid phases. Indeed, information can be obtained on the probe heterogeneity distribution the donors sense various concentrations of acceptor according to their localization. A continuous probability function of having donors with a mean local concentration CA of acceptors in their surroundings should thus be introduced in Eq. (9.36) written in two dimensions ... 

It seems worth pointing out, that 137 and human semm albumin contain no pendant phosphines and the donor atoms in the complexes formed with rhodium can be only O (137) or O, N and perhaps S (HSA), which are not the most suitable for stabilizing low oxidation state metal ions. Still these macroligands gave active and stable catalysts with rhodium, which shows that perhaps in the high local concentration provided by the polymer even these hard donor atoms are able to save the metal ion against hydrolysis or other deterioration. 

In contrast to the electron transfer reactions in question, the efficiency of which is primarily determined by the difference of redox potentials of the excited donor molecule and the acceptor and, hence, by the variation in free energy (21), efficiencies of exothermic energy transfers depend solely on the local concentration of an appropriate quencher. 

Efficient energy transfer between FI and RhB, upon binding into the galleries of surfactant-laden a-ZrP, is partly due to the increase in the local concentrations of the acceptor. At high concentrations of the donor, close packing and orderly arrangement of the donor chromophores can bring an acceptor molecule at... 

NaLS) by copper(II) yields assemblies in which Cu2+ ions constitute the counter ion atmosphere of the micelle (Fig. 4.8). These may be photoreduced to the monovalent state by suitable donor molecules incorporated in the micellar interior. An illustrative example is that where D = N,N -dimethyl 5,11-dihydroindolo 3,3-6 carbazole(DI). When dissolved in NaLS micelles, DI displays an intense fluorescence and the fluorescence lifetime measured by laser techniques is 144 ns. Introduction of Cu2+ as counterion atmosphere induces a 300 fold decrease in the fluorescence yield and lifetime of DI. The detailed laser analysis of this system showed that in Cu(LS) micelles there is an extremely rapid electron transfer from the excited singlet to the Cu2+ ions. This process occurs in less than a nanosecond and hence can compete efficiently with fluorescence and intersystem crossing165. This astonishing result must be attributed to a pronounced micellar enhancement of the rate of the transfer reaction. It is, of course, a consequence of the fact that within such a functional surfactant unit regions with extremely high local concentrations of Cu2+ prevail. (Theoretical estimates predict the counterion concentration in the micellar Stem layer to be between 3 and 6 M). 

It is intermediate thus, ATP is ideally positioned to serve as a phosphate donor or (as ADP) a phosphate acceptor, depending on local concentrations. 

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