Donor/acceptor separation

The rate of ET depends on the donor-acceptor separation distance. In the poly(A/St/Phen)-MV2 + system, the Phen moiety is protected from a close contact with MV2+, but the distance between the compartmentalized Phen and bound MV2+ species is uncertain. This means the impossibility of quantitative discussion on the ET rate in terms of the distance dependence. The spread of the... 

This can be an important process if the acceptor A absorbs in the wavelength region in which the donor D emits. The efficiency of the process is determined by the quantum yield of D emission and by the optical density of A at the donor emission wavelength. The probability that an acceptor molecule will reabsorb the light varies as R 2, where R is the donor-acceptor separation. 

Energy transfer measurements were used, together with fluorescence and absorption spectral data of the donor and acceptor moieties, to calculate the donor-acceptor separation via the Forster equation. The average values of R obtained assuming random donor-acceptor orientations were 21.3 1.6 for (1) and 16.7 + 1.4 for (2). The average separation obtained from molecular models is 21.8 + 2.0 for (1) and 21.5 2.0 for (2). The somewhat low calculated separation between the groups of (2) may be due to nonrandom donor-acceptor orientations. 

Kiefer PM, Hynes JT (2002) Nonlinear free energy relations for adiabatic proton transfer reactions in a polar environment. I. Fixed proton donor—acceptor separation. J Phys Chem A... 

The rate of energy transfer at a very short donor-acceptor separation R by the exchange mechanism has been given by Dexter (1953) as follows ... 

The dipole approximation is valid only for point dipoles, i.e. when the donor-acceptor separation is much larger than the molecular dimensions. At short distances or when the dipole moments are large, it should be replaced by a monopole-monopole expansion. Higher multipole terms should also be included in the calculations. 

V = 10 cm and R = 1.9 nm for ET in the quinone-chlorophyll system. These estimates of donor-acceptor separation are in good agreement with the results of magnetic measurements (17). 

One expects the impact of the electronic matrix element, eqs 1 and 2, on electron-transfer reactions to be manifested in a variation in the reaction rate constant with (1) donor-acceptor separation (2) changes in spin multiplicity between reactants and products (3) differences in donor and acceptor orbital symmetry etc. However, simple electron-transfer reactions tend to be dominated by Franck-Condon factors over most of the normally accessible temperature range. Even for outer-... 

It is important to determine whether some specific donor-acceptor interactions can result in a reasonably large numerical value of V even for large donor-acceptor separations. A tenta-... 

M. Kupryszewska, I. Gryczynski, and A. Kawski, Intramolecular donor-acceptor separations in methionine- and leucine-enkephalin estimated by long-range radiationless transfer of singlet excitation energy, Photochem. Photobiol. 36, 499-502 (1982). 

Both the electronic couphng matrix element and the outer-sphere component of the nuclear reorientation parameter are thought to vary with donor-acceptor separation and orientation [29, 49]. It has been shown in studies of Os and Ru-ammines bridged by polyproline spacers that the distance dependence of X can be greater than that of [50]. Dielectric continuum models of solvent reorganization predict that Xg will increase with... 

FRET interactions are typically characterized by either steady-state or transient fluorescence emission signals from the donor or acceptor species. Efficient nonradiative energy transfer results in donor PL loss associated with acceptor gain in photoluminescence intensity (if the acceptor is an emitter). The rate of this energy transfer is related to the intrinsic lifetime of the isolated donor and depends strongly on the donor-acceptor separation distance ... 

With a precise measurement of the FRET efficiency, it is possible to deduce donor-acceptor separation distances by rearranging Eq. (14.5) (for multiple equidistant acceptors) ... 

The validity of the Forster theory was tested and confirmed in a number of model studies with compounds that contained a donor and an acceptor separated by well-defined rigid spacers. This work has been reviewed 5] In a classical study, a naphthyl group (donor) was attached to the C-terminal and a dansyl group (acceptor) to the N-terminal of poly-L-proline oligomers (1-12 proline residues) 61 These proline oligomers assume a trans helical conformation in ethanol and thus represent spacers of well-defined length (12-46 A). A continuous decrease in the transfer efficiency from 100% at a donor-acceptor separation of... 

Spatial donor-acceptor separation in the certain reference state... 

Hydrogen transport. The approach that they have used to predict proton transport through complex membranes such as Nation is to use ab initio methods to determine the barriers for migration of hydrogen as a function of the donor-acceptor separation and then to employ a statistical method that is based on the ab initio results. This method allows a proton jump among water clusters when the configuration around the proton is appropriate. 

This latter expression is very useful, as Ro is characteristic of each donor-acceptor pair, so it can be calibrated and then be used to predict distances from EET measurements. In Fig. 6 we show a schematic representation of the spectral overlap between donor emission and acceptor absorption given by Eq. 2 as well as a plot of the EET efficiency as a function of the donor-acceptor separation. 

Recently, the effect of the donor-acceptor separation has been studied.76 Both the fluorescence lifetime and quantum yield were found to decrease as the distance between the two porphyrins—Cmeso-Cmeso (cd) and CCmeso-CCmeso (ab)—decreases (Fig. 24). As the two rings get closer to each other, they interact more strongly, and hence nonradiative deactivation becomes more pronounced.75,76... 

The pre-exponential factor A in Eq. 1 is a weak function of the temperature and the reorganization energy, and strongly dependent upon the electronic coupling matrix element V. In the simplest case, V may be assumed to be exponentially dependent upon the through-space donor-acceptor separation r. This yields a distance dependence for electron transfer of ... 

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