Donor-acceptor distances, energy-transfer studies

We recently prepared a series of restricted distance porphyrin-quinone donor-acceptor molecules designed to study the dependence of the rate of electron transfer proceeding from the lowest excited singlet state of the porphyrin on the free energy of reaction.(3-4) We now report measurement... 

FRET occurs when the electronic excitation energy of a donor chromophore is transferred to an acceptor molecule nearby via a through-space dipole-dipole interaction between the donor-acceptor pair.80 The strong dependence of the FRET efficiency on donor-acceptor distance has been widely exploited in studying the structure and dynamics of proteins and nucleic acids, in the detection and visualization of inter-molecular association, and in the development of intermolecular binding assays.81,82... 

As an example of excitation energy transfer studied by time-resolved fluorescence, let us take again the case of the inclusion complex of the multichromophoric cyclodextrin CD-St with oxazine 725 described in Section 7.2.4.2 [15]. Figure 7.9 shows the fluorescence decay of CD-St the very first part of the decay is due to energy transfer [13] from the steroidic naphthalene fluorophores to oxazine 725. Data analysis led to an average decay time for transfer of about 25 ps, which is quite fast, as expected from the short average distance between donor and acceptor ( 9-10 A). 

Energy Transfer Studies of Donor-Acceptor Distances... 

Table III summarizes rate, helicity, and donor-acceptor distance data for the 16-mer bundle. Because of the observable trend in conformation versus rate in the electron transfer studies, it was decided to measure donor-acceptor distances in the 16-mer metalloprotein bundles. In order to study the effects of solution conditions on H, the donor-acceptor distance, Forster energy transfer was used as a spectroscopic ruler, according to...

This hypothesis can be tasted by energy-transfer studies. Donor and acceptor groups are bound to an enzyme and the distance between them is measured when no substrate is present. Then the substrate is added and the distanee is measured again. When such studies are performed, a large change in the distance between the donor and the aeceptor groups takes place upon addition of the substrate. This is ample proof for the induced-fit model. 

That dendrimers are unique when compared with other architectures is confirmed by an investigation on porphyrin core dendrimers and their isomeric linear analogues [63]. The isomers displayed dramatically different hydrodynamic properties, crystallinity, and solubility characteristics when compared to those of their dendritic analogues, and photophysical studies showed that energy transfer from the poly(benzylether) backbone to the core was more efficient in the dendrimer because of the shorter distance between the donor units and the acceptor core. 

The Forster resonance energy transfer can be used as a spectroscopic ruler in the range of 10-100 A. The distance between the donor and acceptor molecules should be constant during the donor lifetime, and greater than about 10 A in order to avoid the effect of short-range interactions. The validity of such a spectroscopic ruler has been confirmed by studies on model systems in which the donor and acceptor are separated by well-defined rigid spacers. Several precautions must be taken to ensure correct use of the spectroscopic ruler, which is based on the use of Eqs (9.1) to (9.3) ... 

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