Forster separation distance

At first glance, it would appear that the results hy Lapp and Vickers (1992) do not agree with the findings of Frobese and Forster (1992) since the recommended separation distance of Lapp and Vickers is far in excess of that proposed by Frobese and Forster. 

It should be pointed out that the Forster calculations are based on the point dipole assumption which may be inaccurate when the separation distance is similar to the molecular size, as is the case for LHCII. In this situation the transition monopole approximation should also be considered. For chla Chang [172] has estimated that this leads to a Forster correction factor of 0.6-2.0 depending on orientation. 

The rates of the Forster energy transfer and Dexter energy transfer depend on the separation distance between the donor and the acceptor, shown qualitatively in Fig. 1.1. The former decreases as the inverse sixth power of the distance, whereas the latter falls off exponentially as the distance increases. Therefore, the Forster energy transfer is able to occur over very large distances, while Dexter energy transfer will give much greater rates at short distances and close contacts. 

As the degree of energy transfer is also dependent on the separation distance between D and A, Forster [99] developed the following expression for the rate constant for energy transfer (kET)... 

When a system contains two fluorescing chromophores such that the emission spectrum of the donor overlaps the absorption spectrum of the acceptor, excitation energy from the donor can be transferred to the acceptor over a considerable separation distance, R. The efficiency, , of this energy transfer would be governed by the equation proposed by Forster [40] ... 

Forster s theory is behind a very nsefnl method for determining the absorption spectrum of a collection of identical chromophores within a separation distance of less than 100 A, placed in a protein, DNA, or some other scaffold. One example is the indole groups in a protein, where the interaction causes a broadening of the absorption spectrum. 

Zhang J, Fu Y, Chowdhury MH et al (2007) Enhanced Forster resonance eneigy transfer on single metal particle. 2. Dependence on donor-acceptor separation distance, particle size, and distance from metal surface. J Phys Chem C 111 11784-11792... 

Another important aspect that needs attention is the optical behavior of a chromophore near a metal surface. The loss in fluorescence intensity by Forster energy transfer mechanisms operating for chromophores in the immediate vicinity of the metal substrate has to be balanced against the decrease in the excitation probability for higher separation distances governed by the exponential decay of the evanescent surface mode normal to the metal substrate. ... 

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) ... 

The Forster formulation predicts R-H dependence on separation distav between the donor and the acceptor. Therefore the rate of transfe expected to increase sharply when distance is reduced and vice ve -., For example, at a distance R = R0/2, the rate of transfer... 

The PAH polymeric layer played an important role in our fluorescence sensor design. First, its positive charges enabled the deposition of anionic dextran that was labeled with the pH indicator fluorescein on the surface of the nanoparticles. More importantly, the PAH polymeric layer separated between the fluorescein molecules and the metal particle. In fact, the thickness of the polymeric layer was over 10 nm, which is larger than the Forster distance required for efficient energy transfer between the fluorophore and the metallic gold particles. 

The energy transfer efficiency is directly proportional to the spectral overlap, and this also directly affects the Forster distance of a particular D-A pair. Figure 10.5 shows the D and A excitation and emission spectra in an ideal energy transfer system, wherein D and A have very distinct excitation spectra (so that A can only be excited by energy transfer and not by direct photon absorption at the wavelengths used to excite D)—the D emission and A excitation spectra overlap strongly—and the D and A emission maxima are well separated, so that the quenching of D fluorescence and the enhancement of A fluorescence can be individually measured.98 99... 

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