Forster efficiency

Q.27.4 A solvent with an ionic strength of 0.15 alters the Ro of fluorochrome pair above to 15 A because of the effect of the local electric field from the ions on the resonance transfer. You reconstitute your pore assembly and notice that when the conductivity of the membrane increases the Forster efficiency drops to 25%. Explain this effect and describe the state of the environment in the pore in the conducting and non-conducting state. 

In these dye-functionalized dendrimers, light absorbed by the numerous peripheral coumarin-2 units is funneled to the coumarin-343 core with remarkably high efficiency (toluene solution 98% for the first three generations 93% for compound 8). Given the large transition moments and the good overlap between donor emission and acceptor absorption, energy transfer takes place by Forster mechanism [34]. 

Hardtke S, Ohl L, Forster R. Balanced expression of CXCR5 and CCR7 on follicular T helper cells determines their transient positioning to lymph node follicles and is essential for efficient B-cell help. Blood 2005 106 1924-1931. 

Figure 6.8. Dependence of the energy transfer efficiency (E = etin the figure) on distance. The slope of 5.9 is in excellent agreement with the r e dependence for Forster-type transfer. From Stryer and Haugh-land.(45) Reprinted by permission of Proc. Nat. Acad. Sci. U.S.

Using the Forster s derivation of see Eq. (1.1), we can write the efficiency in terms of the distances (which is the result that is normally desired) ... 

The FRET efficiency (E) is highly dependent on the distance between donor and acceptor and is defined by Forsters theory [105] ... 

Posokhov, Y. O., Merzlyakov, M., Hristova, K. and Ladokhin, A. S. (2008). A simple proximity correction for Forster resonance energy transfer efficiency determination in membranes using lifetime measurements. Anal. Biochem. 380, 134—6. 

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

Pei et al. [412] reported an alternating fluorene copolymer 331 with 2,2 -bipyridyl in a side chain that emitted at 422 nm. Treating this polymer with Eu3+ chelates formed the polymeric complexes 332-334. Their emission was governed by intramolecular Forster energy transfer, whose efficiency depends on the structure of the ligands and the Eu3+ content (Scheme 2.49) [412], The most effective energy transfer manifested itself in a single red emission band at 612 nm for the complex 332 with a maximum intensity achieved at —25 mol% content of Eu3+. 

Another example of efficient Forster energy transfer in Eu3+ complexes of fluorene copolymers (similar to the alternating copolymers described in Scheme 2.49) was demonstrated by Huang and coworkers [414] for random copolymers. They synthesized copolymers 336 with a different ratio between the fluorene and the benzene units in the backbone and converted them into europium complexes 337 (Scheme 2.50) [414]. The complexes 337 were capable of both blue and red emission under UV excitation. In solution, blue emission was the dominant mode. However, the blue emission was significantly reduced or completely suppressed in the solid state and nearly monochromatic (fwhm 4 nm) red emission at 613 nm was observed. 

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