Efficiency of energy transfer

This is our principal result for the rate of desorption from an adsorbate that remains in quasi-equihbrium throughout desorption. Noteworthy is the clear separation into a dynamic factor, the sticking coefficient S 6, T), and a thermodynamic factor involving single-particle partition functions and the chemical potential of the adsorbate. The sticking coefficient is a measure of the efficiency of energy transfer in adsorption. Since energy supply from the... 

A poly(L-lysine) dendrimer 23 which carries 16 free-base porphyrins in one hemisphere and 16 Zn porphyrins in the other has been synthesized and studied in dimethylformamide solution [54]. In such a dendrimer, energy transfer from the Zn porphyrins to the free-base units can occur with 43% efficiency. When the 32 free base and zinc porphyrins were placed in a scrambled fashion, the efficiency of energy transfer was estimated to be 83% [55]. Very efficient (98%) energy transfer from Zn to free-base porphyrins was also observed in a rigid, snowflake-shaped structure in which three Zn porphyrin units alternate with three free-base porphyrin units [56]. 

The transfer efficiency decreases from a value close to 100% for short oligomers to 16% for the n = 12 oligomer. The efficiency of energy transfer as a function of distance is shown in Figure 6.7. 

Figure 6.7. Efficiency of energy transfer as a function of distance for compounds (3), n = 1-12. From Stryer and Haughland.<45) Reprinted by permission of Proc. Nat. Acad. Sci. U.S.

FRET can take place if the emission spectrum of the donor overlaps with the absorption spectrum of the acceptor and they are located at separation distances within 1-10 nm from each other. The efficiency of energy transfer E can be defined... 

The efficiency of energy transfer (E) is the ratio of the number of energy transfer occurrences from D to A divided by the total number of excitations of a donor molecule. This is the same as the ratio of the rate of energy transfer to the total rate of deactivation of the excited donor. The rate of energy transfer between single donor and acceptor molecules is proportional to 1 /r6DA (Eq. (1.1)) this is a very... 

The efficiency of energy transfer is the same as the quantum yield of energy transfer. It is the number of times that molecules take the energy transfer pathway divided by the number of times that the donor molecules have been excited. This is the same as the ratio of the number of times the excited donor exits by transferring energy to the number of times the excited molecules exit by any process to return to the ground state. In terms of the lifetimes of the donor, this is ... 

We can also count the total number of donor-emitted photons, or measure the corresponding analog intensity, in the presence and absence of energy transfer. From these intensities we can calculate the efficiency of energy transfer. The fluorescence intensity of the donor is proportional to the rate constant through the fluorescence pathway divided by the sum of the rates of leaving the excited state by all pathways. That is,... 

Measuring the acceptor fluorescence or reaction products to determine the efficiency of energy transfer... 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

Photon emission must be a favorable deactivation process of the excited product in relation to other competitive nonradiative processes that may appear in low proportion (Fig. 3). In the case of sensitized CL, both the efficiency of energy transfer from the excited species to the fluorophore and the fluorescence efficiency of the latter must be important. 

The efficiency of energy transfer can also be determined using ... 

Energy-transfer measurements are often used to estimate the distances between sites on biological macromolecules such as proteins, and the effects of conformational changes on these distances. In this type of experiment, the efficiency of energy transfer is used to calculate the distance between donor and acceptor fluorophores in order to obtain structural information about the macromolecule. 

Single-stranded DNA or RNA may adopt hairpin structures in which the distance between two sequences is much shorter than in the absence of hairpin. Figure B9.4.1 shows two synthetic targets, both containing 45 nucleotides, but only the first one is able to form a hairpin via a loop of four thymines. The second one is used as a control. Both contain the complementary sequences for ethidium-13-mer and 11-mer-coumarin separated by the same number of bases. The efficiency of energy transfer from coumarin to ethidium is dose to zero for the control, whereas it is about 25% in the hairpin structure. This value is low but the spatial conformation of this particular three-way junction is only partially known, and the transfer efhdency depends on the relative orientation and/or distance between coumarin and ethidium. 

Thus, the efficiency of energy transfer between donors and acceptors randomly distributed in a plane depends on R0, a, and a, and the transfer efficiency is independent of a. The important point was made that surface density of the acceptor could be 1 per 500 phospholipids for R0 > 30 A. Using these equations for different donor and acceptor concentrations, the data were matched against the different theoretical curves to obtain the R0. An example of the application of the method of Fung and Stryer(81) is the study of energy transfer between the tryptophan of a membrane protein (or peptide models of proteins) and DPH,(83) in which it was shown that efficient energy transfer can occur without any special interaction being required between DPH and the proteins in specific areas of the membrane. 

Fig. 2. Parameters affecting the efficiency of energy transfer. (A) Overlay of FITC emission spectrum and PE absorbance spectrum normalized to maximum fluorescence intensity and maximum optical density, respectively. FITC fluorescence intensity was measured as a function of emissions wavelength using a fluorimeter with an excitation wavelength of 488 nm. PE optical density was measured as a function of wavelength using a spectrophotometer. (B) Schematic representation of energy absorption and the possible pathways for the subsequent energy release (abbreviations as in the text).

The condition for transfer of singlet excitation energy from a fluorophore (donor) to another chromophore (acceptor) to occur is spectral overlap of the donor s fluorescence emission spectrum with the acceptor s absorption spectrum. The efficiency of energy transfer E is defined in eq 15... 

Fig. 1. Concentration dependence of the efficiency of energy transfer A, p-methoxy-acetophenone o, m-methoxyacetophenone A, 3,4-methylenedioxyacetophenone , thioxanthone. (From Chapman and Wampfler48 with permission of the American Chemical Society.)...

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