Energy Transfer Systems

Fluorescence energy transfer is the transfer of excited state energy from a donor to an acceptor. The transfer occurs as a result of transition dipole-dipole interactions between the donor-acceptor pair. The fluorescent sensor investigated, 147(phenanthrene pyrene), had two phenylbotonic acid groups a hexamethylene linker and two different lluorophore groups phenanthrene and pyrene. 

The purpose of constructing sensor 147(phenanthiene pyrene) was to investigate the efficiency of energy transfer from phenanthrene to pyrene as a function of saccharide binding. A similar concept had previously been employed in the 

The fluoreseence titrations of sensor 147(phenanthrene pyrene) (2.5 X 10 mol dm , Xex=299 nm for phenanthrene and Xex=342 nm for pyrene) with different saccharides were carried out in a pH 8.21 aqueous methanolic buffer, as described above (page 86). Absorption V5. concentration plots of sensor 147(phenanthrene pyrene) and the monoboronic add reference compounds 146(pyrene) and 153(phenanthiene) Confirmed that the Jt-Jt stacking of sensor 147(piienanthrene pyrene) was solely iotramolecular. The fluorescence intensity of sensor 147(phenanthrene pyrene) at 417 nm increased with added saccharide when excited at both 299 and 342 nm, while the excimer emission at 460 nm decreased with added saccharide. The change in excimer emission indicates that the Jt-Jt interaction between phenanthrene and pyrene is disrupted on saccharide binding. 

The observed stability constants ( obs) of sensor 147(phenanthrene pyrene) (with Xex=299 nm and ,ex=342 nm) were calculated by fitting the emission intensities at 417 nm V5. concentration of saccharide curves and are given in Table 5. The observed stability constants (A obs) for the diboronic acid sensor 147(phenanthrene pyrene) ( ex=299 and 342 um) with D-glucose were enhanced relative to those of the monoboronic acid reference compounds 146(pyrene) and 153(phenanthrene) while the observed stability constants (Aobs) for the diboronic acid sensor 

147(phenanthrene pyrene) ( ex=299 and 342 Dm) with D-frUCtOSC WCfC rcduCCd relative to those for the monoboronic acid reference compounds 146(pyrene) and 153(piienanthrene)- 

Energy transfer reactions in systems containing the herein investigated molecular wire structures have already been investigated in Erlangen. Two well-characterized examples, which were investigated within the scope of this thesis will be presented in more detail. This lines out the characteristic features of photoinduced energy transfer reactions. 

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As described before, the rr-electrons of porphyrin are delocalized over the molecule and the energy levels of the HOMO and the LUMO are high and low, respectively. The resultant narrow intramolecular HOMO-LUMO gap causes absorption of the entire region of visible light. Usually, porphyrins are red to purple and phthalocyanines are blue to green. Furthermore, the long lifetime of their excited states is appHcable to the construction of photo-induced electron and/or energy transfer systems. 

The flow rate, pressure, and temperature of each stream must be specified. This has already been done in part by constructing the unit ratio material balance. It must be extended to all energy transfer systems by using material and energy balances. 

Jiao GS, Loudet A, Lee HB, Kalinin S, Johansson LBA, Burgess K (2003) Syntheses and spectroscopic properties of energy transfer systems based on squaraines. Tetrahedron 59 3109-3116... 

Figure 8.23 shows emission spectra characteristic of the energy transfer systems studied in Ref. 2. In each case the principal excitation in the ultraviolet is of the donor, Cl. The lowest curve (a) is for a neat Cl solution in glycerol at 3x 10 M. The second curve (b) is the spectrum of a bulk sample containing the donor and an acceptor (R6G at a concentration of 3 x 10 s M). The upper curve (c) shows the spectrum of a 10-/mi-diameter spherical particle of die same material used to obtain curve b. The emission intensity, normalized to the donor peak, is considerably enhanced at the acceptor peak, indicative of extra transfer in the particle compared to the corresponding bulk sample. 

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... 

A complete FRET (Forster resonance energy transfer) system based on chlorophyll in the pores of FSM materials was accomplished by Kuroda s group.149,150 They first functionalized the FSM samples with 3-aminopropyl groups to guarantee an ideal position of the macroscopic chlorin units (in the pore center) and prevent their denaturation. Then they ligated chlorophyll derivatives that possess 3-(triethoxy-silyl)-A-methylpropan-l -amine groups to the pore walls. Zinc (for the FRET donor) and copper (for the FRET acceptor) were chosen as the central ions of the chlorins, which made it possible to initiate and record an efficient FRET process (Fig. 3.14). 

Under these conditions, the quantum yield also reaches a value independent of [A], and is referred to as the limiting quantum yield, ij> , in an energy transfer system. 

In energy transfer systems, lifetimes also play an important role. The mechanistic lifetime of a donor state is the reciprocal of the sum of the first-order and pseudo-first-order rate constants for all steps which depopulate that excited state, of which the latter will... 

Nicotinamide nucleotide transhydrogenases may be divided into two classes. One class is present in certain bacteria, and possibly in some plants, is an easily extractable, water-soluble enzyme is not functionally linked to the energy-transfer system of the bacterial membrane is a fiavoprotein and is specific for the 4B-hydrogen atom of both NADH and NADPH. The other class is present in both certain bacteria and in mitochondria is a firmly membrane-bound water-insoluble enzyme is functionally linked to the energy-transfer system of the bacterial or mitochondrial membrane is not known to be a flavoprotein and is specific for the 4A-hydrogen atom of NADH and the 4B-hydrogen atom of NADPH. For the sake of convenience, the two classes of enzyme will be referred to below as BB-specific and AB-specific transhydrogenases, respectively. 

AB-Specific transhydrogenases are functionally coupled to the energy-transfer system of the membrane in which they are located. This coupling... 

Eigure 22. Energy-transfer systems based on self-assembly of zinc-diporphyrin with trans-dipyridylporphyrins, rfl/zi -dipyridylchlorin, and tetrahydroporphyrin. 

Yu, J., Corripio, A.B, Harrison, D.P, and Copeland, R.J. Analysis of the sorbent energy transfer system (SETS) for power generation and C02 capture. Advances in Environmental Research, 2003, 7, 335. 

We summarize below how we went about modeling EET in the neutral RC based on our model for EET in molecular aggregates. The most significant feature that differentiates the oxidized RC from the neutral reaction center, and any previously reported energy transfer systems we are aware of, is that the acceptor is a dimeric radical. Therefore, the focus of the problem was to determine the electronic energies and origins of the electronic transitions of the oxidized special pair acceptor and to quantify the electronic coupling between each of these relevant transitions and the donor transitions. 

Can we build a 100% efficient energy transfer system Is there such a thing as cost-free energy No, on both counts. It is theoretically impossible, and the laws of thermodynamics, which we discuss in this chapter, teU us why this is so. 

It might thus be expected that this structure has been incorporated in many other proteins wherever there was a need for its characteristic function. A requirement of binding nucleotides to proteins occurs in energy transfer systems and in the molecular reproduction of nucleic acids. Consequently, it is possible that this molecular fossil may be found in such diverse proteins as tRNA synthetase, ribosomal proteins, and virus coat proteins. The recognition of its structure by sequence homology or from X-ray structure determinations may also give guidance as to function where none is known. 

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