Bimolecular fluorescence

Table 2. Bimolecular fluorescence quenching rate constants k, (10 1 mol s " for different TICT molecules with various electron donors [138], The oxidation potential of the donor (in Volts) is also given...

Scheme 1. Bimolecular fluorescence quenching Scheme 2. Fluorescence quenching by reaction. a geminate recombination.

Navarro, G., Carriba, P., Gandia, J., Ciruela, F., Casado, V., Cortes, A., Mallol, J., Canela, E.I. et al. (2008) Detection of heteromers formed by cannabinoid CBl, dopamine D2, and adenosine A2A G-protein-coupled receptors by combining bimolecular fluorescence complementation and bioluminescence energy transfer. Scientific World Journal, 8, 1088-1097. 

This system is organized in vascular plants as a 30 nm diameter plasma membrane complex, known as the rosette TC, that was first described by Mueller and Brown more than 25 years ago [42]. The rosette is composed of an estimated 36 subunits comprising at least three types of cellulose synthase A proteins. All three components have been shown to be essential for correct assembly and enzymatic activity of the complex in Arabidopsis [43, 44]. The physical interaction of these three components has recently been demonstrated by bimolecular fluorescence complementation [45]. The system also offers a beautiful example of the functional aspects of this organization, as cortical microtubules guide the movement of cellulose synthase complexes through the plasma membrane during cell wall biosynthesis [46,47],... 

Type 1 Forster (or fluorescent) resonance energy transfer (FRET)-based biosensors Type 2 Bimolecular fluorescence complementation (BiFC)-based biosensors Type 3 Single FP-based biosensors... 

Bimolecular Fluorescence Complementation (BiFC)-based biosensors... 

Bimolecular fluorescence complementation (BiFC)-based biosensors have been used to visualize a variety of protein—protein interactions in live cells. In this type of biosensor, the FP that is split up and MRE are linked to one portion and the analyte protein is linked to the other portion. When the two proteins interact, the two fragments fuse together, refolding properly into its three-dimensional structure and producing a fluorescence signal. 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

Iq/I — t — KgI0 [Q], in which Kg is the bimolecular rate constant of interaction of quencher Q with the excited states of the PCS, t is the lifetime of excited molecules with no quencher, I0 is the quantum yield of fluorescence in the absence of the quencher, and I is the quantum yield of fluorescence in the presence of the quencher. 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

Research Opportunities. The presence of a long-lived fluorescing state following either 532 nm or 1064 nm excitation of PuF6(g) provides a valuable opportunity to study the extent to which electronic energy in a 5f electron state is available in photochemical and energy transfer reactions. Such gas phase bimolecular reactions would occur in a weak interaction limit governed by van der Waals forces. Seen from the perspective of potential photochemical separations in fluoride volatility... 

Figure 4. Energy diagram for 532 nm excitation of PuF g). The 5f electron states of PuF are shown at the left. The solid arrows indicate photon absorption or emission processes. The wavy arrows indicate nonradiative processes by which excited states of PuFg may be lost. The laser-fluence dependent fluorescence decay found at this excitation wavelength can be explained in terms of a bimolecular reaction between PuFg(g) in its 4550 cm l state and PuF (g) to form PuFj(g). It is assumed that PuF (g) is formed via dissociation of the initially populated PuF state.

Figure 1.5. Femtosecond spectroscopy of bimolecular collisions. The cartoon shown in (a illustrates how pump and probe pulses initiate and monitor the progress of H + COj->[HO. .. CO]->OH + CO collisions. The huild-up of OH product is recorded via the intensity of fluorescence excited hy the prohe laser as a function of pump-prohe time delay, as presented in (h). Potential energy curves governing the collision between excited Na atoms and Hj are given in (c) these show how the Na + H collision can proceed along two possible exit channels, leading either to formation of NaH + H or to Na + H by collisional energy exchange.

As seen from (1) and (2), intermolecular processes may reduce essentially the lifetime and the fluorescence quantum yield. Hence, controlling the changes of these characteristics, we can monitor their occurrence and determine some characteristics of intermolecular reactions. Such processes can involve other particles, when they interact directly with the fluorophore (bimolecular reactions) or participate (as energy acceptors) in deactivation of S) state, owing to nonradiative or radiative energy transfer. Table 1 gives the main known intermolecular reactions and interactions, which can be divided into four groups ... 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

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