Emission decay, biexponential

Proteins having one chromophore per molecule are the simplest and most convenient in studies of fluorescence decay kinetics as well as in other spectroscopic studies of proteins. These were historically the first proteins for which the tryptophan fluorescence decay was analyzed. It was natural to expect that, for these proteins at least, the decay curves would be singleexponential. However, a more complex time dependence of the emission was observed. To describe the experimental data for almost all of the proteins studied, it was necessary to use a set of two or more exponents.(2) The decay is single-exponential only in the case of apoazurin.(41) Several authors(41,42) explained the biexponentiality of the decay by the existence of two protein conformers in equilibrium. Such an explanation is difficult to accept without additional analysis, since there are many other mechanisms leading to nonexponential decay and in view of the fact that deconvolution into exponential components is no more than a formal procedure for treatment of nonexponential curves. 

Kinetics extracted from the transient spectra were fitted to a biexponential function with 3 ps and 18 ps lifetimes, except in the spectral region located between the bleaching and the stimulated emission bands where the decays were fitted to one exponential function with a 1.5 ps lifetime. Decays in the stimulated emission band are biexponential with an average lifetime of about 10 ps, which is comparable to the fluorescence lifetime reported in [3]. 

The biexponential nature of the fluorescence emission of this bichromophore was clearly observed from the decay analysis. There... 

All the emission spectra in solution showed a maximum peak with a shoulder on the red side. It was observed that the photoluminescence A.max in dilute THF solution red shifted from 470 to 474. They also exhibited similar Stokes shifts (about 66 nm) due to their similar backbone. The fluorescence quantum yields ( F) of these molecular wires in dilute THF solution were measured to be 0.25 for 470,0.22 for 471,0.20 for 472,0.20 for 473, and 0.18 for 474, respectively. The excited-state lifetime for OTEs was found to be single-exponential within a few hundred picoseconds. However, in THF solution (10 17 M), the decay of the emission maximum band for these molecular wires was found to be biexponential with two excited-state lifetimes yielding a c2 of <1.2. One excited-state lifetime ranged from 0.42 to 0.30 ns, which was in agreement with that in OTEs. Another one exhibited a... 

Figure 43 Quenching of Ru(bpy)2 + (5 puff) emission by Co(bpy)3+ (1 and5 puff) while both complexes are intercalated in B AZrP galleries. The luminescence decay data were fitted to a biexponential model over four orders of time and intensity values. Alternative models did not fit the data (see text). (From Ref. 86. Copyright 1998 The American Chemical Society.)...

TNP-ATP complex obtained by the single-molecule time-resolved spectroscopy, together with a fluorescence decay curve of TNP-ATP obtained by a bulk measurement. Both curves were well fitted to biexponential functions. The instrument-response function in 195-ps fwhm is also displayed. (B) Representative fluorescence spectrums of two individual enzyme-TNP-ATP complexes showing different emission peaks. A fluorescence spectrum of TNP-ATP obtained from a bulk measurement is also displayed for comparison. All spectrums were normalized to unity at their maximum. (From Ref. 18.)... 

Irrespective of the exact nature of the biexponential fluorescence decay of PdG (emission from two different conformers or bifurcation of the initial irir -state population to two nonradiative decay channels), it is important to note that the subpicosecond excited-state decay, characteristic of guanine or guanosine, is clearly absent in PdG. Thus, the presence of the exocyclic ring, which hinders the out-of-plane deformation of the six-membered ring (C2 in particular), leads to a dramatically reduced internal conversion rate. 

Table I shows examples of the steady-state and time-resolved emission characteristics of [Ru(phen)2(dppz)]2+ upon binding to various DNAs. The time-resolved luminescence of DNA-bound Ru(II) is characterized by a biexponential decay, consistent with the presence of at least two binding modes for the complex (47, 48). Previous photophysical studies conducted with tris(phenanthroline)ruthenium(II) also showed biexponential decays in emission and led to the proposal of two non-covalent binding modes for the complex (i) a surface-bound mode in which the ancillary ligands of the metal complex rest against the minor groove of DNA and (ii) an intercalative stacking mode in which one of the ligands inserts partially between adjacent base pairs in the double helix (36, 37). In contrast, quenching studies using both cationic quenchers such as [Ru(NH3)6]3+ and anionic quenchers such as [Fe(CN)6]4 have indicated that for the dppz complex both binding modes...

Table III shows that the luminescent lifetimes and the relative luminescent intensities for the covalently bound duplex and its noncovalent analogue are similar. As with [Ru(phen)2(dppz)]2+, a biexponential decay in emission is observed for the ruthenated oligonucleotide hybridized to its complement. A small shift in the wavelength of maximum emission is also observed compared to the noncovalent complex. This shift likely reflects the sensitivity in emission to the stacking of the oriented dppz ligand a dependence of the maximum emission wavelength on base...

Experiments on bacteriorhodopsin (BR), which is the basis for a light-driven proton pump in halobacteria, were recently reported [96], The primary photoreaction is believed to be a trans-to-cis isomerization. Absorption of a 620-nm pulse by BR in membranes was followed by measurements of stimulated emission at various probe wavelengths between 695 and 930 nm. The rapid (ti 200 fs) decay of stimulated emission itensity at the bluer wavelengths, slower decay (T2 500 fs) at redder wavelengths, and biexponential decay at intermediate wavelengths were interpreted in terms of partially coherent rotational motion along the Sj potential surface. 

Fig. 13 Top luminescence spectra of 1 x 10 2 M Eu(OTf)3 dissolved in Bmim-NTf2 at different C16H36N+ N3 concentrations. With increasing N3 concentration, the luminescence decreases and the shape of the emission signal changes. Bottom luminescence of lxlO-2 M Eu(OTf)3 dissolved in Bmim-NTf2 reveals a biexponential decay behavior. With the addition of 1 x 10 5 M C16Hi6N N, the decay behavior becomes monoexponential. Image adapted from [242]. Image Copyright American Chemical Society (2008)...

Sandros and Becker [107] studied the cis-to-trans photoisomerization process in 9-styrylanthracenes from the singlet excited state. They found that decay of fluorescence emission from the cis isomer is biexponential and they explained the biexponential decay as involving cis and trans singlet excited states (Scheme 6). Furthermore, they noticed that the trans isomer formed adiabatically can undergo intersystem crossing to the triplet state and can initiate the quantum chain... 

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