Fluorescence shifts

To qualify the environment into which the colorant molecule is embedded, the actual fluorescence spectrum is compared with the one under standard conditions. If the fluorescence emission spectrum is shifted to longer wavelengths (bathochromic shift), it can be concluded that the molecular enviromnent is of a more polar nature or is polarized by the excited fluorophore. Conversely, a fluorescence shift to shorter wavelengths (hypsochromic shift) indicates a transfer of the fluorophore from a polar... 

Callis PR, Burgess BK (1997) Tryptophan fluorescence shifts in proteins from hybrid simulations an electrostatic approach. J Phys Chem 101 9429-9432... 

Vivian JT, Callis PR (2001) Mechanisms of tryptophan fluorescence shifts in proteins. Biophys J 80 2093-2109... 

Structurally these compounds combine a eight-coordinate tetracarboxylate chelating site with stilbene chromophores. Of the six dyes proposed in Ref. 121 only Indo-1 leads to a short-wavelength fluorescence shift when bound to Ca2+. It is also the probe with the smallest D-A interactions as revealed by the shortest wavelength of fluorescence of the free ligand. As a consequence, the decrease of the electron density of the nitrogen binding site is not sufficient to break completely the interaction with Ca2+ in the excited state and the spectrum is considerably blueshifted. 

The method is based on the ability of the excited state of suitable chromophores to both drive and respond to motions in their immediate environment. For example, coumarin excited states have a large dipole moment that causes nearby charged groups to move to stabilize the dipole. As these groups move and reduce the energy of the excited state, the fluorescence shifts toward the red. In simple solutions, this time-resolved Stokes shift (TRSS) experiment measures the time-dependent polarity of the solvent surrounding the coumarin [9]. 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

The presence of PolyPs in these granules is indicated by the following indirect observations. First, volutin granules undergo metachromatic staining by basic dyes, 4,6 -diamino-2-phenylindole (DAPI) fluorescence shifts, and other reactions specific for PolyPs (see... 

Dielectric friction is the measure of the dynamic interaction of a charged or dipolar solute molecule with the surrounding polar solvent molecules. This concept has been applied, by Hynes et al. [339] and others [486], to solvent- and time-dependent fluorescence shifts resulting from the electronic absorption by a solute in polar solvents. If the solvent molecules are strongly coupled to the charge distribution in ground- and excited-state molecules, the relatively slow solvent reorientation can lead to an observable time evolution of the fluorescence spectrum in the nano- to picosecond range. This time-dependent fluorescence (TDF) has been theoretically analysed in terms of dynamic... 

Murakami and Berliner (1983) later reported the existence of a zinc binding site in bovine, human, guinea pig, and rabbit a-lactalbumins, in which the zinc site is physically distinct from the site for binding calcium. This proposal was supported by the fact that when a cation binds to one site, the ensuing conformational shift excludes binding to the other site. All metal ions that were bound to apo-a-iactalbumin at the calcium site caused the same fluorescence shift. Titration of Ca(II) or Mn(II) protein with Zn(II) or Al(III) caused a complete return to apo-a-lactalbumin fluorescence parameters. In contrast, titration of apo-a-iactalbumin with Zn(II) caused no change in fluorescence parameters. 

The shift of the emission band on excitation at the long-wavelength edge shows a time-dependent fluorescence shift of a fluid ethanol solution of 6-... 

Hydroxy-l-naphthoic acid does not show anomalous fluorescence shifts, nor does 2-hydroxy-l-naphthoic acid. 

According to the initial results reported here, a fluorescence enhancement effect for humic material binding with aluminum can be observed, while under different conditions a quenching of fluorescence predominated. Another very interesting fluorescence phenomenon described here is that after complexation with AE, the maximum intensity of the humic material fluorescence shifts to longer excitation wavelengths and to shorter emission wavelengths in an excitation emission matrix (EEM). 

Hunt, C. E. Ansell, R. J., Use of fluorescence shift and fluorescence anisotropy to evaluate the re-binding of template to (s)-propranolol imprinted polymers, Analyst 2006, 131, 678-683... 

Thus, knowledge of the transition moment direction of a phenol band could help in interpreting the fluorescence spectrum of a tyrosine chromophore in a protein in terms of orientation and dynamics. The absorption spectrnm of the first excited state of phenol was observed around 275 nm with a fluorescence peak aronnd 298 nm in water. The tyrosine absorption was reported at 277 nm and the finorescence near 303 nm. Fluorescent efficiency is about 0.21 for both molecules. The fluorescent shift of phenol between protic and aprotic solvents is small, compared to indole, a model for tryptophan-based protein, due to the larger gap between its first and second excited states, which resnlts in negligible coupling . ... 

Solvent effects were found to have minimal influence on the excitation energies of phenol in aqueous solution using a quantum Monte Carlo simulation , which is in line with experimental observations on its absorption spectra " . Reaction field calculations of the excitation energy also showed a small shift in a solution continuum, in qualitative agreement with fluorescent studies of clusters of phenol with increasing number of water molecules " . The largest fluorescent shift of 2100 cm was observed in cyclohexane. 

The first lifetime-limiting mechanism discussed is well illustrated in the experiments of Paramenter and co-workers on the temporal evolution of fluorescence from p-difluorobenzene.7 As the pressure of a quencher gas is increased, thereby increasing T, the pattern of the dispersed fluorescence shifts to weight emission from states that are reached in the early-time dynamics. (Note that early, in this context, is on the order of 100 vibrational periods.)... 

Oj to 02, which is again a measure of the ability to stabilize anionic charge. The fluorescence shifts are given in Table 3.31 and the correlation is shown in Figure 3.32. The orders Li >Na+ andMg +>Ca +>Sr > Ba + show that Lewis acidity decreases with the size of the cation. According to this scale, the tin halides are comparable in acidity to Mg +. The strong Lewis acidity of Sc +, and the lanthanides, such as La + and Yb, has been exploited in various synthetic applications. 

Fig. 3.32. Correlation between fluorescence shift and shift in oxidation energy for several Lewis acids. Reproduced from...

Only limited evidence is available on these questions. For example Mackay and coworkers have estimated an effective dielectric constant of ca. 20 for a variety of microemulsion droplets, based on pK measurements [154], i.e., they appear to be somewhat less polar than normal micelles in water. However fluorescence shifts in microemulsions are similar to those in micelles, suggesting that the polarities of o/w microemulsions and micelles are similar [155]. We do not know whether these differences stem from the different probes used to estimate polarity, or whether there are marked differences between the surfaces of the various microemulsion aggregates. 

Nilsson L, HaUe B Molecular origin of time-dependent fluorescence shifts in proteins. Proc. Natl. Acad. Sci. U. S. A. 2005,102 13867-13872. 

Matsui, J. Kubo, H. Takeuchi, T. Molecularly imprinted fluorescent-shift receptors prepared with 2-(Trifluoromethyl)acrylic acid. Anal. Chem. 2000, 72, 3286-3290. 

It has been calculated that small solute dipoles are even more effectively solvated by solvent quadrupoles than by solvent dipoles. In these terms it is understandable that quadrupolar contributions are more important in the jt than in the E.f(30) scale. Similarly, triethylphosphine oxide, the probe solute of the acceptor number scale, is much smaller than betaine(30) and thus might be more sensitive to quadrupolar solvation. Thus, at long last, the shape of Figure 13.1.2 and similar ones seems rationalized. Note by the way that the quadrupole and CT mechanisms reflect, respectively, inertial and inertialess solvation pathways, and hence could be distinguished by a comparative analysis of absorption and fluorescence shifts (Stokes shift analysis). Flowever, for 4-nilroanisole fluorescence data are not available. 

Ragan DD, Gustavsen R, Schiferl D (1992) Calibration of the ruby R1 and R2 fluorescence shifts as a function of temperature from 0 to 600 K. J Appl Phys 72 5539... 

For the Hg- -Ars cluster the situation is quite different. The experiments observe fluorescence shifted by about 200 cm to the blue (as predicted by our estimates) but none to the red. This is not so easy to understand. All our calculations predict a red-shifted state in the region of -100 to -150 cm". One possible explanation is that for higher clusters the A state predissociates electronically to produce Hg ( Po) atoms. 

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