Decay rate constants, fluorescent probes

The fluorescence lifetimes (t determined at 580 nm) and quantum yields () of SRB were determined in water-dioxane mixtures and a series of alcohols at 25°C. The km value varied with the medium in the range of (4.1-0.7) x 10 s whereas the radiative decay rate constant (kr) was rather insensitive to the medium properties (2.8-1.7) X 10 s" . The relationship between In km and t(30) fall on a straight line and the slope value of the plot was 0.074 0.01. Therefore, the photophysical properties of SRB and Equation (20) are applicable to probing the polarity at a water/oil interface. 

Whereas the observed decay profile no longer is characterized by a single decay rate, the steady-state fluorescence intensity becomes dependent on both 7obs and fc>bs. The typical Stern-Volmer plot is no longer represented by equation 7a, but rather by equation 7b, where fcobs is defined by equation 6b, fc q is the bimolecular quenching rate constant, fco is the probe s mean excited-state unimolecular decay rate constant, fcobs is the mean observed decay rate constant, 70 is the distribution parameter of the Gaussian for the unimolecular decay, and 7obs is the distribution parameter for the observed unimolecular decay rate. 

The fluorescence decay rate constants, k , of several fluorescent probes in various systems (Table I) provide further evidence for hydrophobic microdomains in PA-I8K2 aqueous solutions. All the decays are single exponential decays except for C PN in aqueous solutions of PMA at pH 8. All the values in PA-I8K2 aqueous solutions at pH 4, 8, and 10 are similar to those found in hexanol and much smaller than those in water and aqueous solutions of PMA at pH 8. In summary, PA-I8K2 readily solubilizes pyrene and some positively and negatively charged derivatives of pyrene, especially long-chain derivatives. 

Actually, the apparent rate constant of the photoisomerisation in a viscous media, like biological membranes, was found to be dependent upon the medium relaxation rate. Hence, it is possible to study the dynamics of proteins and biological membranes in the vicinity of the incorporated stilbene probe by monitoring the steady-state fluorescence decay of the stilbene probe with the conventional constant-illumination spectrofluorimeter. The experimental values of a (j>n Icx can be measured independently or can be omitted by comparison with photoisomerisation kinetics of the same probe and similar conditions in a medium with known macro- and microviscosity. A combined analysis of the trans-cis photoisomerisation kinetics of a stilbene probe and its polarization allows the establishment of the mechanism and the estimation of the... 

The triplet-photochrome labeling method has been used to study very rare encounters in a system containing the Erythrosin B sensitiser and SITC photochrome probe (Mekler and Likhtenshtein, 1986). Both types of the molecules were covalently bound to a-chymotrypsin. The photoisomerisation kinetics was monitored by fluorescence decay of the frans-SITS. The rate constants of the triplet-triplet energy transfer between Erythrosin B and SITS (at room temperature and pH 7) were found k,r = 2 xlO7 NT s-1 and ktT = 107 M V. It should be emphasized that the concentration of the triplet sensitiser attached to the protein did not exceed 10 7 M in those experiments, and the collision frequencies were close to 10 s 1 which are 8-9 orders of magnitude less than those measured with the regular luminescence or ESR techniques. 

Excited singlet lifetimes, also called fluorescence lifetimes, of organic molecules are normally smaller than 10 ns. Notable exceptions are polycyclic aromatic hydrocarbons, such as pyrene and naphthalene. Due to their short lifetimes, fluorescent probes can only explore a small volume therefore, competition between the probe s decay to the ground state and dissociation from the supramolecular structure to the homogeneous phase occurs only infrequently. For example, in order to compete with the decay of the excited state, a probe with a 10 ns excited state lifetime must possess a dissociation rate constant from the supramolecular structure that is larger than 10 s. For this reason, fluorescent probes are normally assumed not to relocate during their lifetimes hence, explore a very limited volume. 

The values for the association (kg+) and dissociation (kg ) rate constants of the quenchers can be determined when the conditions discussed in Section II are met. Most of the studies in micelles have been based on the model that leads to Eq. (8), where rate constants are recovered from a four-parameter fit of the fluorescence decay in the presence of quencher. The assumptions of this basic model have been discussed in Section II. This model and inclusion of additional processes, such as probe and quencher migration, have been employed for over... 

In addition to comparing overall quenching rate constants, it is also possible to recover the values of the quencher association and dissociation rate constants from quenching experiments. The same model that was employed for fluorescent probes can be employed. This model considered that the probe was immobile. The general solution to this model is given by Eq. (8), which has four parameters defined by the rate constants for the processes described in Fig. 1. However, the experimental results showed that the triplet state decayed by pseudo-first-order kinetics, suggesting that once the quenchers enter the supramolecular system, quenching occurred with an efficiency of unity. Under these conditions, Eq. (5) can be applied. In addition, if the condition that [H] holds, Eq. (5) can be reduced to... 

A direct access to the dynamic properties of the probe can be derived from the fluorescence decay measurements by the determination of the rate constant of excimer formation Measurement of the monomer fluorescence decay alone enables the determination of Besides... 

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