Environment polarity

A luminescent unit extensively used to functionalize dendrimers is the so-called dansyl (5-dimethylamino-l-naphthalenesulphonamido group). Dendrimers (up to the third generation, compound 9) containing a single dansyl unit attached off center [39] show that this fluorescent unit, which is very sensitive to environment polarity, is progressively shielded from interaction with water molecules as the dendrimer generation increases. 

The intrinsic UV fluorescence of proteins is dominated by the tryptophan indole rings. The absorption maximum is 280-290 nm with the fluorescence maximum ranging from 315-355 nm, depending on the local environment of the indole side-chains. Quantum yields range from 0.04 to 0.50 0.10 is a common value. As the local environment polarity or dielectric constant increases, the fluorescence maximum shifts up to 355 nm, such as for an indole ring in water or buffer. Trp moieties in highly hydro-phobic environments fluoresce at 315-320 nm. Thus the fluorescence emission maximum (and the quantum yield) provide indirect information as to the local environment of the Trp fluors. 

The important determining factor for the relative energy of the charge-separated state A B to the hole-pair state AB is certainly the nature of the environment (polar solvent). Note that, although the emission from the TICT state has been observed in the gas phase, most of the observations have been made in polar solvents. We can again use the... 

In other media like micelles, cyclodextrin, binary solvent mixtures, and proteins (47-55), lifetime distributions are routinely used to model the decay kinetics. In all of these cases the distribution is a result of the (intrinsic or extrinsic) fluorescent probe distributing simultaneously in an ensemble of different local environments. For example, in the case of the cyclodextrin work from our laboratory (53-55), the observed lifetime distribution is a result of an ensemble of 1 1 inclusion complexes forming and coexisting. These complexes are such that the fluorescent probe is located simultaneously in an array of environments (polarities, etc.) in, near, and within the cyclodextrin cavity, which manifest themselves in a distribution of excited-state lifetimes (53-55). In the present study our experimental results argue for a unimodal lifetime distribution for PRODAN in pure CF3H. The question then becomes, how can a lifetime distribution be manifest in a pure solvent ... 

Polarized spectra can be valuable in providing information on the symmetry of a species without requiring a detailed knowledge of their electronic stracture. In crystalline environments, polarized information is inherently available, but its utility is dependent on knowing the orientation(s) of the chromophores with respect to the crystal axes. In general, it is only possible to measure polarized spectra along the symmetry axes of a crystal. These directions may differ from the directions of the transition moments of the chromophores and the spectra that are measured then need to be deconstructed into polarized spectra of the chromophores themselves. ... 

Equilibrium Denaturation. A variety of different techniques can be employed to monitor protein conformational changes in the presence of denaturants. Activity measurements reflect the extent of alterations of the active site environment. However, enzyme activity measurements may be affected the presence of denaturant in the assay mixture. The denaturation curves obtained by this method are difficult to inte ret and can only be taken as a first approximation of the unfolding transition. U.V. difference spectra indicate conformational changes by monitoring the degree of solvent exposure of aromatic amino-acid side chains. Finally, fluorescence intensity measurements can reveal the nature of the environment (polar, non-polar) of the four tryptophans of p-lactamase. 

Klitgaard-Kristensen, D., Sejrup, H. P. HAFLIDASON, H. 2002. Distribution of recent calcareous benthic foraminifera in the northern North Sea and relation to the environment. Polar Research, 21, 275-282. 

The photoisomerization of / -ionone (114) and / -ionylidene (115) derivatives in aqueous solution was inhibited in the presence of P-CD because of the restrictions imposed on the rotation of the double bond by the cavity. The only photoreaction observed was 1,5-H migration, which was probably promoted by a change in the nature of the lowest excited state from n,it to K,7t as induced by lowering the environment polarity [300]. 

More the dipole of the environment is important, i.e., more the polarity of the environment is important, more energy should be released by the fluorophore to reorient the dipole of the microenvironment. Thus, more the environment polarity increases, less energy will be left for the photon emission and more the emission maximum of the fluorophore will be shifted to the red. 

The fluorescent probe by itself shows no thermal quenching (Fig. 4.43a). Therefore thermal quenching is not related to environment polarity alone. The presence of a structured matrix around the fluorophore induces the sensitivity of the fluorophore to temperature. The origin of this thermal quenching is the fast motions of the microenvironment surrounding the fluorophore. 

The measurements of the fluorescence emission spectra of the proteins (data not showed) revealed that the fluorescence of both proteins is blue shifted relative to the tryptophan fluorescence (353 nm) in a buffer solution (Banishev et al., 2008a). This is due to a decrease in the tryptophan environment polarity in the proteins. The maximum of the HSA fluorescence (332 nm) is blue shifted in comparison with BSA (342 nm). Since the fluorescence spectrum of the tryptophan residues reflects the polarity of their nearest environment, and since the properties of the environments of Trp>-212 in BSA and Trp-214 in HSA are similar (Eftink et al., 1977), such a shift can be related to the fact that BSA contains tryptophan Trp>-134 located in the environment with a higher polarity (in comparison with Trp)-212). Thus, the total fluorescence spectrum of BSA is red shifted. This result will be necessary for choosing the registration wavelength in measuring the acceptor and donor fluorescence when the nonlinear and kinetic curves will be measured (Section 6.1). 

Additionally improvement to the description of the solva-tochromic shifts can be achieved by employing a perturbative approach for accounting for a state-specific response of the polarizable environment [55-58]. In this formalism, after the excited electronic states are obtained (at fixed value of environment polarization), their one-electron densities are used to repolarize the environment A differential polarization energy corresponding to a particular electronic state is provided by the following expression ... 

The QM/continuum models are characterized by a representation of the environment as a structureless dielectric, solely characterized by its macroscopic dielectric constant, e, which determines the environment polarization as a response to the presence of the In the most widespread of these... 

Another remarkable feature of the pyrene fluorescence properties is that an increase in the pyrene environment polarity results in a dramatic increase in the intensity of the 0-0 vibronic band (7i peak), while the peak is solvent insensitive. Thus, a transition from a nonpolar to a polar environment results in a dramatic increase of the /1//3 ratio in emission spectrum [167]. 

The MM force fields consider a set of connected atoms (point masses). The atomic interactions are divided in two families the bonded ones (stretching, bending, torsion) and the non-bonded ones (mainly electrostatic and van der Waals interactions). Some elaborated force fields allow the atomic point charges to vary according to their environment (polarization). Albeit the lack of explicit description of the electrons and nuclei is a strong limitation of MM methods, the main restriction of the MM representation is the predefined and fixed connectivity between the atoms. 

In the microemulsion systems the primary alcohols are frequently considered as cosurfactants, which are usually weakly amphiphilic molecules that help the amphiphilic surfactants to reduce the surface tension of the interface between the immiscible components of the system. In this way they usually enhance and emphasize the internal structure of the system at the colloidal level. Remarkably, the short-chain alcohols, which are sufficiently soluble in water, themselves show surfactant-like behavior in plain binary water mixtures. As was shown by Kahlweit et al. [87], this specific behavior can be observed from the break in the curves of surface tension versus molar fraction of alcohol in water. Similar breaks were observed by Zana et al. [7] in the curves of fluorescence intensity versus molar fraction of alcohol, where changes in the environment polarity are sensed by the pyrene fluorescence probe. Interestingly, with increasing the length of the... 

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