Solvating probe molecule fluorescence

The Relationship of C(t) to the Solvent Coordinate. The fluorescence frequency of a solvating probe molecule whose spectrum is dominated by solvent interactions (see below) can be expressed by the following equation... 

The central question in liquid-phase chemistry is How do solvents affect the rate, mechanism and outcome of chemical reactions Understanding solvation dynamics (SD), i.e., the rate of solvent reorganization in response to a perturbation in solute-solvent interachons, is an essential step in answering this central question. SD is most often measured by monitoring the time-evolution in the Stokes shift in the fluorescence of a probe molecule. In this experiment, the solute-solvent interactions are perturbed by solute electronic excitation, Sq Si, which occurs essenhaUy instantaneously on the time scale relevant to nuclear motions. Large solvatochromic shifts are found whenever the Sq Si electroiuc... 

The time-resolved spectroscopy is a sensitive tool to study the solute-solvent interactions. The technique has been used to characterize the solvating environment in the solvent. By measuring the time-dependent changes of the fluorescence signals in solvents, the solvation, rotation, photoisomerization, or excimer formation processes of a probe molecule can be examined. In conventional molecular solutions, many solute-solvent complexes. 

The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section I.A assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. 

Fluorescence spectroscopy was utilized as the diagnostic to evaluate these phenomena. It will be shown that a single probe molecule, a derivative of dimethylaminonaphthalenesulfonamide, (dansyl, J.) can function both as an environmental probe to evaluate the degree of solvation of polymer chains in the gel phase and also serve as a sensitive indicator for the diffusion of ionic reagents through the crosslinked gel network. 

A recent study reports the solvation dynamics of coumarin 153 and 6-propionyl-2-dimethylaminon-aphthalene (PRODAN) in [emim][BF4] and [bmim] [BF4]. The fluorescence decay profiles showed biphasic solvation dynamics as was observed for molten tetraalkylammonium salts.- The results were interpreted as indicating a short-lived component resulting from the motion of the anion, followed by a longer-lived component resulting from the motion of both the anion and the cation. In the same papers, estimates of the value of (30) were made for these solvents based on comparisons of the values for each probe molecule in conventional solvents. The values obtained (48.9 from coumarin 153 and 47.1 kcal mol" from PRODAN for [bmim][BF4]) were somewhat lower than those measured directly using Reichardt s dye.- ... 

Femtosecond spectroscopy has an ideal temporal resolution for the study of ultrafast water motions from femtosecond to picosecond time scales [33-36]. Femtosecond solvation dynamics is sensitive to both time and length scales and can be a good probe for protein hydration dynamics [16, 37-50]. Recent femtosecond studies by an extrinsic labeling of a protein with a dye molecule showed certain ultrafast water motions [37-42]. This kind of labeling usually relies on hydrophobic interactions, and the probe is typically located in the hydrophobic crevice. The resulting dynamics mostly reflects bound water behavior. The recent success of incorporating a synthetic fluorescent amino acid into the protein showed another way to probe protein electrostatic interactions [43, 48]. 

We recently developed a systematic method that uses the intrinsic tryptophan residue (Trp or W) as a local optical probe [49, 50]. Using site-directed mutagenesis, tryptophan can be mutated into different positions one at a time to scan protein surfaces. With femtosecond temporal and single-residue spatial resolution, the fluorescence Stokes shift of the local excited Trp can be followed in real time, and thus, the location, dynamics, and functional roles of protein-water interactions can be studied directly. With MD simulations, the solvation by water and protein (residues) is differentiated carefully to determine the hydration dynamics. Here, we focus our own work and review our recent systematic studies on hydration dynamics and protein-water fluctuations in a series of biological systems using the powerful intrinsic tryptophan as a local optical probe, and thus reveal the dynamic role of hydrating water molecules around proteins, which is a longstanding unresolved problem and a topic central to protein science. 

The goal of this work was to show how the fluorescence probe with the general structure I (Scheme 1) behaves in swollen polydimethylsiloxane networks and how the solvation can be examined of linear polydimethylsiloxanes in the presence of covalent bonded probes. This molecule consists of a donor part, e.g., a dialkylamino group, and an acceptor, which is usually an ester group. The following points are important to discuss for a better understanding of the fluorescence behavior... 

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast Internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example . The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge... 

The role of vibrational relaxation and solvation dynamics can be probed most effectively by fluorescence experiments, which are both time- and frequency-resolved,66-68 as indicated at the end of Sec. V. We have recently developed a theory for fluorescence of polar molecules in polar solvents.68 The solvaion dynamics is related to the solvent dielectric function e(co) by introducing a solvation coordinate. When (ai) has a Lorentzian dependence on frequency (the Debye model), the broadening is described by the stochastic model [Eqs. (113)], where the parameters A and A may be related to molecular... 

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