Solvation dynamics probe

Joo T, Jia Y, Yu J-Y, Lang M J and Fleming G R 1996 Third-order nonlinear time domain probes of solvation dynamics J. Chem. Phys. 104 6089... 

FIG. 3 Solvation dynamics dependence of coumarin 314 probe molecule orientation at the air-water interface. Signals are generated with a 420 nm pump photon and probed by surface second harmonic signal with 840 nm (SH at 420), x Sx element. The normalized change in SH field is plotted vs. pump delay, r is derived from a single exponential fit to the data, (a) Pump polarization S (inplane), (b) Pump polarization P (out-of-plane). (Reprinted from Ref 24 with permission from the American Chemical Society.)... 

The solvation dynamics of the three different micelle solutions, TX, CTAB, and SDS, exhibit time constants of 550, 285, 180 ps, respectively. The time constants show that solvent motion in these solutions is significantly slower than bulk water. The authors attribute the observed time constants to water motion in the Stern layer of the micelles. This conclusion is supported by the steady-state fluorescence spectra of the C480 probe in these solutions. The spectra exhibit a significant blue shift with respect the spectrum of the dye in bulk water. This spectral blue shift is attributed to the probe being solvated in the Stern layer and experiencing an environment with a polarity much lower than that of bulk water. 

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... 

Fig. 2 shows the effect of creating hydrogen-bonding complexes between HPTA and oxygen-bases on the solvation correlation function of HPTA, C(t) [10]. Utilizing a pump-probe set-up described elsewhere [11], with 400 nm excitation, the dynamic stokes shift of HPTA was analyzed with about 50fs time-resolution. The hydrogen-bonded HPTA exhibited much faster dynamics than the solvation dynamics of the uncomplexed HPTA in pure DCM. 

An additional piece of information can be obtained by studying a synthetic compound derived from the GFP chromophore (1-28) fluorescing at room temperature. In Fig. 3a we show the chemical structure of the compound that we studied in dioxan solution by pump-probe spectroscopy. If we look at the differential transmission spectra displayed in Fig. 3b, we observed two important features a stimulated emission centered at 508 nm and a huge and broad induced absorption band (580-700 nm). Both contributions appear within our temporal resolution and display a linear behavior as a function of the pump intensity in the low fluences limit (<1 mJ/cm2). We note that the stimulated emission red shifts with two characteristic time-scales (500 fs and 10 ps) as expected in the case of solvation dynamics. We conclude that in the absence of ESPT this chromophore has the same qualitative dynamical behavior that we attribute to the relaxed anionic form. 

Ongoing studies using resonant excitation and different dyes will clarify these points. Additional studies using pump-probe as well as photon echo techniques are planned, that should increase the time resolution and give a more complete picture of solvation dynamics at water-ZrOj and water-Ti02 interfaces. 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

Summary of Published C(t) Measurements. Transient solvation dynamics of a variety of excited state probe molecules have been studied in the last few years. Table 1 summarizes the measurements that have been made at room temperature. Table 2 gives a brief summary of the low temperature results. 

A number of theoretical models for solvation dynamics that go beyond the simple Debye Onsager model have recently been developed. The simplest is an extension of Onsager model to include solvents with a non-Debye like (dielectric continuum and the probe can be represented by a spherical cavity. Newer theories allow for nonspherical probes [46], a nonuniform dielectric medium [45], a structured solvent represented by the mean spherical approximation [38-43], and other approaches (see below). Some of these are discussed in this section. Attempts are made where possible to emphasize the comparison between theory and experiment. 

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]. 

When solvation time is much shorter than the lifetimes, both constructions of c(f) give very similar results because v (f) vsc. However, when solvation dynamics becomes slower, such as in proteins, on a time scale close to the lifetimes, the contribution of v/(f) is significant, and Eq. (6) must be used to construct c(t). For all results reported here, we used Eq. (6). Note that for the molecular dye probe with only single lifetime emission, Vi(t) = vt 0) = vss = v(oo) and Eq. (6) becomes equal to Eq. (1). 

In this section, we review our first examinations of tryptophan probing sensitivity and water dynamics in a series of important model systems from simple to complex, which range from a tripeptide [70], to a prototype membrane protein melittin [70], to a common drug transporter human serum albumin [71], and to lipid interface of a nanochannel [86]. At the end, we also give a special case that using indole moiety of tryptophan probes supramolecule crown ether solvation, and we observed solvent-induced supramolecule folding [87]. The obtained solvation dynamics in these systems are linked to properties or functions of these biological-relevant macromolecules. 

Figure 19 shows the typical fluorescence transients of TBE from more than 10 gated emission wavelengths from the blue to the red side. At the blue side of the emission maximum, all transients obtained from four Trp-probes in the cubic phase aqueous channels drastically slow down compared with that of tryptophan in bulk water. The transients show significant solvation dynamics that cover three orders of magnitude on time scales from sub-picosecond to a hundred picoseconds. These solvation dynamics can be represented by three distinct decay components The first component occurs in about one picosecond, the second decays in tens of picoseconds, and the third takes a hundred picoseconds. The constmcted hydration correlation functions are shown in Fig. 20a with anisotropy dynamics in Fig. 20b. Surprisingly, three similar time scales (0.56-1.431 ps, 9.2-15 ps, and 108-140 ps) are obtained for all four Trp-probes, but their relative amplitudes systematically change with the probe positions in the channel. Thus, for the four Trp-probes studied here, we observed a correlation between their local hydrophobicity and the relative contributions of the first and third components from Trp, melittin, TME to TBE, the first components have contributions of 40%, 35%, 26%, and 17%, and the third components vary from 32%, to 38%, 43%, and 53%, respectively. The...

Extensive studies in reverse micelles revealed a similar water distribution [127-130], which is consistent with the distinct water model proposed by Finer [150]. For example, when the molar ratio (wo) of water to the surfactant is 6.8 in lecithin reverse micelles with a corresponding diameter of 37 A, three solvation time scales of 0.57 (13%), 14 (25%), and 320 ps (62%) were observed using coumarin 343 as the molecular probe. At w0 = 4.8 with a 30-A water core diameter, only a single solvation dynamic was observed at 217 ps, which indicates that all water molecules are well ordered inside the aqueous pool. The lecithin in these reverse micelles have charged headgroups, which have much stronger interactions with water than the neutral headgroups of monoolein in the... 

With the probe position moving toward the center of the aqueous channel, we detected more ultrafast and less slow solvation components. Note the negligible change of the quasi-bound water contributions, which indicates the complete detection of the two layers of quasi-bound water by all four Trp-probes. For TME, the fluorescence emission peak shifts to 338 nm, and its location moves to the lipid interface (Fig. 18). We did observe a smaller fraction of slow solvation dynamics decreasing from 53% in TBE to 43% in TME and an increase of the ultrafast component from 17% to 26%. The corresponding anisotropy dynamics drops from 726 to 440 ps with a less hindered local motion at the lipid interface. 

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