Stokes shifts hydration

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. 

In Fig. 39, the three Stokes shifts show distinct relationships with tryptophan s emission maxima and ascertain the dominance of solvation response from surface water hydration. The first component AE shows a monotonic increase... 

MD simulations with either protein or water constrained at the instant of photoexcitation were performed for both isomer 1 and isomer 2. For isomer 1, because surface water relaxation dominates the slow component of the total Stokes shift, in Fig. 44a we show the result of simulations of isomer 1 with an ensemble of frozen protein configurations to examine the role of protein fluctuations. Clearly the long component of indole-water interactions disappears when the protein is constrained. This result shows that without protein fluctuations, indole-water relaxation over tens of picoseconds does not occur. Thus, although surface hydrating water molecules seem to drive the global solvation and, from the dynamics of the protein and water contributions, are apparently responsible for the slowest component of the solvation Stokes shift for isomer 1 (Fig. 42), local protein fluctuations are still required to facilitate this rearrangement process. When the protein is frozen, the ultrafast... 

Because the dynamics of phospholipid membranes have been well characterized using AF probes [8-13], fluorescence results obtained with hydrated human SC were compared to aqueous suspensions of unilamellar distearoyl-phosphatidylcholine (DSPC) vesicles. DSPC was also used because its phase transition temperature (55°C) is close to that of SC lipids (65 C). The microenvironment inside DSPC and SC membranes was studied by measuring fluorescence lifetimes, and shifts in emission maxima were compared to excitation maxima (Stokes shifts), along with quenching of a series of AF probes by iodide. Stokes shifts (Av) [6] were calculated as ... 

Tables 1 and 2 summarize Stokes-shift (Av) and lifetime data, respectively, for a series of AF probes in DSPC and human SC. Since Stokes-shift data in hexane should reach a minimum value due to the absence of a dipole moment and hydrogen-bonding in this solvent [7], findings with hexane were compared to those of SC and DSPC in Table 1. The data show that while the Av of 2-AF through 12-AF in DSPC were similar to values obtained in hexane, much smaller values were obtained in fully hydrated SC. Consequently, the...

Molecular dynamics (MD) simulations fluorescence Stokes shift (FSS), 381 globule state hydration... 

The large Stokes shift observed between the absorption and emission maxima of [Ce(H2 0)g] doped in La(ethylsulfate)3 9H2O crystal was attributed to a deformation in the TCTP structure in the excited state. In the rapidly fluctuating solution environment, this deformation leads to dissociation of one of the equatorial waters followed by a shift of one of the vertex waters to an equatorial position to form the excited state [Ce(H2 0)g]. The appearance of a new band was the evidence for the formation of this excited state species of lower hydration (Jorgensen and Brinen 1963, Kaizu et al. 1985, Miyakawa et al. 1988). 

Most recently, Mizuno et al. presented a femtosecond version (250 fs time resolution, 160 cm spectral resolution) of the RR experiment to probe the O-H band of the electron as it hydrates following 2 X 4.66 eV photon excitation. Mizuno et al. conclude that the precursor of the hydrated electron that undergoes continuous blue shift on the time scale of 1-2 ps also yields a downshifted O-H stretch signal whose resonance enhancement follows the efficiency of Raman excitation as the absorption spectrum of the s-like state shifts to the blue (thus indirectly confirming its identity as a hot s-like state). The comparison of anti-Stokes and Stokes Raman intensities indicates that the local temperature rise is < 100 K at 250 fs. This estimate agrees with the estimates based on the evolution of the spectral envelope during the thermalization, using the dependence of the absorption maximum of thermalized electron on the bath temperature. 

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