Solvation dynamics, ultrafast component

Femtosecond solvation dynamics experiments in water [147] clearly hint at the existence of a bimodal response of the solvent to a change in solute charge density that is produced by photon absorption for instance. Water appears to show an ultrafast component in the fl/ kT timescale and a slow component due to diffusive motions whose timescale would be in the 1/y range. 

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. 

Figure 2 shows the spectral response functions (5,(r), Eq. 1) derived firom the spectra of Fig. 1. In order to adequately display data for these varied solvents, whose dynamics occur on very different time scales, we employ a logarithmic time axis. Such a representation is also useful because a number of solvents, especially the alcohols, show highly dispersive response functions. For example, one observes in methanol significant relaxation taking place over 3-4 decades in time. (Mdtiexponential fits to the methanol data yield roughly equal contributions from components with time constants of 0.2, 2, and 12 ps). Even in sinqrle, non-associated solvents such as acetonitrile, one seldom observes 5,(r) functions that decay exponentially in time. Most often, biexponential fits are required to describe the observed relaxation. This biexponential behavior does not reflect any clear separation between fast inertial dynamics and slower diffusive dynamics in most solvents. Rather, the observed spectral shift usually appears to sirrply be a continuous non-exponential process. That is not to say that ultrafast inertial relaxation does not occur in many solvents, just that there is no clear time scale separation observed. Of the 18 polar solvents observed to date, a number of them do show prominent fast components that are probably inertial in origin. For example, in the solvents water [16], formamide, acetoniuile, acetone, dimethylformamide, dimethylsulfoxide, and nitromethane [8], we find that more than half of the solvation response involves components with time constants of 00 fs.

Using fs resolution, two residence times of water at the surface of two proteins have been reported (Fig. 7.6) [21]. The natural probe tryptophan amino acid was used to follow the dynamics of water at the protein surface. For comparison, the behavior in bulk water was also studied. The experimental result together with the theoretical simulation-dynamical equilibrium in the hydration shell, show the direct relationship between the residence time of water molecules at the surface of proteins and the observed slow component in solvation dynamics. For the two biological systems studied, a bimodal decay for the hydration correlation function, with two primary relaxation times was observed an ultrafast time, typically 1 ps or less, and a longer one typically 15-40 ps (Fig. 7.7) [21]. Both times are related to the residence period of water at the protein surface, and their values depend on the binding energy. Measurement of the OH librational band corresponding to intermolecular motion in nanoscopic pools of water and methanol... 

Arzhantsev, S., Ito, N., Heitz, M., Maroncelli, M. (2003). Solvation dynamics of coumarin 153 in several classes of ionic liquids cation dependence of the ultrafast component. Chem. Phys. Lett. Vol. 381. p>p. 278-286. 

Since the solvation time correlation function is known both from experiments and from computer simulations, we can easily carry out the above exercise. When this is done, the theory predicts a lack of, or weak, dependence of the electron transfer rate on solvent dynamics, for weakly adiabatic reactions the reason being the dominance of the ultrafast component in SD of water, so the solvent moves too fast to offer any retardation ... 

Simulation studies of solvation dynamics (SD) in SCW were reported for the first time by Rey andLaria [7]. Their studies indicated a biphasic decay of solvation energy, with an ultrafast decay, ratiier similar to the one observed for bulk water. This is rather surprising because here density is low and the extended HB network is non-existent, thereby eliminating the contributions fi om the libration and intermolecular vibration modes. Their results were subsequently corroborated by theoiy, which shows that the ultrafast component arises here from the fast rotational motion of small water molecules [8]. 

The emission of Trp 19 in melittin shifts to the red side peaking at 341 nm (Fig. 18), and the probe location slightly moves away from the lipid interface toward the channel center. Consistently, we observed a larger fraction of the ultrafast solvation component (35%) and a smaller contribution of slow ordered-water motion (38%). Melittin consists of 26 amino acid residues (Fig. 9), and the first 20 residues are predominantly hydrophobic, whereas the other 6 near the carboxyl terminus are hydrophilic under physiological conditions. This amphipathic property makes melittin easily bound to membranes, and extensive studies from both experiments [156-161] and MD simulations [162-166] have shown the formation of an 7-helix at the lipid interface. Self-assembly of 7-helical melittin monomers is believed to be important in its lytic activity of membranes [167-169]. Our observed hydration dynamics are consistent with previous studies, which support the view that melittin forms an 7-helix and inserts into the lipid bilayers and leaves the hydrophilic C-terminus protruding into the water channel. The orientational relaxation shows a completely restricted motion of Trp 19, and the anisotropy is constant in 1.5 ns (Fig. 20b), which is consistent with Trp 19 located close to the interface around the headgroups and rigid well-ordered water molecules. 

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... 

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