Solvation water relaxation

Figure 43. Solvation dynamics from MD simulations for isomer 2. (a) The linear-response calculated time-resolved Stokes shifts for indole-protein, indole-water, and their sum. (b) Direct nonequilibrium simulations of the time-resolved Stokes shifts for indole-water, indole-protein, and their sum. Note the lack of slow component in indole-water relaxation in both (a) and (b), which is opposite to isomer 1 in Fig. 42. Also shown is the indole-water (within 5 A of indole) with coupled long-time negative solvation, (c) Relaxation between indole-lys79 and indole-glu4. The interaction energy changes from these two residues nearly cancel each other, (d) The distance changes between the indole and two charged residues, but both residues move away from the indole ring.

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... 

Figure 44. Solvation dynamics from constrained MD simulations, (a) Comparison of indole-water relaxation with and without frozen protein structure for isomer 1. The slow component of the water response nearly disappears, which indicates that slow water relaxation needs protein fluctuations, (b) Comparison of indole—protein relaxation with and without frozen water for isomer 2. Similarly, the slow component of the indole—protein disappears, which indicates that the protein relaxation also requires water fluctuations.

A rather simple experimental teehnique involving measurement of the time-dependent fluorescence Stokes shift (TDFSS) after an initial exeitation has been applied to measure SD in a large number of liquids. TDFSS oceurs due to dipolar solvation of the excited probe and thus gives an estimate of the solvation timeseales. In an important paper, Jimenez et al. reported the results of SD of the exeited state of the dye coumarin 343 (C343) in liquid water [14]. Their result is shown in Figure 3.13. The initial part of the solvent response of water was found to be extremely fast (few tens of femtoseconds) and it constituted more than 60% of the total solvation energy relaxation. The subsequent relaxation was found to occur in the picosecond timescale. The decay of the solvation time correlation function, S t)y was fitted to a function of the following form... 

The observation of slow, confined water motion in AOT reverse micelles is also supported by measured dielectric relaxation of the water pool. Using terahertz time-domain spectroscopy, the dielectric properties of water in the reverse micelles have been investigated by Mittleman et al. [36]. They found that both the time scale and amplitude of the relaxation was smaller than those of bulk water. They attributed these results to the reduction of long-range collective motion due to the confinement of the water in the nanometer-sized micelles. These results suggested that free water motion in the reverse micelles are not equivalent to bulk solvation dynamics. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

In addition, water motion has been investigated in reverse micelles formed with the nonionic surfactants Triton X-100 and Brij-30 by Pant and Levinger [41]. As in the AOT reverse micelles, the water motion is substantially reduced in the nonionic reverse micelles as compared to bulk water dynamics with three solvation components observed. These three relaxation times are attributed to bulklike water, bound water, and strongly bound water motion. Interestingly, the overall solvation dynamics of water inside Triton X-100 reverse micelles is slower than the dynamics inside the Brij-30 or AOT reverse micelles, while the water motion inside the Brij-30 reverse micelles is relatively faster than AOT reverse micelles. This work also investigated the solvation dynamics of liquid tri(ethylene glycol) monoethyl ether (TGE) with different concentrations of water. Three relaxation time scales were also observed with subpicosecond, picosecond, and subnanosecond time constants. These time components were attributed to the damped solvent motion, seg-... 

Notwithstanding Platzman s theory, most calculations of radiation-chemical yields in water and aqueous solutions were performed using the free-radical model (see Magee, 1953 Samuel and Magee, 1953 Ganguly and Magee, 1956). The hypothesis was that the recapture time of the electron would be shorter than the dielectric relaxation time. Therefore, recombination would outcompete solvation. 

Many natural materials are porous but also proton-rich such as wood or other plant products. Relaxation of liquids in these materials has features in common with both inorganic matrices and the protein systems discussed above. The class of porous polysaccharide materials used for size exclusion chromatography provides an example one commercial product is Sephadex. The material swells on solvation to form a controlled pore gel. The main application involves excess liquid, generally water, which flows through the gel bed carrying solutes of various size. The large solutes are excluded from the pore interior and elute rapidly while the smaller ones equilibrate with the pore interior and elute later. The solvent generally samples the pore interior as well as the bulk phase. 

In this discussion, I have intentionally not discussed the solvation of the electron in water. These results are quite confusing because of the overlap of the electronic relaxation of the excited solvated electron and the rearrangement of the solvent. In an alcohol, these terms are well separated and so discussion is simplified. In addition, we are not aware of any study on the solvation of an anion in water. 

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