Solvation dynamics

At a protein surface, the time dependence of the solvation energy of a newly created probe derives contributions Ifom many sources, not only from the surface and the bulk water molecules but also from the amino acid side-chains and from ions (as they always tend to be present in experimental systems). This makes the analysis of the SD of a protein solution difficult. 

There have been two different interpretations of the slow dynamics observed in the SD of the lysozyme hydration layer. The first attributes the intermediate time-scales (30 0 ps) to slow water. Bagchi and co-workers employed the dynamic exchange model to relate the observed slow dynamics to the timescale of the fluctuation of water in the hydration layer [11]. In an alternative interpretation. Song et al. used the formulation developed by Song and Marcus that relates the solvation time correlation function to the DR of the medium. They attributed the 

In a series of important studies, Zewail and co-workers examined the SD of exeited tryptophan as a natural probe in several proteins by using the TDFSS teehnique [13]. The advantage of using tryptophan as a probe was twofold. First, it was a natural probe, so the conformation of the protein was not disturbed and the solvation of the native state was explored. Second, one could study proteins where the tryptophan is partly or fully exposed to water, and so SD studies allowed one to directly probe the response of biological water. They found a slow component in the solvation time correlation function, which was in the range 20-40 ps. This was more than an order of magnitude slower than the bulk response. 

In a series of important studies Bhattacharyya and co-workers studied the SD studies of a covalently bound probe to protein glutaminyl-tRNA synthetase 

Some earlier studies had reported that SD in protein environments was nonexponential with a long component with a long time constant of the order of 10 ns. Such a slow timescale component appears to be due to the ultraslow motion of the large solute probe or due to the slow conformational fluctuation of proteins. 

Given a solute in equilibrium with solvent molecules, a sudden change in the solute s electronic structure due to an absorption of electromagnetic radiation or an electron transfer will generally create a nonequilibrium state. The solvent electronic and nuclear degrees of freedom will respond to reestablish equilibrium. These solvent dynamics can be monitored experimentally. Assuming an instantaneous response of the solvent electronic degrees of freedom, the slower solvent response involves translation, rotation, and vibration of the solvent molecules, which can be followed by classical molecular dynamics. Because experimental and theoretical studies of solvation dynamics can reveal important phenomena needed to understand solvent dynamics and solute-solvent interactions, they have been reviewed extensively. Solvation 

While TRF and other techniques have been used extensively to probe solvent dynamics in bulk liquids377,390,489,490,499 micelles and reverse 

The solvation dynamics of coumarin 314 (C314) adsorbed at the air-water interface was measured using TRSHG and found to be similar to that in bulk water (0.8 ps). Experiments with polarized pump pulses in the direction parallel and perpendicular to the interface showed that the solvation dynamics depend on the solute orientation, being faster when the pump pulse is parallel to the interface.The solvation dynamics of C314 adsorbed at the air-water interface in the presence of neutral, anionic, and cationic surfactants show that electric field of the dye and interactions with specific hydrophilic groups can slow down interfacial water dynamics. 

The equilibrium calculation of solvation dynamics involves computing the equilibrium time correlation function  

While in many cases solvent dynamics follow the linear response approximation, even for large perturbations away from equilibrium, it is expected to fail when the equilibrium fluctuations do not sample important regions of phase space in which the nonequilibrium dynamics takes Thus, 

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Solvent dynamical effects on relaxation and reaction process were considered in Chapters 13 and 14. These effects are usually associated with small amplitude solvent motions that do not appreciably change its configuration. However, the most important solvent effect is often equilibrium in nature — modifying the free energies of the reactants, products, and transition states, thereby affecting the free energy of activation and sometime even the course of the chemical process. Solvation energies relevant to these modifications can be studied experimentally by calorimetric and spectroscopic methods, and theoretically by methods of equilibrium statistical mechanics. 

There are many things for which it s not enough To specify one cause, although the fact Is that there s only one. But just suppose You saw a corpse somewhere, you d better name Every contingency—how could you say Whether he died of cold, or of cold still. 

Of poison, or disease The one thing sure Is that he s dead. It seems to work this way In many instances... 

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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... 

Van der Zwan G and Hynes J T 1983 Nonequilibrium solvation dynamics in solution reaction J. Chem. Phys. 78 4174-85... 

Stratt R M and Maroncelli M 1996 Nonreactive dynamics in solution the emerging molecular view of solvation dynamics and vibrational relaxation J. Phys. Chem. 100 12 981... 

Jarzeba W, Walker G C, Johnson A E and Barbara P F 1991 Nonexponential solvation dynamics of simple liquids and mixtures Chem. Phys. 152 57-68... 

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Riter R E, Edington M D and Beck W F 1996 Protein-matrix solvation dynamics in a subunit of C-phycocyanin J. Phys. Chem. 100 14 198-205... 

Passino S A, Nagasawa Y, Joo T and Fleming G R 1997 Three-pulse echo peak shift studies of polar solvation dynamics J. Phys. Chem. A 101 725-31... 

Homoelle B J, Edington M D, Diffey W M and Beck W F 1998 Stimulated photon-echo and transientgrating studies of protein-matrix solvation dynamics and interexciton-state radiationless decay in a phycocyanin and allophycocyanin J. Phys. Chem. B 102 3044-52... 

Dynamic light-scattering experiments or the analysis of some physicochemical properties have shown that finite amounts of formamide, A-methylformamide, AA-dimethyl-formamide, ethylene glycol, glycerol, acetonitrile, methanol, and 1,2 propanediol can be entrapped within the micellar core of AOT-reversed micelles [33-36], The encapsulation of formamide and A-methylformamide nanoclusters in AOT-reversed micelles involves a significant breakage of the H-bond network characterizing their structure in the pure state. Moreover, from solvation dynamics measurements it was deduced that the intramicellar formamide is nearly completely immobilized [34,35],... 

Solvation dynamics have been measured for a wide range of polar (and some not so polar) solvents. In all solvents, two distinct types of motion comprise the response [7]. The... 

For the remainder of this chapter, we discuss results for various studies of interfacial solvation dynamics. We first discuss studies at liquid liquid interfaces at planar interfaces and in microheterogeneous media in Section II. In Section III, we discuss solvation dynamics at liquid solid interfaces. In Section IV, we review theoretical models and simulations of solvation dynamics at liquid interfaces. Finally, we conclude with a discussion of future studies. 

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. 

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

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-... 

Pant and Levinger have measured the solvation dynamics of water at the surface of semiconductor nanoparticles [48,49]. In this work, nanoparticulate Zr02 was used as a model for the Ti02 used in dye-sensitized solar photochemical cells. Here, the solvation dynamics for H2O and D2O at the nanoparticle surface are as fast or faster than bulk water motion. This is interpreted as evidence for reduced hydrogen bonding at the particle interface. 

Chandra and his coworkers have developed analytical theories to predict and explain the interfacial solvation dynamics. For example, Chandra et al. [61] have developed a time-dependent density functional theory to predict polarization relaxation at the solid-liquid interface. They find that the interfacial molecules relax more slowly than does the bulk and that the rate of relaxation changes nonmonotonically with distance from the interface They attribute the changing relaxation rate to the presence of distinct solvent layers at the interface. Senapati and Chandra have applied theories of solvents at interfaces to a range of model systems [62-64]. 

The overall picture arising from a comprehensive view of the solvation dynamics studies at interfaces that have been done can be summarized interfacial dynamics differ from bulk solution and cannot simply be considered the same as the bulk. In most cases, the structure of the interface appears to impact the dynamics by slowing them down. However, in a few cases, the dynamics appear to speed up. 

Piquemal J-P, Perera L, Cisneros GA, Ren P, Pedersen LG, Darden TA (2006) Towards accurate solvation dynamics of divalent cations in water using the polarizable Amoeba force field from energetics to structure. J Chem Phys 125 054511... 

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