The isotope effect in solvation dynamics and nonadiabatic relaxation: A quantum simulation study of the photoexcited solvated electron in D2O

Structure of the aqueous electron

A cavity or excluded-volume structure best explains the experimental properties of the aqueous or “hydrated” electron.

Site-specific hydration dynamics of globular proteins and the role of constrained water in solvent exchange with amphiphilic cosolvents

The thermodynamic driving forces for protein folding, association, and function are often determined by protein-water interactions. With a novel covalently bound labeling approach, we have used sensitive vibrational probes, site-selectively conjugated to two lysozyme variants-in conjunction with ultrafast two-dimensional infrared (2D-IR) spectroscopy-to investigate directly the protein-water interface. By probing alternatively a topologically flat, rigid domain and a flexible domain, we find direct experimental evidence for spatially heterogeneous hydration dynamics. The hydration environment around globular proteins can vary from exhibiting bulk-like hydration dynamics to dynamically constrained water, which results from stifled hydrogen bond switching dynamics near extended hydrophobic surfaces. Furthermore, we leverage preferential solvation exchange to demonstrate that the liberation of dynamically constrained water is a sufficient driving force for protein-surface association reactions. These results provide an intuitive picture of the dynamic aspects of hydrophobic hydration of proteins, illustrating an essential function of water in biological processes.

Probing deuterium isotope effect on structure and solvation dynamics of human serum albumin

The deuterium isotopic effect on the structure and solvation dynamics of the protein, human serum albumin (HSA), has been studied by using circular dichroism (CD), femtosecond up-conversion, FRET, and single-molecule spectroscopy. The CD spectra suggest that D(2)O affects the structure of HSA, leading to a 20% decrease in the helical structure. The FRET study indicates that the distance of C153 from the lone tryptophan residue of HSA is quite similar (≈21 Å) in H(2)O and D(2)O, and hence, the location of the probe in the protein remains the same in the two solvents. The single-molecule study suggests that coumarin 153 (C153) binds almost exclusively (>96%) to one site of HSA. Solvation dynamics of C153 in HSA is found to be markedly retarded in D(2)O compared with H(2)O. In H(2)O, the solvation of C153 bound to HSA is found to be biexponential with one component of 7 ps (30%) and a long component of 350 ps (70%). In D(2)O, we detected a short component of 4 ps (41%) and a long component of 950 ps (59%). Thus, the ultraslow component of the solvation dynamics of C153 bound to HSA in D(2)O (950 ps) is 2.5-fold slower than that in H(2)O (350 ps). The marked deuterium isotope effect has been ascribed to water molecules confined in the protein environment and to a lesser extent to the structural modification of protein by D(2)O.

Structure and Dynamics of the Solvated Electron in Alcohols from Resonance Raman Spectroscopy

Resonance Raman (RR) spectroscopy is used to probe the structure and excited-state dynamics of the solvated electron in the primary liquid alcohols methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), and n-butanol (n-BuOH). The strong resonance enhancements (>or=10(4) relative to pure solvent) of the libration, CO stretch, COH bend, CH3 bend, CH2 bend, and OH stretch reveal significant Franck-Condon coupling of the intermolecular and intramolecular vibrational modes of the solvent to the electronic excitation of the solvated electron. All enhanced bands are fully accounted for by a model of the solvated electron that is comprised of several nearby solvent molecules that are only perturbed by the presence of the electron; no new molecular species are required to explain our data. The 340 cm(-1) downshift observed for the OH stretch frequency of e-(MeOH), relative to pure solvent, strongly suggests that the methanol molecules in the first solvent shell have the hydroxyl group directed linearly toward the excess electron density. The smaller downshifts observed for e-(EtOH), e-(n-PrOH), and e-(n-BuOH) are explained in terms of a OH group that is bent 28-40 degrees from linear. The Raman cross sections and absorption spectra are modeled, lending quantitative support for the inhomogeneous origin of the broad absorption spectra, the necessity of OH local motion in all enhanced Raman modes of the alcohols, and the dominant librational response of the solvent upon photoexcitation of the electron.

Quantized time correlation function approach to nonadiabatic decay rates in condensed phase: Application to solvated electrons in water and methanol

A new, alternative form of the golden rule formula defining the nonadiabatic transition rate between two quantum states in condensed phase is presented. The formula involves the quantum time correlation function of the energy gap, of the nonadiabatic coupling, and their cross terms. Those quantities can be inferred from their classical counterparts, determined via molecular dynamics simulations. The formalism is applied to the problem of the nonadiabatic p-->s relaxation of an equilibrated p-electron in water and methanol. We find that, in both solvents, the relaxation is induced by the coupling to the vibrational modes and the quantum effects modify the rate by a factor of 2-10 depending on the quantization procedure applied. The resulting p-state lifetime for a hypothetical equilibrium excited state appears extremely short, in the sub-100 fs regime. Although this result is in contrast with all previous theoretical predictions, we also illustrate that the lifetimes computed here are very sensitive to the simulated electronic quantum gap and to the strongly correlated nonadiabatic coupling.

Electronic Relaxation Dynamics of Water Cluster Anions

The electronic relaxation dynamics of water cluster anions, (H(2)O)(n)(-), have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p<--s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (tau(D)2(O)/tau(H)2(O) approximately 2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n > or = 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 < or = n < or = 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.

ULTRAFAST SOLVATION DYNAMICS EXPLORED BY FEMTOSECOND PHOTON ECHO SPECTROSCOPIES

Chemical reaction and optical dynamics in the liquid phase are strongly affected by specific solute-solvent interactions. The dynamical part of this coupling leads to energy fluctuations. The associated energy gap dynamics can be probed by using various nonlinear optical spectroscopies. We discuss various forms of photon echo--time-integrated, time-gated, and heterodyne-detected photon echo--as well as Fourier transform spectral interferometry. It is shown that for solutions of the dye molecule DTTCI, a system-bath correlation function can be acquired that provides a quantitative description of all (non)linear spectroscopic experiments. The deduced correlation function is projected onto the multimode Brownian oscillator model, which allows for a physical interpretation of the multiple-time correlation function and a determination of the spectral density relevant to the solvation process. The following applications of photon echo to condensed phase dynamics are discussed: enhanced vibrational mode suppression, Liouville pathways interference, and dynamical Stokes shift. Recent results of echo-peak shift experiments on the hydrated electron are also presented. The review concludes that photon echo should be useful as a novel tool to explore transition state dynamics.