Ismael A. Heisler

Low-Frequency Modes of Aqueous Alkali Halide Solutions: Glimpsing the Hydrogen Bonding Vibration

Salty Stretch What happens at the molecular level when salt dissolves in water? Much of the data characterizing the geometry and dynamics of ion solvation shells has come from indirect observation of the surrounding water structure. Using a time domain Raman technique based on the interference of four ultrashort polarized light pulses, Heisler and Meech (p. 857) have now mapped directly the stretching vibrations associated with the weak hydrogen bonding interactions between bulk water molecules and chloride, bromide, or iodide ions. An optical scattering technique is used to map the weak bonding interaction between water and dissolved halide ions. The solvation of ions in aqueous media is a fundamental process in biology and chemistry. Here, we report direct time-domain observations of the hydrogen bond vibrational mode formed between a halide ion (chloride, bromide, or iodide) and the surrounding water molecules. The frequency of the hydrogen bond mode is sensitive to both the atomic weight and the concentration of the ion. The peak frequencies fall in the 125 to 175 wave-number range, a spectral region accessed through time-domain polarization-resolved coherent Raman scattering using a diffractive optic method. The polarized Raman response observed is discussed in terms of the structure of the anion’s solvation shell and modeled through calculations on water chloride clusters.

Ultrafast dynamics in the power stroke of a molecular rotary motor

Light-driven molecular motors convert light into mechanical energy through excited-state reactions. Unidirectional rotary molecular motors based on chiral overcrowded alkenes operate through consecutive photochemical and thermal steps. The thermal (helix inverting) step has been optimized successfully through variations in molecular structure, but much less is known about the photochemical step, which provides power to the motor. Ultimately, controlling the efficiency of molecular motors requires a detailed picture of the molecular dynamics on the excited-state potential energy surface. Here, we characterize the primary events that follow photon absorption by a unidirectional molecular motor using ultrafast fluorescence up-conversion measurements with sub 50 fs time resolution. We observe an extraordinarily fast initial relaxation out of the Franck-Condon region that suggests a barrierless reaction coordinate. This fast molecular motion is shown to be accompanied by the excitation of coherent excited-state structural motion. The implications of these observations for manipulating motor efficiency are discussed.

Reactive Dynamics in Confined Liquids : Ultrafast Torsional Dynamics of Auramine O in Nanoconfined Water in Aerosol OT Reverse Micelles

The effects of confinement on the ultrafast torsional reaction of auramine O in aqueous solution are investigated through ultrafast fluorescence up-conversion with 50 fs time resolution. The aqueous solution is confined in nanoscale water droplets by an ionic surfactant. The torsional motion is orders of magnitude slower in the confined droplets than in bulk aqueous solution. The dynamics become faster with increasing radius of the nanodroplet but never reach the bulk value, even when the radius is as large as 10 nm. Time-dependent fluorescence spectra were constructed and subsequently analyzed using a one-dimensional generalized Smoluchowski equation. An accurate description of the data was achieved using a time-dependent diffusion coefficient. This is suggested to arise because the medium friction reflects dynamics on a broad range of time scales spanning the reaction dynamics. The friction recovered suggests strongly hindered motion in the confined droplet and can be qualitatively related to solvation dynamics measured in AOT, consistent with auramine O torsional dynamics being accompanied by intramolecular charge redistribution.

THz Spectra and Dynamics of Aqueous Solutions Studied by the Ultrafast Optical Kerr Effect

The nature and extent of the effects that hydrophilic and hydrophobic solutes have on the dynamics of water molecules continues to be an area of intense experimental and theoretical investigation. In this work, we use the ultrafast optical Kerr effect to measure the picosecond dynamics and THz Raman spectral densities of a series of aqueous solutions. The solutes studied are the hydrophilic urea and formamide and the hydrophobic trimethylamine N-oxide and tetramethylurea. Measurements are made as a function of concentration between <0.1 M and >4 M. At low concentrations (<0.5 M), the THz spectrum resembles that of bulk water, but the picosecond relaxation time, reflecting dynamics in the water H-bonded network, is increased relative to bulk water for all four solutes. The extent to which water relaxation is slowed down depends on the nature of the solute, and is more pronounced for hydrophilic than for hydrophobic solutes. At concentrations above 1 M, a range of solute-solvent and solute-solute interactions gives rise to diverse solute dependent changes in the THz spectral density and to a further slowing down of the picosecond relaxation. The hydrophobic trimethylamine N-oxide has remarkably little effect on the spectral density of water, which may indicate solute self-association and the formation of water pools in more concentrated solutions. For hydrophilic urea and formamide, the THz spectral density suggests that water structure is disrupted at concentrations where most water molecules are part of a solvation shell. At such high concentrations, modes associated with the H-bonded solute make a significant contribution to the spectral density at around 100 cm(-1). The hydrophobic tetramethylurea solute makes a substantial contribution to the spectral density, complicating the interpretation, but a line shape analysis suggests that it also does not strongly perturb the water structure.

Low-Frequency Modes of Aqueous Alkali Halide Solutions: An Ultrafast Optical Kerr Effect Study

A detailed picture of aqueous solvation of ions is central to the understanding of diverse phenomena in chemistry and biology. In this work, we report polarization resolved THz time domain measurements of the Raman spectral density of a wide range of aqueous salt solutions. In particular, the isotropic Raman spectral density reveals the frequency of the hydrogen bond formed between the halide ion and water. The frequency of this mode is measured for the series Cl(-), Br(-), and I(-) as a function of concentration, cation size, and charge. The frequencies extrapolated to zero concentration permit an estimation of the force constant of the mode, which is found to decrease with increasing halide mass and to be similar to the force constant associated with the water-water hydrogen bond. This result is consistent with recent calculations. The extrapolation of the frequency of the chloride hydrogen bond to zero concentration reveals a dependence of the frequency on the nature of the cation. This is ascribed to an interaction between the solvated anion and cation even at the lowest concentration studied here (<0.15 M). It is suggested that this behavior reflects the influence of the electric field of the cation on the hydrogen bond of an adjacent anion. Such interactions should be taken into account when modeling experimental data recorded at concentrations of ions in excess of 0.1 M. These measurements of the isotropic Raman spectral density are compared with those for the anisotropic response, which reflects the frequencies of the full range of hydrogen bonds in aqueous salt solutions. The anisotropic spectral density recovered can be modeled in terms of a concentration-dependent population of water-water H-bonds with a frequency unaffected by the ions, the halide-water hydrogen bonds, and a low-frequency collision-induced contribution.

Ultrafast Dynamics and Hydrogen-Bond Structure in Aqueous Solutions of Model Peptides

The dynamics of water molecules in the hydration layers of proteins are critical for biological function. Here the molecular dynamics in aqueous solutions of model hydrophilic and amphiphilic dipeptides are studied as a function of concentration using the ultrafast optical Kerr effect (OKE). The OKE is a direct time-domain method which yields both picosecond time scale molecular dynamics and low-frequency (Terahertz) Raman spectra, which contain information on the hydrogen-bonded structure of aqueous solutions. Two distinct concentration regimes are identified, above and below 0.4 M peptide concentration. In the low-concentration regime the tetrahedral water structure is largely preserved but the structural dynamics in water are slowed significantly by interaction with the peptide. The slow down is more marked for the hydrophilic than the amphiphilic peptide. Suppression of water structural dynamics observed is greater than that reported for retardation of the water reorientation in NMR, reflecting the different dynamics probed by these different methods. Above 0.4 M the tetrahedral water structure is more strongly perturbed, a contribution to the THz Raman spectrum from the solvated peptide is observed, and structural dynamics in the solution are markedly slowed. This is assigned to slow relaxation within an H-bonded network of peptide molecules. The strong concentration dependence observed goes some way toward explaining disagreements between different measurements of the dynamics of peptide solvation which have appeared in the literature.

Water Dynamics at Protein Interfaces: Ultrafast Optical Kerr Effect Study

The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at ~80 cm(-1), which is assigned to a bending of the protein amide chain.

Chemically modulating the photophysics of the GFP chromophore.

There is growing interest in engineering the properties of fluorescent proteins through modifications to the chromophore structure utilizing mutagenesis with either natural or unnatural amino acids. This entails an understanding of the photophysical and photochemical properties of the modified chromophore. In this work, a range of GFP chromophores with different alkyl substituents are synthesized and their electronic spectra, pH dependence, and ultrafast fluorescence decay kinetics are investigated. The weakly electron donating character of the alkyl substituents leads to dramatic red shifts in the electronic spectra of the anions, which are accompanied by increased fluorescence decay times. This high sensitivity of electronic structure to substitution is also characteristic of some fluorescent proteins. The solvent viscosity dependence of the decay kinetics are investigated, and found to be consistent with a bimodal radiationless relaxation coordinate. Some substituents are shown to distort the planar structure of the chromophore, which results in a blue shift in the electronic spectra and a strong enhancement of the radiationless decay. The significance of these data for the rational design of novel fluorescent proteins is discussed.

Structural Dynamics of Hydrated Phospholipid Surfaces Probed by Ultrafast 2D Spectroscopy of Phosphate Vibrations

The properties of biomembranes depend in a decisive way on interactions of phospholipids with hydrating water molecules. To map structural dynamics of a phospholipid-water interface on the length and time scale of molecular motions, we introduce the phospholipid symmetric and asymmetric phosphate stretch vibrations as probes of interfacial hydrogen bonds and electrostatic interactions. The first two-dimensional infrared spectra of such modes and a line shape analysis by density matrix theory reveal two distinct structural dynamics components; the first 300 fs contribution is related to spatial fluctuations of charged phospholipid head groups with additional water contributions at high hydration levels; the second accounts for water-phosphate hydrogen bonds persisting longer than 10 ps. Our results reveal a relatively rigid hydration shell around phosphate groups, a behavior relevant for numerous biomolecular systems.

Ultrafast dynamics of protein proton transfer on short hydrogen bond potential energy surfaces: S65T/H148D GFP.

Ultrafast proton transfer dynamics on a short H-bond in a protein were studied through the time-resolved fluorescence of the S65T/H148D green fluorescent protein (GFP) mutant. In response to the change in chromophore pK(a) upon excitation, the donor-proton-acceptor structure evolves on a sub-100 fs time scale, followed by picosecond time scale vibrational cooling and host structure reorganization.