Stephen R. Meech

Optically-heterodyne-detected optical Kerr effect (OHD-OKE): Applications in condensed phase dynamics

The ultrafast optically-heterodyne-detected optical Kerr effect (OHD-OKE) is established as a relatively simple tool for recording the ultrafast dynamics of liquids with high temporal resolution and excellent signal-to-noise ratios. The principles and practice of the OHD-OKE method are outlined. Its application in recording the dynamics of several molecular liquids is described. The data are discussed in terms of the underlying microscopic molecular motions. Orientational motion--both librational and diffusive--is responsible for a significant fraction of the dynamics. Other potential contributions are discussed, but these are less readily assigned. The application of OHD-OKE measurements in interpreting ultrafast studies of the optical dynamics of solutions is discussed. Finally the extension of OHD-OKE methods to record the dynamics of more complex, heterogeneous, media is described.

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.

An Alternate Proton Acceptor for Excited State Proton Transfer in Green Fluorescent Protein: Rewiring GFP

The neutral form of the chromophore in wild-type green fluorescent protein (wtGFP) undergoes excited-state proton transfer (ESPT) upon excitation, resulting in characteristic green (508 nm) fluorescence. This ESPT reaction involves a proton relay from the phenol hydroxyl of the chromophore to the ionized side chain of E222, and results in formation of the anionic chromophore in a protein environment optimized for the neutral species (the I* state). Reorientation or replacement of E222, as occurs in the S65T and E222Q GFP mutants, disables the ESPT reaction and results in loss of green emission following excitation of the neutral chromophore. Previously, it has been shown that the introduction of a second mutation (H148D) into S65T GFP allows the recovery of green emission, implying that ESPT is again possible. A similar recovery of green fluorescence is also observed for the E222Q/H148D mutant, suggesting that D148 is the proton acceptor for the ESPT reaction in both double mutants. The mechanism of fluorescence emission following excitation of the neutral chromophore in S65T/H148D and E222Q/H148D has been explored through the use of steady state and ultrafast time-resolved fluorescence and vibrational spectroscopy. The data are contrasted with those of the single mutant S65T GFP. Time-resolved fluorescence studies indicate very rapid (< 1 ps) formation of I* in the double mutants, followed by vibrational cooling on the picosecond time scale. The time-resolved IR difference spectra are markedly different to those of wtGFP or its anionic mutants. In particular, no spectral signatures are apparent in the picosecond IR difference spectra that would correspond to alteration in the ionization state of D148, leading to the proposal that a low-barrier hydrogen bond (LBHB) is present between the phenol hydroxyl of the chromophore and the side chain of D148, with different potential energy surfaces for the ground and excited states. This model is consistent with recent high-resolution structural data in which the distance between the donor and acceptor oxygen atoms is < or = 2.4 A. Importantly, these studies indicate that the hydrogen-bond network in wtGFP can be replaced by a single residue, an observation which, when fully explored, will add to our understanding of the various requirements for proton-transfer reactions within proteins.

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.

Chemically Optimizing Operational Efficiency of Molecular Rotary Motors

Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited-state processes. Here the excited-state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck-Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency.

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.