R. Kramer Campen

Protons and Hydroxide Ions in Aqueous Systems.

Understanding the structure and dynamics of waters constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

Evidence of microbial consortia metabolizing within a low-latitude mountain glacier

Carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ) concentrations of ice samples from a South American mountain glacier are larger than those in coeval polar ice cores. Of the six possible mechanisms that may cause the composition of air measured in ice cores to depart from that of the overlying troposphere (gravitational fractionation, thermal fractionation, gases trapped in refrozen meltwater, atmospheric heterogeneity, abiotic chemical reactions, and biologically mediated chemical reactions), only metabolism of a consortium of microorganisms within the ice can explain our data.

Determining In Situ Protein Conformation and Orientation from the Amide-I Sum-Frequency Generation Spectrum: Theory and Experiment

Vibrational sum-frequency generation (VSFG) spectra of the amide-I band of proteins can give detailed insight into biomolecular processes near membranes. However, interpreting these spectra in terms of the conformation and orientation of a protein can be difficult, especially in the case of complex proteins. Here we present a formalism to calculate the amide-I infrared (IR), Raman, and VSFG spectra based on the protein conformation and orientation distribution. Based on the protein conformation, we set up the amide-I exciton Hamiltonian for the backbone amide modes that generate the linear and nonlinear spectroscopic responses. In this Hamiltonian, we distinguish between nearest-neighbor and non-nearest-neighbor vibrational couplings. To determine nearest-neighbor couplings we use an ab initio 6-31G+(d) B3LYP-calculated map of the coupling as a function of the dihedral angles. The other couplings are estimated using the transition-dipole coupling model. The local-mode frequencies of hydrogen-bonded peptide bonds and of peptide bonds to proline residues are red-shifted. To obtain realistic hydrogen-bond shifts we perform a molecular dynamics simulation in which the protein is solvated by water. As a first application, we measure and calculate the amide-I IR, Raman, and VSFG spectra of cholera toxin B subunit docked to a model cell membrane. To deduce the orientation of the protein with respect to the membrane from the VSFG spectra, we compare the experimental and calculated spectral shapes of single-polarization results, rather than comparing the relative amplitudes of VSFG spectra recorded for different polarization conditions for infrared, visible, and sum-frequency light. We find that the intrinsic uncertainty in the interfacial refractive index--essential to determine the overall amplitude of the VSFG spectra--prohibits a meaningful comparison of the intensities of the different polarization combinations. In contrast, the spectral shape of most of the VSFG spectra is independent of the details of the interfacial refractive index and provides a reliable way of determining molecular interfacial orientation. Specifically, we find that the symmetry axis of the cholera toxin B subunit is oriented at an angle of 6° ± 17° relative to the surface normal of the lipid monolayer, in agreement with 5-fold binding between the toxins five subunits and the receptor lipids in the membrane.

Disaccharide Topology Induces Slowdown in Local Water Dynamics

Molecular level insight into water structure and structural dynamics near proteins, lipids, and nucleic acids is critical to the quantitative understanding of many biophysical processes. Unfortunately, understanding hydration and hydration dynamics around such large molecules is challenging because of the necessity of deconvoluting the effects of topography and chemical heterogeneity. Here we study, via classical all-atom simulation, the water structure and structural dynamics around two biologically relevant solutes large enough to have significant chemical and topological heterogeneity but small enough to be computationally tractable: the disaccharides kojibiose and trehalose. We find both molecules to be strongly amphiphilic (as quantified from normalized local density fluctuations) and to induce nonuniform local slowdown in water translational and rotational motions. Detailed analysis of the rotational slowdown shows that, while the rotational mechanism is similar to that previously identified in other aqueous systems by Laage, Hynes, and coworkers, two novel characteristics are observed: broadening of the transition state during hydrogen bond exchange (water rotation) and a subpopulation of water for which rotation is slowed because of hindered access of the new accepting water molecule to the transition state. Both characteristics are expected to be generic features of water rotation around larger biomolecules and, taken together, emphasize the difficulty in transferring insight into water rotation around small molecules to much larger amphiphilic solutes.

Mechanism of vibrational energy dissipation of free OH groups at the air–water interface

Significance Interfaces of liquid water play an important role in a wide variety of biological, environmental, and technological processes. These interfaces have many unusual macroscopic properties, e.g., the high surface tension, the tendency to sequester ions at the air–water interface, and the high proton conductivity, that are difficult to understand given simple descriptions of bulk liquid water. To understand what differentiates interfacial water from the bulk, insights into the molecular level structure and dynamics of water at interfaces are crucial. We quantify the dynamics of vibrational energy dissipation of interfacial water using ultrafast time-resolved surface specific vibrational spectroscopy and show that the relaxation pathways differ markedly from bulk water and reflect surface structural dynamics. Interfaces of liquid water play a critical role in a wide variety of processes that occur in biology, a variety of technologies, and the environment. Many macroscopic observations clarify that the properties of liquid water interfaces significantly differ from those of the bulk liquid. In addition to interfacial molecular structure, knowledge of the rates and mechanisms of the relaxation of excess vibrational energy is indispensable to fully understand physical and chemical processes of water and aqueous solutions, such as chemical reaction rates and pathways, proton transfer, and hydrogen bond dynamics. Here we elucidate the rate and mechanism of vibrational energy dissipation of water molecules at the air–water interface using femtosecond two-color IR-pump/vibrational sum-frequency probe spectroscopy. Vibrational relaxation of nonhydrogen-bonded OH groups occurs at a subpicosecond timescale in a manner fundamentally different from hydrogen-bonded OH groups in bulk, through two competing mechanisms: intramolecular energy transfer and ultrafast reorientational motion that leads to free OH groups becoming hydrogen bonded. Both pathways effectively lead to the transfer of the excited vibrational modes from free to hydrogen-bonded OH groups, from which relaxation readily occurs. Of the overall relaxation rate of interfacial free OH groups at the air–H2O interface, two-thirds are accounted for by intramolecular energy transfer, whereas the remaining one-third is dominated by the reorientational motion. These findings not only shed light on vibrational energy dynamics of interfacial water, but also contribute to our understanding of the impact of structural and vibrational dynamics on the vibrational sum-frequency line shapes of aqueous interfaces.

Optically probing Al—O and O—H vibrations to characterize water adsorption and surface reconstruction on α-alumina: An experimental and theoretical study

Oxide/water interfaces are ubiquitous in a wide variety of applications and the environment. Despite this ubiquity, and attendant decades of study, gaining molecular level insight into water/oxide interaction has proven challenging. In part, this challenge springs from a lack of tools to concurrently characterize changes in surface structure (i.e., water/oxide interaction from the perspective of the solid) and O-H population and local environment (i.e., water/oxide interaction from the water perspective). Here, we demonstrate the application of surface specific vibrational spectroscopy to the characterization of the interaction of the paradigmatic α-Al2O3(0001) surface and water. By probing both the interfacial Al-O (surface phonon) and O-H spectral response, we characterize this interaction from both perspectives. Through electronic structure calculation, we assign the interfacial Al-O response and rationalize its changes on surface dehydroxylation and reconstruction. Because our technique is all-optical and interface specific, it is equally applicable to oxide surfaces in vacuum, ambient atmospheres and at the solid/liquid interface. Application of this approach to additional alumina surfaces and other oxides thus seems likely to significantly expand our understanding of how water meets oxide surfaces and thus the wide variety of phenomena this interaction controls.

The Free OD at the Air/D2O Interface is Structurally and Dynamically Heterogeneous

Air/water interfaces are both ubiquitous in the environment and technology and a useful model for hydrophobic solvation more generally. Previous experimental and computational studies have highlighted that molecular level markers of such an extended hydrophobic surface are broken hydrogen bonds and, as a result, OH groups that are not hydrogen bond donors: free OH. Understanding both the time-averaged structure and structural dynamics of these free OH thus plays a critical role in developing a quantitative, molecular level understanding of hydrophobic solvation. Here we show, by combining polarization-dependent vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulation, that the free OD of D2O at the air/D2O interface is structurally and dynamically heterogeneous: that longer lived free OD groups tend to point closer to the surface normal, have a narrower orientational distribution, and are closer to the vapor phase. Knowledge of this structural heterogeneity should help link existing descriptions of hydrophobic solvation that focus either on the termination of the bulk hydrogen bond network or local density fluctuations. In addition the results of this study clarify that schemes to increase signal-to-noise ratios in VSF measurements by delaying the visible pulse relative to the infrared should be used only with independent constraints on the systems structural dynamics.

Experimentally probing the libration of interfacial water: the rotational potential of water is stiffer at the air/water interface than in bulk liquid

Most properties of liquid water are determined by its hydrogen-bond network. Because forming an aqueous interface requires termination of this network, one might expect the molecular level properties of interfacial water to markedly differ from water in bulk. Intriguingly, much prior experimental and theoretical work has found that, from the perspective of their time-averaged structure and picosecond structural dynamics, hydrogen-bonded OH groups at an air/water interface behave the same as hydrogen-bonded OH groups in bulk liquid water. Here we report the first experimental observation of interfacial waters libration (i.e. frustrated rotation) using the laser-based technique vibrational sum frequency spectroscopy. We find this mode has a frequency of 834 cm(-1), ≈165 cm(-1) higher than in bulk liquid water at the same temperature and similar to bulk ice. Because libration frequency is proportional to the stiffness of waters rotational potential, this increase suggests that one effect of terminating bulk waters hydrogen bonding network at the air/water interface is retarding rotation of water around intact hydrogen bonds. Because in bulk liquid water the libration plays a key role in stabilizing reaction intermediates and dissipating excess vibrational energy, we expect the ability to probe this mode in interfacial water to open new perspectives on the kinetics of heterogeneous reactions at aqueous interfaces.

The Nature of the Dielectric Response of Methanol Revealed by the Terahertz Kerr Effect

The dielectric response of liquids in the terahertz (THz) and sub-THz frequency range arises from low-energy collective molecular motions, which are often strongly influenced by intermolecular interactions. To shed light on the microscopic origin of the THz dielectric response of the simplest alcohol, methanol, we resonantly excite this liquid with an intense THz electric-field pulse and monitor the relaxation of the induced optical birefringence. We find a unipolar THz-Kerr-effect signal which, in contrast to aprotic polar liquids, shows a weak coupling between the THz electric field and the permanent molecular dipole moment of the liquid. We assign this weak coupling to the restricted translational rather than rotational nature of the excited mode. Our approach opens a new avenue to the assignment of the dielectric spectrum of liquids to a microscopic origin.

Characterization of water dissociation on α-Al2O3(102): theory and experiment

The interaction of water with α-alumina (i.e. α-Al2O3) surfaces is important in a variety of applications and a useful model for the interaction of water with environmentally abundant aluminosilicate phases. Despite its significance, studies of water interaction with α-Al2O3 surfaces other than the (0001) are extremely limited. Here we characterize the interaction of water (D2O) with a well defined α-Al2O3(11[combining macron]02) surface in UHV both experimentally, using temperature programmed desorption and surface-specific vibrational spectroscopy, and theoretically, using periodic-slab density functional theory calculations. This combined approach makes it possible to demonstrate that water adsorption occurs only at a single well defined surface site (the so-called 1-4 configuration) and that at this site the barrier between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV. A subset of OD stretch vibrations are parallel to this dissociation coordinate, and thus would be expected to be shifted to low frequencies relative to an uncoupled harmonic oscillator. To quantify this effect we solve the vibrational Schrödinger equation along the dissociation coordinate and find fundamental frequencies red-shifted by more than 1500 cm(-1). Within the context of this model, at moderate temperatures, we further find that some fraction of surface deuterons are likely delocalized: dissociatively and molecularly absorbed states are no longer distinguishable.