Session E26: Chemical Physics of Hydrogen Bonding III

Water at protein surfaces studied with femtosecond nonlinear spectroscopy

We report on an investigation of the structure and dynamics of water molecules near protein surfaces with femtosecond nonlinear spectroscopic techniques. We measured the reorientation dynamics of water molecules near the surface of several globular protein surfaces, using polarization-resolved femtosecond infrared spectroscopy. [1] We found that water molecules near the protein surface have a much slower reorientation than water molecules in bulk liquid water. The number of slow water molecules scales scales with the size of the hydrophobic surface of the protein. When we denature the proteins by adding an increasing amount of urea to the protein solution, we observe that the water-exposed surface increases by \textasciitilde 50{\%} before the secondary structure of the proteins changes. This finding indicates that protein unfolding starts with the protein structure becoming less tight, thereby allowing water to enter. With surface vibrational sum frequency generation (VSFG) spectroscopy, we studied the structure of water at the surface of antifreeze protein III.[2] The measured VSFG spectra showed the presence of ice-like water layers at the ice-binding site of the protein in aqueous solution, at temperatures well above the freezing point. This ordered ice-like hydration layers at the protein surface likely plays an important role in the specific recognition and binding of anti-freeze protein III to nascent ice crystallites, and thus in its anti-freeze mechanism. [1] C.C.M. Groot and H.J. Bakker, \textit{Proteins Take up Water Before Unfolding}, J. Phys. Chem. Lett. \textbf{7}, 1800--1804 (2016). [2] K. Meister, S. Strazdaite, A.L. DeVries, S. Lotze, L.L.C. Olijve, I.K. Voets, and H.J. Bakker, \textit{Observation of ice-like water layers at an aqueous protein surface}, Proc. Natl. Acad. Sci. USA \textbf{111}, 17732-17736 (2014).   

Cold cluster snapshots of the Grotthuss proton relay mechanism in water

The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded network. However, the vibrational spectroscopic signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D$^{\mathrm{+}}$(D$_{\mathrm{2}}$O)$_{n}$, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton transfer event. By complexing the solvated hydronium ``Eigen'' cluster, D$_{\mathrm{3}}$O$^{\mathrm{+}}$(D$_{\mathrm{2}}$O)$_{\mathrm{3}}$, with increasingly strong H-bond acceptor molecules (H$_{\mathrm{2}}$, N$_{\mathrm{2}}$, CO, H$_{\mathrm{2}}$O), we are able to track the frequency of every O-D stretch in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule.   

Mechanism of Selective Adsorption of Ions to Aqueous Interfaces: Graphene/Water vs. Air/Water

The behavior of ions at aqueous interfaces has been a subject of much controversy for over a century. By exploiting the strong charge-transfer-to-solvent (CTTS) resonances of selected anions in aqueous electrolytes, their adsorption properties have measured by deep UV-SHG spectroscopy methods for both air/water and graphene/water interfaces. Temperature and concentration dependences determined by both experiment and computer simulations for the air/water case reveal that the strong interfacial adsorption observed for weakly hydrated ions is enthalpically driven by hydration forces and impeded by a novel entropy effect(capillary wave suppression). Extension of this approach to the water-graphene interface reveals a surprising similarity to the air-water case, albiet with different mechanistic details. Our recent development of a broadband deep UV SFG spectroscopy technique has produced detailed CTTS spectra of interfacial ions, for which comparisons with bulk CTTS spectra provide additional new insights.\\ \\D.E. Otten, P. Shaffer, P. Geissler, R.J. Saykally, \textit{Elucidating the Mechanism of Selective Ion Adsorption to the Liquid Water Surface}, PNAS \textbf{109}, 701(2012).   

The origin of the extremely different solubilities of polyethers in water

\noindent Abstract: The solubilities of polyethers in water are surprisingly counter-intuitive. The best-known example is the difference between polyethylene glycol (\mbox{[-CH$_2$-CH$_2$-O-]$_n$}) which dissolves up to 0.5~kg/l, and polyoxymethylene (\mbox{[-CH$_2$-O-]$_n$}) which is {\it completely insoluble} in water, exactly the opposite of what one expects from the ratio of hydrophobic to hydrophilic parts in these molecules. Similar anomalies exist for oligomeric and cyclic polyethers. To solve this apparent mystery, we use femtosecond vibrational and GHz dielectric spectroscopy with complementary {\it ab initio} calculations and molecular dynamics simulations. We find that the dynamics of water molecules solvating polyethers is fundamentally different depending on their C/O composition. The calculations and simulations show that this because the partial charge on the O atoms depends on the number of C atoms by which they are separated. Our results show that quantum effects can have a major impact on aqueous solubilities.   

Vibrational Characterization of the Hydrogen Bonding Network of a Microsolvated Ruthenium Polypyridyl Electrocatalytic Water Oxidation Intermediate

Detailed molecular-level understanding of the electrocatalytic mechanism of homogeneous water oxidation remains elusive due to the difficulty of studying reaction intermediates by traditional analytical methods. Our experimental apparatus combines an electrospray ionization source, two cryogenic ion traps, and a time-of-flight photofragmentation infrared spectrometer for the controlled formation and vibrational interrogation of solvated ionic clusters. We have utilized this approach to acquire infrared spectra of stepwise-solvated [Ru(tpy)(bpy)(OH)]$^{2+}$ (tpy = 2,2$'$;6$'$,2$"$-terpyridine, bpy = 2,2$'$-bipyridine), an electrochemical intermediate of the well-known molecular catalyst, [Ru(tpy)(bpy)(OH$_{2}$)]$^{2+}$. Through comparison with the spectra of microsolvated [Ru(tpy)(bpy)(OH$_{2}$)]$^{2+}$, insights into the role of the hydrogen bonding network on catalytic structure and activity, specifically proton-coupled electron transfer (PCET), will be discussed.   

Methane Clathrate at Gas-Water Interface Studied by Sum Frequency Vibrational Spectroscopy

Methane clathrate (hydrate) is a rich resource of naturally occurring crystalline substance comprising a guest methane molecule embedded in a water cage at low temperature and high pressure. Despite its great importance in energy, climate effect and gas/oil transportation, the initial formation process of gas clathrate remains elusive, particularly in experiment. In this work, using sum-frequency vibrational spectroscopy (SFVS), we reported our experimental observation of methane clathrate formed at water-gas interface, where both water and methane molecules are abundant in the hetero-interfacial region, in a large range of temperature and pressure. The surface vibrational spectrum of gas clathrate was obtained for the first time. The resultant SF spectrum reveals that clathrate-like water network develops at interface much earlier than the thermodynamic phase transition point of the bulk clathrate. Our results provide molecular-level understandings of structure and formation process of gas hydrate at the interface, which has important bearing on inhibition of blockage in gas (oil) pipelines, carbon dioxide sequestration, chemical energy storage and so on.   

Combining Ion Mobility Mass Spectrometry and Molecular Dynamics - a look at the conformational behaviour during water droplet evaporation.

Intrinsically Disordered Proteins (IDPs) lack stable secondary and tertiary structure yet fulfil a myriad of functions within their native environments. Determination of the conformational landscape of such heterogeneous ensemble is challenging and mass spectrometry (MS) and ion mobility mass spectrometry (IM-MS) offer unique benefits for structural biology, complementing existing techniques. IM-MS is able to determine the conformational spread of a given protein or protein-complex in vacuo in terms of a collision cross section. In order to best relate the gas phase conformational diversity to that present in solution it is essential to consider how desolvation alters the stable conformations and the increased importance of self-solvation in a solvent-free environment. Here we present the results of MD and IM-MS experiments from oncogenic IDPs, namely the C-terminus of the KID domain of p27 and the N-terminal transactivation domain of p53. For both proteins the experimental charge state distribution is used to prepare starting structures with different protonation states. Independent MD trajectories of each charge state are performed with explicit solvent which is then `evaporated' as water molecules move further from the protein core simulating the desolvation process.   

Optical fingerprints of solid-liquid interfaces: a joint ATR-IR and \emph{first principles} investigation

Despite the importance of understanding the structural and bonding properties of solid-liquid interfaces for a wide range of (photo-)electrochemical applications, there are presently no experimental techniques available to directly probe the microscopic structure of solid-liquid interfaces. To develop robust strategies to interpret experiments and validate theory, we carried out attenuated total internal reflection (ATR-IR) spectroscopy measurements and \emph{ab initio} molecular dynamics (AIMD) simulations of the vibrational properties of interfaces between liquid water and well-controlled prototypical semiconductor substrates. We show the Ge(100)/H$_2$O interface to feature a reversible potential-dependent surface phase transition between Ge-H and Ge-OH termination. The Si(100)/H$_2$O interface is proposed as a model system for corrosion and oxidation processes. We performed AIMD calculations under finite electric fields, revealing different pathways for initial oxidation. These pathways are predicted to exhibit unique spectral signatures. A significant increase in surface specificity can be achieved utilizing an angle-dependent ATR-IR experiment, which allows to detect such signatures at the interfacial layer and consequently changes in the hydrogen bond network.