Unveiling the Amphiphilic Nature of TMAO by Vibrational Sum Frequency Generation Spectroscopy

By combining heterodyne-detected sum-frequency generation (SFG) spectroscopy, <i>ab initio</i> molecular dynamics (AIMD) simulation, and a post-vibrational self-consistent field (VSCF) approach, we reveal the orientation and surface activity of the amphiphile trimethylamine-<i>N</i>-oxide (TMAO) at the water/air interface. Both measured and simulated C–H stretch SFG spectra show a strong negative and a weak positive peak. We attribute these peaks to the symmetric stretch mode/Fermi resonance and antisymmetric in-plane mode of the methyl group, respectively, based on the post-VSCF calculation. These positive and negative features evidence that the methyl groups of TMAO are oriented preferentially toward the air phase. Furthermore, we explore the effects of TMAO on the interfacial water structure. The O–H stretch SFG spectra manifest that the hydrogen bond network of the aqueous TMAO-solution/air interface is similar to that of the amine-<i>N</i>-oxide (AO) surfactant/water interface. This demonstrates that, irrespective of the alkyl chain length, the AO groups have a similar impact on the hydrogen bond network of the interfacial water. In contrast, we find that adding TMAO to water makes the orientation of the free O−H groups of the interfacial water molecules more parallel to the surface normal. Invariance of the free O–H peak amplitude despite the enhanced orientation of the topmost water layer illustrates that TMAO is embedded in the topmost water layer, manifesting the clear contrast of the hydrophobic methyl group and the hydrophilic AO group of TMAO

Reduced Near-Resonant Vibrational Coupling at the Surfaces of Liquid Water and Ice

We study the resonant interaction of the OH stretch vibrations of water molecules at the surfaces of liquid water and ice using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. By studying different isotopic mixtures of H₂O and D₂O, we vary the strength of the interaction, and we monitor the resulting effect on the HD-SFG spectrum of the OH stretch vibrations. We observe that the near-resonant coupling effects are weaker at the surface than in the bulk, for both water and ice, indicating that for both phases of water the OH vibrations are less strongly delocalized at the surface than in the bulk

Lipid Carbonyl Groups Terminate the Hydrogen Bond Network of Membrane-Bound Water

We present a combined experimental sum-frequency generation (SFG) spectroscopy and <i>ab initio</i> molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine <i>N</i>-oxide (AO) surfactant monolayers. Simulated O–H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the similar zwitterionic AO interface. The positive peaks at the water/PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen bond network to reach, with its (<i>up</i>-oriented) O–H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group in the interfacial water structure at the membrane interface, namely, stabilizing the water hydrogen bond network

Ultrafast Reorientational Dynamics of Leucine at the Air–Water Interface

Ultrafast dynamics of protein side chains are involved in important biological processes such as ligand binding, protein folding, and hydration. In addition, the dynamics of a side chain can report on local environments within proteins. While protein side chain dynamics have been probed for proteins in solution with nuclear magnetic resonance and infrared methods for decades, information about side chain dynamics at interfaces is lacking. At the same time, the dynamics and motions of side chains can be particularly important for interfacial binding and protein-driven surface manipulation. We here demonstrate that ultrafast reorientation dynamics of leucine amino acids at interfaces can be recorded in situ and in real time using polarization- and time-resolved pump–probe sum frequency generation (SFG). Combined with molecular dynamics simulations, time-resolved SFG was used to probe the reorientation of the isopropyl methyl groups of l-leucine at the air–water interface. The data show that the methyl units reorient diffusively at an in plane rate of <i>D</i>_φ = 0.07 rad²/ps and an out of plane rate of <i>D</i>_θ = 0.05 rad²/ps

Conical Ionic Amphiphiles Endowed with Micellization Ability but Lacking Air–Water and Oil–Water Interfacial Activity

Micellization in water and reduction of the surface tension at water interfaces with air and oil are two archetypical properties of surfactants, caused by self-aggregation and Gibbs monolayer formation at the interfaces, respectively. We present here a new type of amphiphiles that possess a conical shape consisting of a hydrophobic apex and five ionic termini at the base of the cone. The conical shape and the high charge density cooperatively impede monolayer formation at the interfaces, hence preventing foaming and emulsification. On the other hand, the conical shape strongly assists micelle formation in water and hemimicelle formation on a solid surface to promote dissolution of nanoparticles such as magnetic nanoparticles and nanocarbons in water. The well-defined shape and charge locations distinguish the new amphiphiles from known polymer amphiphiles that show similar surface activity

Water Bending Mode at the Water–Vapor Interface Probed by Sum-Frequency Generation Spectroscopy: A Combined Molecular Dynamics Simulation and Experimental Study

We present a combined molecular dynamics simulation and experimental study on the water bending mode at the water–vapor interface using sum-frequency generation (SFG) spectroscopy. The SFG spectrum simulated using an ab initio-based water model shows good agreement with the experimental data. The imaginary part of the SFG response shows a negative peak at ∼1650 cm^–1 and a positive peak at ∼1730 cm^–1. Our results reveal that these widely (∼80 cm^–1) separated peaks result from the interference of two closely spaced (∼29 cm^–1) peaks of opposite sign. The positive peak at ∼1689 cm^–1 originates from water with two donor hydrogen atoms with the HOH angular bisector pointing down toward the bulk, and the negative peak at ∼1660 cm^–1 from water with free O–H groups, pointing up. The small frequency difference of 29 cm^–1 indicates that the HOH bending mode frequency of interfacial water is relatively insensitive to the number of hydrogen bonds

Water in Contact with a Cationic Lipid Exhibits Bulklike Vibrational Dynamics

Water in contact with lipids is an important aspect of most biological systems and has been termed “biological water”. We used time-resolved infrared spectroscopy to investigate the vibrational dynamics of lipid-bound water molecules, to shed more light on the properties of these important molecules. We studied water in contact with a positively charged lipid monolayer using surface-specific two-dimensional sum frequency generation vibrational spectroscopy with subpicosecond time resolution. The dynamics of the O–D stretch vibration was measured for both pure D₂O and isotopically diluted D₂O under a monolayer of 1,2-dipalmitoyl-3-trimethylammonium-propane. It was found that the lifetime of the stretch vibration depends on the excitation frequency and that efficient energy transfer occurs between the interfacial water molecules. The spectral diffusion and vibrational relaxation of the stretch vibration were successfully explained with a simple model, taking into account the Förster transfer between stretch vibrations and vibrational relaxation via the bend overtone. These observations are very similar to those made for bulk water and as such lead us to conclude that water at a positively charged lipid interface behaves similarly to bulk water. This contrasts the behavior of water in contact with negative or zwitterionic lipids and can be understood by noting that for cationic lipids the charge-induced alignment of water molecules results in interfacial water molecules with O–D groups pointing toward the bulk

Oppositely Charged Ions at Water–Air and Water–Oil Interfaces: Contrasting the Molecular Picture with Thermodynamics

The surface-active ions tetraphenylarsonium (Ph₄As⁺) and tetraphenylboron (Ph₄B^–) have a similar structure but opposite charge. At the solution–air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph₄As⁺ compared with Ph₄B^–, in addition to opposite alignment of interfacial water molecules. At the water–oil interface, the interfacial tension is 7 mN/m lower for Ph₄As⁺ than for Ph₄B^– salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements

Molecular Insight into the Slipperiness of Ice

Measurements of the friction coefficient of steel-on-ice over a large temperature range reveal very high friction at low temperatures (−100 °C) and a steep decrease in the friction coefficient with increasing temperature. Very low friction is only found over the limited temperature range typical for ice skating. The strong decrease in the friction coefficient with increasing temperature exhibits Arrhenius behavior with an activation energy of <i>E</i>_a ≈ 11.5 kJ mol^–1. Remarkably, molecular dynamics simulations of the ice–air interface reveal a very similar activation energy for the mobility of surface molecules. Weakly hydrogen-bonded surface molecules diffuse over the surface in a rolling motion, their number and mobility increasing with increasing temperature. This correlation between macroscopic friction and microscopic molecular mobility indicates that slippery ice arises from the high mobility of its surface molecules, making the ice surface smooth and the shearing of the weakly bonded surface molecules easy