Xiangke Chen

Interfacial Water Structure Associated with Phospholipid Membranes Studied by Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy

Phase-sensitive vibrational sum frequency generation is employed to investigate the water structure at phospholipid/water interfaces. Interfacial water molecules are oriented preferentially by the electrostatic potential imposed by the phospholipids and have, on average, their dipole pointing toward the phospholipid tails for all phospholipids studied, dipalmitoyl phosphocholine (DPPC), dipalmitoyl phosphoethanolamine (DPPE), dipalmitoyl phosphate (DPPA), dipalmitoyl phosphoglycerol (DPPG), and dipalmitoyl phospho-l-serine (DPPS). Zwitterionic DPPC and DPPE reveal weaker water orienting capability relative to net negative DPPA, DPPG, and DPPS. Binding of calcium cations to the lipid phosphate group reduces ordering of the water molecules.

Shedding light on water structure at air-aqueous interfaces: ions, lipids, and hydration

An account is given of the current state of understanding of aqueous salt, acid, and lipid/water surfaces, interfacial depth, and molecular organization within the air-solution interfacial region. Water structure, hydration, surface propensity of solutes, and surface organization are discussed. In this perspective, vibrational sum frequency generation spectroscopic studies of aqueous surfaces are interpreted. Comment on future directions within the field of aqueous surface structure is provided.

Na+ and Ca2+ Effect on the Hydration and Orientation of the Phosphate Group of DPPC at Air−Water and Air−Hydrated Silica Interfaces

Hydration and orientation of the phosphate group of dipalmitoylphosphatidylcholine (DPPC) monolayers in the liquid-expanded (LE) phase and the liquid-condensed (LC) phase in the presence of sodium ions and calcium ions was investigated with vibrational sum frequency generation (SFG) spectroscopy at the air-aqueous interface in conjunction with surface pressure measurements. In the LE phase, both sodium and calcium affect the phosphate group hydration. In the LC phase, however, sodium ions affect the phosphate hydration subtly, while calcium ions cause a marked dehydration. Silica-supported DPPC monolayers prepared by the Langmuir-Blodgett method reveal similar hydration behavior relative to that observed in the corresponding aqueous subphase for the case of water and in the presence of sodium ions. However, in the presence of calcium ions the phosphate group dehydration is greater than that from the corresponding purely aqueous CaCl(2) subphase. The average tilt angles from the surface normal of the PO(2)(-) group of DPPC monolayers on the water surface and on the silica substrate calculated from SFG data are found to be 59 degrees +/- 3 degrees and 72 degrees +/- 5 degrees , respectively. Orientation of the phosphate group is additionally affected by the presence of ions. These findings show that extrapolation of results obtained from model membranes from liquid surfaces to solid supports may not be warranted since there are differences in headgroup organization on the two subphases.

Direct comparison of phase-sensitive vibrational sum frequency generation with maximum entropy method: Case study of water

We present a direct comparison of phase sensitive sum-frequency generation experiments with phase reconstruction obtained by the maximum entropy method. We show that both methods lead to the same complex spectrum. Furthermore, we discuss the strengths and weaknesses of each of these methods, analyzing possible sources of experimental and analytical errors. A simulation program for maximum entropy phase reconstruction is available at: http://lbp.epfl.ch/.

Phase-Sensitive Sum Frequency Revealing Accommodation of Bicarbonate Ions, and Charge Separation of Sodium and Carbonate Ions within the Air/Water Interface

Interfacial water structure plays a key role in many chemical, biological, and environmental processes. Here, in addition to conventional VSFG, we employ phase-sensitive sum frequency generation (PS-SFG) to investigate the average direction of the transition dipole of interfacial water molecules that is intrinsically contained in the sign of the second-order nonlinear susceptibility, χ((2)). The orientation of water at air/aqueous inorganic salt interfaces of Na(2)CO(3) and NaHCO(3) was inferred from the direct measurement of the transition dipole moment of the interfacial water molecules. It is found that bicarbonate and its counterion sodium do not significantly perturb the interfacial water structure, whereas carbonate strongly orients water so that the water hydrogens point down toward the bulk solution. This is consistent with the picture of carbonate anions residing many layers below the water surface with a preference for the sodium cations to be above the anions and thereby closer to the topmost layer of the water surface.

Surface organization of aqueous MgCl2 and application to atmospheric marine aerosol chemistry

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl2 surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl2 concentration. At low concentrations (< 1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (> 3.1 M). Finally, the MgCl2 concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg2+ to the PA headgroup.

Interactions of Dimethylsulfoxide with a Dipalmitoylphosphatidylcholine Monolayer Studied by Vibrational Sum Frequency Generation

The interactions between phospholipid monolayers and dimethylsulfoxide (DMSO) molecules were investigated by vibrational sum frequency generation (VSFG) spectroscopy in a Langmuir trough system. Both the head and the tail groups of dipalmitoylphosphatidylcholine (DPPC) as well as DMSO were probed to provide a comprehensive understanding of the interactions between DPPC and DMSO molecules. A condensing effect is observed for the DPPC monolayer on a concentrated DMSO subphase (>20 mol %). This effect results in a well-ordered conformation for the DPPC alkyl chains at very large mean molecular areas. Interactions between DMSO and DPPC headgroups were also studied. DMSO-induced dehydration of the DPPC phosphate group is revealed at DMSO concentration above 10 mol %. The average orientation of DMSO with DPPC versus dipalmitoylphosphate sodium salt (DPPA) monolayers was compared. The comparison revealed that DMSO molecules are perturbed and reorient because of the interfacial electric field created by the charged lipid headgroups. The orientation of the DPPC alkyl chains remains nearly unchanged in the liquid condensed phase with the addition of DMSO. This suggests that DMSO molecules are expelled from the condensed monolayer. In addition, implications for the DMSO-induced permeability enhancement of biological membranes from this work are discussed.

Interfacial Molecular Organization at Aqueous Solution Surfaces of Atmospherically Relevant Dimethyl Sulfoxide and Methanesulfonic Acid Using Sum Frequency Spectroscopy and Molecular Dynamics Simulation

The molecular organization at the aqueous dimethyl sulfoxide (DMSO) and methanesulfonic acid (MSA) surfaces was investigated using vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulation. The molecular orientation of surface DMSO and MSA is deduced based on the VSFG spectra of both C-H stretch and S-O stretch regions. The S-O stretch region was studied for the first time and is shown to be critical in molecular orientation determination. On average, the CH(3) groups of DMSO and MSA are preferentially pointing outward into the air. However, the DMSO S═O group points slightly inward away from the surface, while the SO(3) vector of dissociated MSA points nearly straight down. In addition, MD simulations reveal that the orientation distribution of surface DMSO is relatively broad in contrast with a narrow distribution of surface MSA, which agrees with the experiment findings.

Water structure at aqueous solution surfaces of atmospherically relevant dimethyl sulfoxide and methanesulfonic acid revealed by phase-sensitive sum frequency spectroscopy.

Interfacial water structures of aqueous dimethyl sulfoxide (DMSO) and methanesulfonic acid (MSA) were studied by Raman, infrared, and conventional and phase-sensitive vibrational sum frequency generation (VSFG) spectroscopies. Through isotopic dilution, we probed bulk water hydrogen bonding strength using the vibrational frequency of dilute OD in H(2)O. As indicated by the frequency shift of the OD frequency, it is shown that DMSO has little influence on the average water hydrogen bonding strength at low concentrations in contrast with an overall weakening effect for MSA. For the water structure at the surface of aqueous solutions, although conventional VSFG spectra suggest only slight structural changes with DMSO and a red shift of hydrogen-bonded water OH frequency, phase-sensitive VSFG reveals more thoroughly structural changes in the presence of both DMSO and MSA. In the case of DMSO, reorientation of interfacial water molecules with their hydrogens pointing up toward the oxygen of the S=O group is observed. For MSA, the interfacial water structure is affected by both the dissociated methanesulfonate anions and the hydronium ions residing at the surface. Both the methanesulfonate anions and the hydronium ions have surface preference; therefore, the electric double layer (EDL) formed at the surface is relatively thin, which leads to partial reorientation of interface water molecules with net orientation of water hydrogens up. Surface DMSO molecules are more effective at reorienting surface water relative to MSA molecules.