Zishuai Huang

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.

Binding of Mg2+ and Ca2+ to Palmitic Acid and Deprotonation of the COOH Headgroup Studied by Vibrational Sum Frequency Generation Spectroscopy

At the air/liquid interface, cation binding specificity of alkaline earth cations, Mg(2+) and Ca(2+), with the biologically relevant ligand carboxylate (COO(-)) using vibrational sum frequency generation spectroscopy is reported. The empirical evidence strongly supports that the ionic binding strength is much stronger for Ca(2+) to COO(-) than that for Mg(2+). We conclude that at a near-neutral pH, the mechanism that governs Ca(2+) binding to COO(-) is accompanied by commensurate deprotonation of the carboxyl headgroup. In addition, surface molecular structure and ion concentration influence the cation binding behavior at the air/liquid interface. In a 0.1 M Ca(2+)(aq) solution, Ca(2+) initially favors forming ionic complexes in a 2:1 bridging configuration (2Ca(2+):1COO(-)) but 1:1 chelating bidentate complexes (1Ca(2+):1COO(-)) gradually emerge as secondary species as the system reaches equilibrium. As the Ca(2+) concentration rises to 0.3 M, the primary complexed species exists in the 2:1 bridging configuration. Unlike Ca(2+), Mg(2+) at 0.1 and 0.3 M favors a solvent-separated ionic complex with COO(-).

Interfacial water structure and effects of Mg2+ and Ca2+ binding to the COOH headgroup of a palmitic acid monolayer studied by sum frequency spectroscopy.

The interfacial hydrogen-bonding network that uniquely exists in between a palmitic acid (PA) monolayer and the underneath surface water molecules was studied using vibrational sum frequency generation (VSFG) spectroscopy. Perturbations due to cation binding of Mg(2+) and Ca(2+) were identified. The polar ordering of the interfacial water molecules under the influence of the surface field of dissociated PA headgroups was observed. Only a fraction of PA molecules are deprotonated at the air/water interface with a neat water subphase, yet the submonolayer concentration of negatively charged PA headgroups induces considerable polar ordering on the interfacial water molecules relative to the neat water surface without the PA film. Upon addition of calcium and magnesium chloride salts to the subphase of the PA monolayer, the extent of polar ordering of the interfacial water molecules was reduced. Ca(2+) was observed to have the greater impact on the interfacial hydrogen-bonding network relative to Mg(2+), consistent with the greater binding affinity of Ca(2+) toward the carboxylate group relative to Mg(2+) and thereby modifying the interfacial charge. At high-salt concentrations, the already disrupted hydrogen-bonding network reorganizes and reverts to its original hydrogen-bonding structure as that which appeared at the neat salt solution surface without a PA monolayer.

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/.

Cation effects on interfacial water organization of aqueous chloride solutions. I. Monovalent cations: Li+, Na+, K+, and NH4(+).

The influence of monovalent cations on the interfacial water organization of alkali (LiCl, NaCl, and KCl) and ammonium chloride (NH4Cl) salt solutions was investigated using surface-sensitive conventional vibrational sum frequency generation (VSFG) and heterodyne-detected (HD-)VSFG spectroscopy. It was found in the conventional VSFG spectra that LiCl and NH4Cl significantly perturb water’s hydrogen-bonding network. In contrast, NaCl and KCl had little effect on the interfacial water structure and exhibited weak concentration dependency. The Im χs(2)(ωIR) spectra from HD-VSFG further revealed that, for all chloride solutions, the net transition dipole moments of hydrogen-bonded water molecules (O → H) are oriented more toward the vapor phase relative to neat water. This suggests the presence of an interfacial electric field generated from the formation of an ionic double layer in the interfacial region with a distribution of Cl(-) ions located above the countercations, in agreement with predictions from MD simulations. The magnitude of this electric field shows a small but definite cation specificity and follows the order Li(+) ≈ Na(+) > NH4(+) > K(+). The observed trend was found to be in good agreement with previously published surface potential data.

Salty Glycerol versus Salty Water Surface Organization: Bromide and Iodide Surface Propensities

Salty NaBr and NaI glycerol solution interfaces are examined in the OH stretching region using broadband vibrational sum frequency generation (VSFG) spectroscopy. Raman and infrared (IR) spectroscopy are used to further understand the VSFG spectroscopic signature. The VSFG spectra of salty glycerol solutions reveal that bromide and iodide anions perturb the interfacial glycerol organization in a manner similar as that found in aqueous halide salt solutions, thus confirming the presence of bromide and iodide anions at the glycerol surface. Surface tension measurements are consistent with the surface propensity suggested by the VSFG data and also show that the surface excess increases with increasing salt concentration, similar to that of water. In addition, iodide is shown to have more surface prevalence than bromide, as has also been determined from aqueous solutions. These results suggest that glycerol behaves similarly to water with respect to surface activity and solvation of halide anions at its air/liquid interface.

Influence of salt purity on Na+ and palmitic acid interactions.

The influence of salt purity on the interactions between Na(+) ions and the carboxylate (COO(-)) head group of palmitic acid (PA) monolayers is studied in the COO(-) and OH stretching regions using broad-band vibrational sum frequency generation (VSFG) spectroscopy. Ultrapure (UP) and ACS grade NaCl salts are used for aqueous solution preparation after proper pretreatment. The time evolution of VSFG spectra of PA monolayers on solutions made from these two grades of salts is different, which reveals that the salt purity has a significant impact on the interactions between Na(+) ions and the COO(-) group of PA. The trace metal impurities in ACS grade salt, which are more abundant than those in UP grade salt, are responsible for this difference due to their stronger affinity for the carboxylate group relative to Na(+) and further affects the interfacial water structure. These results suggest that the alkali salt grade even after pretreatment is critical in the studies of alkali cation-carboxylate interactions and comparison of relative binding affinities of different cations.