Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two-Dimensional Heterodyne-Detected VSFG Spectroscopy

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular-level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate-ion (TPB⁻)/water and tetraphenylarsonium-ion (TPA⁺)/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of the anionic TPB⁻ interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen-bonded OH groups, which are hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of the cationic TPA⁺ interface only show the presence of usual hydrogen-bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.     

Solvation of Calcium–Phosphate Headgroup Complexes at the DPPC/Aqueous Interface

The effects of sodium (Na⁺) and calcium (Ca²⁺) cations on model zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayers spread on metal chloride salt solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne‐detected (HD)‐VSFG spectroscopy. VSFG and HD‐VSFG spectra in the OH stretching region reveal cation‐specific effects on the interfacial water′s H‐bonding network, knowledge of which has been limited to date. It is found that low‐concentrated Ca²⁺ more strongly perturbs interfacial water organization relative to highly concentrated Na⁺. At higher Ca²⁺ concentrations, the water H‐bonding network at the DPPC/CaCl₂ interface reorganizes and the resulting spectrum closely follows that of the bare air/CaCl₂ interface up to ～3400 cm^?1. Most interesting is the appearance of a negative band at ～3450 cm^?1 in the DPPC/CaCl₂ Im χ_s⁽²⁾ spectra, likely arising from an asymmetric solvation of Ca²⁺–phosphate headgroup complexes. This gives rise to an electric field that orients the net OH transition moments of a subset of OH dipoles toward the bulk solution.     

Femtosecond Hydrogen Bond Dynamics of Bulk‐like and Bound Water at Positively and Negatively Charged Lipid Interfaces Revealed by 2D HD‐VSFG Spectroscopy

Interfacial water in the vicinity of lipids plays an important role in many biological processes, such as drug delivery, ion transportation, and lipid fusion. Hence, molecular‐level elucidation of the properties of water at lipid interfaces is of the utmost importance. We report the two‐dimensional heterodyne‐detected vibrational sum frequency generation (2D HD‐VSFG) study of the OH stretch of HOD at charged lipid interfaces, which shows that the hydrogen bond dynamics of interfacial water differ drastically, depending on the lipids. The data indicate that the spectral diffusion of the OH stretch at a positively charged lipid interface is dominated by the ultrafast (<～100 fs) component, followed by the minor sub‐picosecond slow dynamics, while the dynamics at a negatively charged lipid interface exhibit sub‐picosecond dynamics almost exclusively, implying that fast hydrogen bond fluctuation is prohibited. These results reveal that the ultrafast hydrogen bond dynamics at the positively charged lipid–water interface are attributable to the bulk‐like property of interfacial water, whereas the slow dynamics at the negatively charged lipid interface are due to bound water, which is hydrogen‐bonded to the hydrophilic head group.     

Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D‐HD‐SFG Spectroscopy

Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface—whereas in bulk water the coupling is homogeneous. For strongly hydrogen‐bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near‐surface region. For weakly hydrogen‐bonded OH groups that absorb around 3500 cm^?1, which are assigned to the outermost, yet hydrogen‐bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen‐bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.     

Synthesis,properties and solid state structure of 5‐diphenylphosphino‐2‐hydroxy‐1,3‐xylyl‐18‐crown‐5

5‐Diphenylphosphino‐2‐hydroxy‐1,3‐xylyl‐18‐crown‐5 has been synthesized from 5‐bromo‐2‐hydroxy‐18‐crown‐5 by reacting it in sequence at low temperature with n‐butyl lithium and methyl diphenylphosphonite. The phosphorous donor properties of this phenol phosphine (OH derivative) and the corresponding phenoxide (O^? derivative) have been studied in the presence and absence of alkali metal ions by determining the frequencies of the A₁ ν(CO) bands of Ni(CO)₃L complexes. For the OH and O^? derivatives, the latter generated by addition of CsOH to the former, the ν(CO) bands are observed at 2067.6 and 2063.4 cm^?1, respectively, providing the trend predicted by Hammett parameters for OH and O^? substituents. Addition of Na⁺ or K⁺ to the OH derivative has little effect on this stretching frequency, but the former ion shifts the O^? derivative band to 2067.7 cm^?1 A solid state structure has been obtained of the OH derivative, and two independent molecules were found in the unit cell. Both have a single water molecule hydrogen bonded to two across‐ring oxygen atoms and the phenol hydrogen. The crown ether ring has the usual gauche and anti arrangements for the C‐C and C? O bonds.     

Interaction of water in polymers: Poly(ethylene‐co‐vinyl acetate) and poly(vinyl acetate)

We studied the interaction of water in poly(ethylene‐co‐vinyl acetate) of various vinyl acetate compositions and poly(vinyl acetate), on the basis of the infrared spectrum of the water dissolved therein. The spectrum shows a very sharp and distinct band at about 3690 cm^?1 (named as A), and less‐sharp two bands around 3640 (B) and 3550 cm^?1 (C), the A band being outstanding especially at a low vinyl acetate composition. As the vinyl acetate composition increases, the A band decreases in intensity relative to the C band, whereas the B band increases contrarily. Analysis of the spectral change has elucidated that one‐bonded water (of which one OH is hydrogen‐bonded to the C?O of an ester group and the other OH is free) and two‐bonded water (each OH of which is hydrogen‐bonded to one C?O) coexist in the copolymer and that two‐bonded water increases in relative population with increasing vinyl acetate composition. Dissolved water is entirely two‐bonded in poly(vinyl acetate), in which C?O groups are densely distributed in the matrix. We proved that dissolved water in polymers is hydrogen‐bonded through one or two OH groups to the possessed functional groups but does not cluster. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 777–785, 2005     

Excited‐State Hydrogen Bonding Strengthening and Weakening with Intramolecular Charge Transfer in Resorufin‐Water Complexes: A TD‐DFT Study

The geometric structures, infrared spectra and hydrogen bond binding energies of the various hydrogen‐bonded Res^?‐water complexes in states S0 and S1 have been calculated using the density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods, respectively. Based on the changes of the hydrogen bond lengths and binding energies as well as the spectral shifts of the vibrational mode of the hydroxyl groups, it is demonstrated that hydrogen bonds HB‐II, HB‐III and HB‐IV are strengthened while hydrogen bond HB‐I is weakened in the four singly hydrogen‐bonded Res^?‐Water complexes upon photoexcitation. When the four hydrogen bonds are formed simultaneously between one resorufin anion and four water molecules in the Res^?‐4Water complex, all the hydrogen bonds are weakened in both the ground and excited states compared with those in the corresponding singly hydrogen‐bonded Res^?‐Water complexes. Furthermore, in complex Res^?‐4Water, hydrogen bonds HB‐II and HB‐IV are strengthened while hydrogen bonds HB‐I and HB‐III are weakened after the electronic excitation. The hydrogen bond strengthening and weakening in the various hydrogen‐bonded Res^?‐water complexes should be due to the redistribution of the charges among the four heteroatoms (O_1‐3 and N₁) within the resorufin molecule upon the optical excitation.     

Electrochemical Investigation for Cu2+ Oscillatory Phenomena at the Liquid/Liquid Interface with a Specific Adsorption of Ion Pair Model

In this paper, a novel current oscillatory phenomenon for Cu²⁺ at the water/1,2‐dichloroethane interface is reported with cyclic voltammetry and potential‐step chronoamperometry. The small irregular current spikes were only observed near the site of the oxidation peak of CuCl₂^? and were mainly related to the Cu²⁺concentration in the aqueous phase. Our experimental results demonstrated that the current oscillation is caused by specific adsorption of ion pairs at the W/DCE interface between Cu²⁺ in the aqueous phase and TPB^? in the organic phase. Therefore, a specific adsorption of ion pair model has been formulated for the current oscillation at the liquid/liquid interface. The DFT calculation method was used to explain the mechanism of ion pair formation. The calculation results suggested that the TPB^?Cu²⁺TPB^? ion pair has the lowest‐energy state, thus providing qualitative support for the ion pair model. A probable mechanism for the observed current oscillation was also discussed in this paper. At the same time, a spectrophotometric experiment was performed to evidence a strong attractive interaction between Cu²⁺ and TPB^?.     

Head‐to‐Tail Intermolecular Hydrogen Bonding of OH and NH Groups with Fluoride

To explore the anion‐recognition ability of the phenolic hydroxyl group and the amino hydrogen, we synthesized three different acridinedione (ADD) based anion receptors, 1 , 2 and 3 , having OH, NH, and combination of OH and NH groups, respectively. Absorption, emission and ¹H NMR spectral studies revealed that receptor 1 , having only a phenolic OH group, shows selective deprotonation of the hydroxyl proton towards F^?, which results in an “ON–OFF”‐type signal in the fluorescence spectral studies. Receptor 2 , which only has an amino hydrogen, also shows deprotonation of the amino hydrogen with F^?, whereas receptor 3 (having both OH and NH groups) shows head‐to‐tail intermolecular hydrogen bonding of OH and NH groups with F^? prior to deprotonation. The observation of hydrogen bonding of the OH and NH groups in a combined solution of 1 and 2 with F^? in a head‐to‐tail hetero‐intermolecular fashion, and the absence of head‐to‐head and tail‐to‐tail intermolecular hydrogen bonding in 1 and 2 with F^?, prove that the difference in the acidity of the OH and NH protons leads to the formation of an intermolecular hydrogen‐bonding complex with F^? prior to deprotonation. The presence of this hydrogen‐bonding complex was confirmed by absorption spectroscopy, 3D emission contour studies, and ¹H NMR titration.     

Hybrid Curdlan Poly(γ ‐Glutamic Acid) Nanoassembly for Immune Modulation in Macrophage

A nanoformulation composed of curdlan, a linear polysaccharide of 1,3‐β‐linked d ‐glucose units, hydrogen bonded to poly(γ ‐glutamic acid) (PGA), was developed to stimulate macrophage. Curdlan/PGA nanoparticles (C‐NP) are formulated by physically blending curdlan (0.2 mg mL^?1 in 0.4 m NaOH) with PGA (0.8 mg mL^?1). Forster resonance energy transfer (FRET) analysis demonstrates a heterospecies interpolymer complex formed between curdlan and PGA. The ¹H‐NMR spectra display significant peak broadening as well as downfield chemical shifts of the hydroxyl proton resonances of curdlan, indicating potential intermolecular hydrogen bonding interactions. In addition, the cross peaks in ¹H‐¹H 2D‐NOESY suggest intermolecular associations between the OH‐2/OH‐4 hydroxyl groups of curdlan and the carboxylic‐/amide‐groups of PGA via hydrogen bonding. Intracellular uptake of C‐NP occurs over time in human monocyte‐derived macrophage (MDM). Furthermore, C‐NP nanoparticles dose‐dependently increase gene expression for TNF‐α, IL‐6, and IL‐8 at 24 h in MDM. C‐NP nanoparticles also stimulate the release of IL‐lβ, MCP‐1, TNF‐α, IL‐8, IL‐12p70, IL‐17, IL‐18, and IL‐23 from MDM. Overall, this is the first demonstration of a simplistic nanoformulation formed by hydrogen bonding between curdlan and PGA that modulates cytokine gene expression and release of cytokines from MDM.     

Trimodal Control of Ion‐Transport Activity on Cyclo‐oligo‐(1→6)‐β‐D‐glucosamine‐Based Artificial Ion‐Transport Systems

Cyclo‐oligo‐(1→6)‐β‐D ‐glucosamines functionalized with hydrophobic tails are reported as a new class of transmembrane ion‐transport system. These macrocycles with hydrophilic cavities were introduced as an alternative to cyclodextrins, which are supramolecular systems with hydrophobic cavities. The transport activities of these glycoconjugates were manipulated by altering the oligomericity of the macrocycles, as well as the length and number of attached tails. Hydrophobic tails of 3 different sizes were synthesized and coupled with each glucosamine scaffold through the amide linkage to obtain 18 derivatives. The ion‐transport activity increased from di‐ to tetrameric glucosamine macrocycles, but decreased further when flexible pentameric glucosamine was introduced. The ion‐transport activity also increased with increasing length of attached linkers. For a fixed length of linkers, the transport activity decreased when the number of such tails was reduced. All glycoconjugates displayed a uniform anion‐selectivity sequence: Cl^?>Br^?>I^?. From theoretical studies, hydrogen bonding between the macrocycle backbone and the anion bridged through water molecules was observed.     

Role of tetrapropylammonium cations in gel-phase silicalite synthesis

Silicalite synthesis from tetrapropylammonium (TPA⁺) sodium silicate gels was studied by X-ray diffraction, elemental analysis, ion exchange, ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. Based on this information we confirm a hydrogel—solid transformation mechanism for silicalite crystallization. The initial synthesis gel is a highly articulated silicate network containing pockets of water with solvated Na⁺ and TPA⁺ cations. As the silica condenses and becomes more hydrophobic, water and solvated cations are expelled. The condensed silicate gel then encapsulates the hydrophobic TPA⁺ cations in cages resembling the channel intersections of silicalite before X-ray crystalline silicalite is observed. Crystallization occurs within the gel via rearrangement of the TPA⁺-occluded silicate cages by the breaking and reformation of siloxane bonds into the more stable silicalite structure. Rates of nucleation and crystallization both increase with increasing TPA⁺ gel content. The amount of silicalite which forms is limited by the amount of TPA⁺, which must be present in the ratio of one TPA⁺ per channel intersection.     

Cetyltrimethylammonium Bromide as a Surface Modifier in Paper Separation of 2,4‐D and Silvex Herbicides from Aqueous Solutions

The paper capillary permeation adsorption (PCPA) separation of 2,4‐D and silvex herbicides from water by the addition of cetyltrimethylammonium bromide (CTAB) was studied. The effect of pH, CTAB concentration, and the type of PCPA treatment on separatability has been investigated. A nearly 100% separatability was obtained for each of 2,4‐D and silvex at pH values larger than 7 and 5, respectively. The separatability is greater than that without an addition of CTAB. It was confirmed that 2,4‐D and silvex are adsorbed as molecules on the fiber surface that contains ion pairs CTA⁺COO^? formed by the combination of CTA⁺ cations with the carboxyl groups bonded in the fiber surface.     

Infrared spectroscopic study of the interactions of nylon‐6 with water

We studied the interactions of nylon‐6 with water by following the Fourier transform infrared spectra of a hydrated thin film during dehydration. Very small changes in the spectra caused by the interactions were clearly revealed by the application of spectral subtraction. The water was found to interact with amide groups to form hydrogen bonds with non‐hydrogen‐bonded or free C?O and NH groups in the amorphous portion in the first hydration sphere. This was deduced from an analysis of minus and plus peaks appearing around the absorptions of the NH stretching, amide I band, and amide II bands in the difference spectra between the spectra during dehydration and the one at the most dehydration. The interactions of the amide groups with water were significantly stronger than the hydrogen bond between CO and NH in the crystalline portion, according to the magnitude of the frequency shift of relevant bands. Water, as the interacting counterpart, showed a distorted OH stretching absorption with two close peaks at about 3450 cm^?1. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1722–1729, 2003     

Void‐Assisted Ion‐Paired Proton Transfer at Water–Ionic Liquid Interfaces

At the water–trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P_14,6,6,6][FAP]) ionic liquid interface, the unusual electrochemical transfer behavior of protons (H⁺) and deuterium ions (D⁺) was identified. Alkali metal cations (such as Li⁺, Na⁺, K⁺) did not undergo this transfer. H⁺/D⁺ transfers were assisted by the hydrophobic counter anion of the ionic liquid, [FAP]^?, resulting in the formation of a mixed capacitive layer from the filling of the latent voids within the anisotropic ionic liquid structure. This phenomenon could impact areas such as proton‐coupled electron transfers, fuel cells, and hydrogen storage where ionic liquids are used as aprotic solvents.     

Near infrared analysis of blends of sulfonated polystyrene ionomers and poly(ε‐caprolactam)

Near infrared spectroscopy (NIR) was used to characterize the nature of specific interactions in blends of lightly sulfonated polystyrene ionomers (M‐SPS where M = Zn⁺², Mn⁺², or Li⁺) and polycaprolactam (PA6). The assignments of the NIR overtone bands that arise due to the interactions between the cation of the ionomer, and the amide groups were made using spectra of model compounds. The relative populations of the different environments of the N? H groups were qualitatively determined by deconvoluting the NIR spectra into five absorbances representing hydrogen‐bonded N? H in crystalline and amorphous phases and an ion‐amide complex. The ion‐amide complex was specific for the blends. The interpolymer interactions were sensitive to composition and temperature, but qualitatively the behavior was the same for all three ionomer salts investigated. © 2008 Wiley Periodicals, Inc. JPolym Sci Part B: Polym Phys 46: 1602–1610, 2008     

Effect of Different Solvent Characteristics on the Proton‐Ligand and Sm+3‐Ligand Formation Constant of Butanamide‐3‐[2‐[(2‐Hydroxyphenyl Methylene) Amino]Ethylimino]N‐Phenyl(BHPAP) in Mixed Solvents

Complexes equilibrium of (BHPAP) with proton and Sm⁺³ ion has been measured in various mixed aqueous solvents, viz.; methanol‐water, ethanol‐water, acetone‐water and dioxane‐water. Based on potentiometric equilibrium measurements of hydrogen ion concentration at 30 °C, ionic strength 0.1 M KNO₃ and in the above various mixed solvents, the values of protonation constant of BHPAP‐Sm⁺³ complex have been evaluated. The variation of protonation and stability constants with the inverse of dielectric constant or mole fraction of solvent was studied. Application of Fuoss expression and consideration of electrostatic and non‐electrostatic effects are made to explain the above constants. The solid complexes were isolated for each Pr⁺³ and Nd⁺³‐BHPAP. Elemental analysis, conductance, infrared spectra, and electronic spectra for these solutions and TG, DTG and DTA measurements characterized these solids. The ligand behaves towards the metal ion as a dibasic tetradentate ligand.     

Proton Order–Disorder Phenomena in a Hydrogen‐Bonded Rhodium–η5‐Semiquinone Complex: A Possible Dielectric Response Mechanism

A newly synthesized one‐dimensional (1D) hydrogen‐bonded (H‐bonded) rhodium(II)–η⁵‐semiquinone complex, [Cp*Rh(η⁵‐p‐HSQ‐Me₄)]PF₆ ([ 1 ]PF₆; Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl; HSQ=semiquinone) exhibits a paraelectric–antiferroelectric second‐order phase transition at 237.1 K. Neutron and X‐ray crystal structure analyses reveal that the H‐bonded proton is disordered over two sites in the room‐temperature (RT) phase. The phase transition would arise from this proton disorder together with rotation or libration of the Cp* ring and PF₆^? ion. The relative permittivity ε_b′ along the H‐bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of ¹³C CP/MAS NMR spectra demonstrates that the proton is dynamically disordered in the RT phase and that the proton exchange has already occurred in the low‐temperature (LT) phase. Rate constants for the proton exchange are estimated to be 10^?4–10^?6 s in the temperature range of 240–270 K. DFT calculations predict that the protonation/deprotonation of [ 1 ]⁺ leads to interesting hapticity changes of the semiquinone ligand accompanied by reduction/oxidation by the π‐bonded rhodium fragment, producing the stable η⁶‐hydroquinone complex, [Cp*Rh³⁺(η⁶‐p‐H₂Q‐Me₄)]²⁺ ([ 2 ]²⁺), and η⁴‐benzoquinone complex, [Cp*Rh⁺(η⁴‐p‐BQ‐Me₄)] ([ 3 ]), respectively. Possible mechanisms leading to the dielectric response are discussed on the basis of the migration of the protonic solitons comprising of [ 2 ]²⁺ and [ 3 ], which would be generated in the H‐bonded chain.     

Hydro­gen bonding in 1‐carbamoyl­guanidinium methyl­phospho­nate monohydrate

The title compound, C₂H₇N₄O⁺·CH₄O₃P⁻·H₂O, crystallized with one carbamoyl­guanidinium cation, one methyl­phos­phonate anion and one water mol­ecule in the asymmetric unit. All H atoms of the carbamoyl­guanidinium ion are involved in a hydrogen‐bonded network. The CH₃PO₂(OH) anions, together with the water mol­ecules, build O—H⋯O hydrogen‐bonded ribbons around a 2₁ screw axis parallel to the b axis. Neighbouring ribbons are not directly connected via hydrogen bonding. The carbamoyl­guanidinium cations are linked to these ribbons by N—H⋯O bridges and build a slightly buckled layer structure, the interlayer distance being b/2.     

Elucidation of artefacts in proton transfer reaction time‐of‐flight mass spectrometers

We present an effective procedure to differentiate instrumental artefacts, such as parasitic ions, memory effects, and real trace impurities contained in inert gases. Three different proton transfer reaction mass spectrometers were used in order to identify instrument‐specific parasitic ions. The methodology reveals new nitrogen‐ and metal‐containing ions that up to date have not been reported. The parasitic ion signal was dominated by [N₂]H⁺ and [NH₃]H⁺ rather than by the common ions NO⁺ and O₂⁺. Under dry conditions in a proton transfer reaction quadrupole interface time‐of‐flight mass spectrometer (PTR‐QiTOF), the ion abundances of [N₂]H⁺ were elevated compared with the signals in the presence of humidity. In contrast, the [NH₃]H⁺ ion did not show a clear humidity dependency. On the other hand, two PTR‐TOF1000 instruments showed no significant contribution of the [N₂]H⁺ ion, which supports the idea of [N₂]H⁺ formation in the quadrupole interface of the PTR‐QiTOF. Many new nitrogen‐containing ions were identified, and three different reaction sequences showing a similar reaction mechanism were established. Additionally, several metal‐containing ions, their oxides, and hydroxides were formed in the three PTR instruments. However, their relative ion abundancies were below 0.03% in all cases. Within the series of metal‐containing ions, the highest contribution under dry conditions was assigned to the [Fe(OH)₂]H⁺ ion. Only in one PTR‐TOF1000 the Fe⁺ ion appeared as dominant species compared with the [Fe(OH)₂]H⁺ ion. The present analysis and the resulting database can be used as a tool for the elucidation of artefacts in mass spectra and, especially in cases, where dilution with inert gases play a significant role, preventing misinterpretations.