Hyper-Rayleigh Scattering as a New Chiroptical Method: Uncovering the Nonlinear Optical Activity of Aromatic Oligoamide Foldamers.

Molecular helices based on self-organized aromatic oligoamide foldamers have been designed and prepared in their two enantiomeric forms in order to probe their second-order nonlinear chiroptical properties in solution. The quinoline oligoamides were rationally functionalized by electron-donating and electron-withdrawing groups to afford a gradual increase of the electronic polarization of the helical architectures. Their hyper-Rayleigh scattering (HRS) responses in solution were accordingly assessed, using either linearly polarized or circularly polarized incident light. Both methods allowed us to observe nonlinear optical activity that was quantified, for the first time for molecular systems, through circular differential scattering intensity ratios. The hyper-Rayleigh optical activity study reveals important charge-transfer differences within the aromatic oligomers, depending on the helix handedness and on the extent of electronic polarization induced by the appended substituents. The origin of the enantiomeric difference is discussed considering both achiral and chiral contributions. Overall, using aromatic oligoamide foldamers as a chiral model, we demonstrate the capabilities of HRS as a complementary chiroptical method, ideally suited for the analysis of various chiral molecular and supramolecular systems in solution. The reliability and chiral discrimination sensitivity of the method can be further improved through dynamic measurements using standard polarization modulation and heterodyning techniques.

Ice-binding site of surface-bound type III antifreeze protein partially decoupled from water.

Type III antifreeze proteins (AFP III) have been widely recognized as one class of ice-binding proteins produced by several biological organisms to withstand freezing conditions. Besides their ability to restrict ice growth through their ice-binding site (IBS), AFP III have also been shown to possess a great propensity for hydrophobic surfaces such as the air-water interface. Yet, it is not known whether AFP III adsorb with a specific orientation and how hydrophobic interactions affect the IBS. Molecular insights on the accessibility of the IBS and its interactions with water are important for understanding AFP III action in vivo but also for their application as ice-inhibiting agents for deicing, frozen food storage, as well as for long-term blood and organ cryo-preservation. Here, the orientation of fish AFP III adsorbed at the air-water interface has been studied using a combination of molecular dynamics (MD) simulations and vibrational sum-frequency generation (SFG) spectroscopy together with spectral calculations. The SFG/MD analysis indicated that when AFP III adsorbs at the air-water interface, it mostly retains its native state and orients with a tilt angle of 120° with respect to the surface normal. We found that the IBS is only partially solvated, leaving the pyramidal ice plane binding domain exposed to the vapor phase. These findings suggest that interactions with hydrophobic interfaces (e.g., cell membranes, polymers) could lead to the partial decoupling of the IBS from water and, to some extent, to a loss of AFP III antifreezing activity.

Effect of pH and Salt on Surface pKa of Phosphatidic Acid Monolayers.

The pH-induced surface speciation of organic surfactants such as fatty acids and phospholipids in monolayers and coatings is considered to be an important factor controlling their interfacial organization and properties. Yet, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface pKa) of the protic functional group(s) present. Here, we use three independent methods-compression isotherms, surface tension pH titration, and infrared reflection-absorption spectroscopy (IRRAS)-to study the protonation state of dipalmitoylphosphatidic acid (DPPA) monolayers on water and NaCl solutions. By examining the molecular area expansion at basic pH, the pKa to remove the second proton of DPPA (surface pKa2) at the aqueous interface is estimated. In addition, utilizing IRRAS combined with density functional theory calculations, the vibrational modes of the phosphate headgroup were directly probed and assigned to understand DPPA charge speciation with increasing pH. We find that all three experimental techniques give consistent surface pKa2 values in good agreement with each other. Results show that a condensed DPPA monolayer has a surface pKa2 of 11.5, a value higher than previously reported (∼7.9-8.5). This surface pKa2 was further altered by the presence of Na+ cations in the aqueous subphase, which reduced the surface pKa2 from 11.5 to 10.5. It was also found that the surface pKa2 value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface pKa2 as low as 9.2 for DPPA monolayers in the two-dimensional gaseous phase over NaCl solutions. The experimentally determined surface pKa2 values are also found to be in agreement with those predicted by Gouy-Chapman theory, validating these methods and proving that surface charge density is the driving factor behind changes to the surface pKa2.

Solvent-Shared Ion Pairs at the Air-Solution Interface of Magnesium Chloride and Sulfate Solutions Revealed by Sum Frequency Spectroscopy and Molecular Dynamics Simulations.

The ion distribution and ion pairing properties of Mg2+, SO42-, NO3-, and Cl- in the interfacial region of MgSO4, Mg(NO3)2, and MgCl2 solutions were investigated using vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulations. An electric field reversal relative to Mg(NO3)2 and MgCl2 solutions is observed at the interface of a MgSO4 solution. We show that, although magnesium cations are expected to have preference for bulk solvation, solvent-shared ion pairs (SIPs) exist in the interfacial region in which Mg2+ cations are closer to the solution surface than sulfate anions. While interfacial SIPs are few, they dominate the electric field effect observed. Thus, SIPs play a significant role in determining the electric field direction and magnitude at the air-aqueous interface. In addition to impact on the fundamental understanding of aqueous surfaces and interfacial ion-ion interactions, these findings have implications for atmospheric aerosol chemistry and thundercloud electrification.

Surface organization of a DPPC monolayer on concentrated SrCl2 and ZnCl2 solutions.

Transition metals are known to be enriched in organic-coated marine aerosols, but the impact these cations have on their surface properties is not well understood. Here the effect of Zn2+ enrichment on the surface properties of a dipalmitoylphosphatidylcholine (DPPC) monolayer was investigated and compared to that of the alkaline earth metal Sr2+, an ion not enriched in aerosols. Phase behavior of the DPPC film on concentrated aqueous solutions was probed with surface pressure-area isotherms while domain morphology was monitored with Brewster angle microscopy (BAM). Infrared reflection-absorption spectroscopy (IRRAS) and vibrational sum frequency generation (VSFG) spectroscopy were used to assess the impact of cations on the conformation and orientation of alkyl chains as well as the hydration state of the carbonyl and phosphatidylcholine (PC) moieties. Results of compression isotherms and BAM show that Zn2+ strongly interacts with DPPC molecules, and induces condensation of the monolayer while Sr2+ only weakly interacts with the monolayer in expanded phases. Conformational order and orientation of alkyl chains in the condensed phase are not significantly altered by either cation. IRRAS indicates that Sr2+ has weak interactions with the PC headgroup. Zn2+ ions cause dehydration of carbonyl groups and binds to the phosphate group in a 2 : 1 bridging complex. Findings here suggest that Sr2+ is not enriched in aerosols because it behaves similar to a monovalent ion and only weakly interacts with the monolayer, while enrichment of Zn2+ is due to strong binding to the lipid film.

Surface Potential of DPPC Monolayers on Concentrated Aqueous Salt Solutions.

The presence and exchange of electrical charges on the surfaces of marine aerosols influence their ability to act as cloud condensation nuclei and play a role in thundercloud electrification. Although interactions exist between surface-active inorganic ions and organic compounds, their role in surface charging of marine aerosols is not well understood. In this study, the surface potential of dipalmitoylphosphatidylcholine (DPPC) monolayers, a zwitterionic phospholipid found in the sea surface microlayer, is measured on concentrated (0.3-2.0 M) chloride salt solutions containing marine-relevant cations (Na(+), K(+), Mg(2+), Ca(2+)) to model and elucidate the electrical properties of organic-covered marine aerosols. Monovalent cations show only a weak effect on the surface potential of DPPC monolayers in the condensed phase compared to water. In contrast, Mg(2+) and Ca(2+) increase the surface potential, indicating different cation binding modes and affinities for the PC headgroup. Moreover, it is found that for divalent chloride salt solutions, the PC headgroup and interfacial water molecules make the largest dipolar contribution to the surface potential. This study shows that for equal charge concentrations, divalent cations impact surface potential of DPPC monolayers more strongly than monovalents likely through changes in the PC headgroup orientation induced by their complexation along with the lesser ordering of interfacial water molecules caused by phosphate group charge screening.

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

The effects of sodium (Na(+) ) and calcium (Ca(2+) ) 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 Ca(2+) more strongly perturbs interfacial water organization relative to highly concentrated Na(+) . At higher Ca(2+) 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 χs ((2)) spectra, likely arising from an asymmetric solvation of Ca(2+) -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.

Relative Order of Sulfuric Acid, Bisulfate, Hydronium, and Cations at the Air-Water Interface.

Sulfuric acid (H2SO4), bisulfate (HSO4(-)), and sulfate (SO4(2-)) are among the most abundant species in tropospheric and stratospheric aerosols due to high levels of atmospheric SO2 emitted from biomass burning and volcanic eruptions. The air/aqueous interfaces of sulfuric acid and bisulfate solutions play key roles in heterogeneous reactions, acid rain, radiative balance, and polar stratospheric cloud nucleation. Molecular-level knowledge about the interfacial distribution of these inorganic species and their perturbation of water organization facilitates a better understanding of the reactivity and growth of atmospheric aerosols and of the aerosol surface charge, thus shedding light on topics of air pollution, climate change, and thundercloud electrification. Here, the air/aqueous interface of NaHSO4, NH4HSO4, and Mg(HSO4)2 salt solutions as well as H2SO4 and HCl acid solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne-detected (HD) VSFG spectroscopy. VSFG spectra of all acid solutions show higher SFG response in the OH-bonded region relative to neat water, with 1.1 M H2SO4 being more enhanced than 1.1 M HCl. In addition, VSFG spectra of bisulfate salt solutions highly resemble that of the dilute H2SO4 solution (0.26 M) at a comparable pH. HD-VSFG (Im χ((2))) spectra of acid and bisulfate salt solutions further reveal that hydrogen-bonded water molecules are oriented preferentially toward the bulk liquid phase. General agreement between Im χ((2)) spectra of 1.1 M H2SO4 and 1.1 M HCl acid solutions indicate that HSO4(-) ions have a similar surface preference as that of chloride (Cl(-)) ions. By comparing the direction and magnitude of the electric fields arising from the interfacial ion distributions and the concentration of each species, the most reasonable relative surface preference that can be deduced from a simplified model follows the order H3O(+) > HSO4(-) > Na(+), NH4(+), Mg(2+) > SO4(2-). Interestingly, contrary to some other near-neutral salt solution interfaces (e.g., chlorides and nitrates), cation-specific effects are here overshadowed by hydronium ions.

Reduced Condensing and Ordering Effects by 7-Ketocholesterol and 5ß,6ß-Epoxycholesterol on DPPC Monolayers.

The exposure of organic-coated marine aerosols containing cholesterol (Chol) to radiation and/or an oxidizing atmosphere results in the formation of oxidized derivatives or oxysterols and will likely change aerosol surface properties. However, the intermolecular interactions between oxysterols and other lipid components and their influence on the surface properties of marine aerosols are not well-known. To address this question, the interfacial behavior and domain morphology of model Langmuir monolayers of two ring-substituted oxysterols, 7-ketocholesterol (7-KChol) and 5ß,6ß-epoxycholesterol (5,6ß-EChol), mixed with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were investigated by means of compression isotherms and Brewster angle microscopy (BAM) over a broad range of surface pressures and sterol molar ratios. Mixed DPPC/cholesterol (Chol) monolayers were also measured for comparison. The results of compression experiments showed that the condensing effect induced on mixed DPPC/sterol monolayers at low surface pressures and for intermediate molar ratios (0.3 ≤ X(sterol) ≤ 0.7) was weaker for oxysterols than for Chol. Additionally, mixed DPPC/oxysterol monolayers exhibited markedly smaller (∼2-3-fold) interfacial rigidity. Examination of the excess free energy of mixing further revealed that DPPC monolayers containing 7-KChol and Chol were thermodynamically more stable at high surface pressures than those with 5,6ß-EChol, indicating that the strength of interactions between DPPC and 5,6ß-EChol was the smallest. Finally, BAM images in the LE-LC phase of DPPC revealed that in comparison to Chol the addition of small amounts of oxysterols results in larger and less numerous domains, showing that oxysterols are not as effective in fluidizing the condensed phase of DPPC. Taken together, these results suggest that the strength of van der Waals interactions of DPPC alkyl chains with sterols follows the sterol hydrophobicity, with Chol being the most hydrophobic and oxysterols more hydrophilic due to their ketone and epoxy moieties. The difference in the condensing ability and stability of 7-KChol and 5,6ß-EChol on DPPC likely originates from the distinct molecular structure and position of oxidation on the steroid nucleus. As suggested by recent MD simulations, depending on the oxidation position, ring-substituted oxysterols have a broader angular distribution of orientation than Chol in bilayers, which could be responsible for the observed reduction in condensing ability.

Extracting Infrared Spectra of Protein Secondary Structures Using a Library of Protein Spectra and the Ramachandran Plot.

Infrared (IR) spectra from 1200 to 1800 cm(-1) of the pure α-helix and ß-sheet secondary structures have been extracted using a covariant least-squares procedure which relates a library of 40 infrared (IR) solution protein spectra from the work of Dong, Carpenter, and Caughey and amino acid fractions of the proteins based on assignments by STRIDE (secondary structure identification) of Eisenhaber and Argos. The excitonic splitting of the ß-sheet structures is determined for this library of solution proteins. The method is extended to find a set of spectral basis functions that analyze IR spectra of protein samples for α-helix and ß-sheet content. A rigorous error analysis including covariance, the correlations between the input library spectra, was used to justify the results and avoid less meaningful results. The utility of the results on α-helix and ß-sheet regions is demonstrated by detecting protein changes due to cancer in imaging Fourier transform IR (FTIR) spectra of liver tissue slices. This work ends with a method to extract IR spectra of less prominent torsional angle distributions.

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.

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.

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.

Sulfate adsorption at the buried hematite/solution interface investigated using total internal reflection (TIR)-Raman spectroscopy.

Sulfate adsorption at buried mineral/solution interfaces is of great interest in geochemistry and atmospheric aerosol chemistry due to the sulfate anion's environmental ubiquity and the wide role of physical and chemical phenomena that it impacts. Here we present the first application of total internal reflection-Raman (TIR-Raman) spectroscopy, a surface sensitive spectroscopy, to probe sulfate ion behavior at the buried hematite/solution interface. Hematite is the most thermodynamically stable iron oxide polymorph and as such is widely found in nature. Our results demonstrate the feasibility of a TIR-Raman approach to study simple, inorganic anion adsorption at buried interfaces. Moreover, our data suggest that inner-sphere sulfate adsorption proceeds in a bidentate fashion at the hematite surface. These results help clarify long-standing questions as to whether sulfate forms inner-sphere adsorption complexes at hematite surfaces in a mono- or bidentate fashion based on attenuated total reflection-infrared (ATR-IR) observations. Our results are discussed with perspective to this debate and the applicability of TIR-Raman spectroscopy to address ambiguities of ion adsorption to mineral surfaces.

Surface Prevalence of Perchlorate Anions at the Air/Aqueous Interface.

Air/aqueous interfaces provide a unique environment for many chemical, environmental, and biological processes. To gain insight, molecular-level understanding of the interfacial water organization and ion distributions at these interfaces is required. Here, the air/aqueous interface of NaClO4 salt solutions was investigated by means of conventional and heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. It is found that perchlorate (ClO4(-)) ions exist in the interfacial region and prefer to reside on average above their counterions. This finding is inferred from the average orientation of the OH transition dipole moment of interfacial water molecules governed by the direction of the net electric field arising from the interfacial ion distributions. At the air/aqueous interface of NaClO4 salt solutions, the net dipole moments of hydrogen-bonded water molecules are oriented preferentially toward the vapor phase. Contrary to some other salts (e.g., sulfates), the presence of ClO4(-) may cause a full reversal in the direction of the interfacial electric field at a higher concentration (≥1.7 M). Another interpretation for the positive Im χ((2)) spectra of NaClO4 salt solutions could be an increase in the population of water species contributing positively to the net OH transition dipole moment. Regardless of the mechanism, this effect becomes even more pronounced with increasing salt concentration.

Nonfouling poly(ethylene oxide) layers end-tethered to polydopamine.

Nonfouling surfaces capable of reducing protein adsorption are highly desirable in a wide range of applications. Coating of surfaces with poly(ethylene oxide) (PEO), a water-soluble, nontoxic, and nonimmunogenic polymer, is most frequently used to reduce nonspecific protein adsorption. Here we show how to prepare dense PEO brushes on virtually any substrate by tethering PEO to polydopamine (PDA)-modified surfaces. The chain lengths of hetero-bifunctional PEOs were varied in the range of 45-500 oxyethylene units (M(n) = 2000-20,000). End-tethering of PEO chains was performed through amine and thiol headgroups from reactive polymer melts to minimize excluded volume effects. Surface plasmon resonance (SPR) was applied to investigate the adsorption of model protein solutions and complex biologic medium (human blood plasma) to the densely packed PEO brushes. The level of protein adsorption of human serum albumin and fibrinogen solutions was below the detection limit of the SPR measurements for all PEO chains end-tethered to PDA, thus exceeding the protein resistance of PEO layers tethered directly on gold. It was found that the surface resistance to adsorption of lysozyme and human blood plasma increased with increasing length and brush character of the PEO chains end-tethered to PDA with a similar or better resistance in comparison to PEO layers on gold. Furthermore, the chain density, thickness, swelling, and conformation of PEO layers were determined using spectroscopic ellipsometry (SE), dynamic water contact angle (DCA) measurements, infrared reflection-absorption spectroscopy (IRRAS), and vibrational sum-frequency-generation (VSFG) spectroscopy, the latter in air and water.

From Conventional to Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy: Probing Water Organization at Aqueous Interfaces.

Elucidation of water organization at aqueous interfaces has remained a challenging problem. Conventional vibrational sum frequency generation (VSFG) spectroscopy and its most recent extension, phase-sensitive VSFG (PS-VSFG), have emerged as powerful experimental methods for unraveling structural information at various aqueous interfaces. In this Perspective, we briefly describe the two possible VSFG detection modes, and we point out features that make these methods highly suited to address questions about water organization at air/aqueous interfaces. Several important aqueous interfacial systems are discussed to illustrate the versatility of these methods. Remaining challenges and exciting prospective directions are also presented.

Sample cells for probing solid/liquid interfaces with broadband sum-frequency-generation spectroscopy.

Two sample cells designed specifically for sum-frequency-generation (SFG) measurements at the solid/liquid interface were developed: one thin-layer analysis cell allowing measurement of films on reflective metallic surfaces through a micrometer layer of solution and one spectroelectrochemical cell allowing investigation of processes at the indium tin oxide/solution interface. Both sample cells are described in detail and data illustrating the capabilities of each are shown. To further improve measurements at solid/liquid interfaces, the broadband SFG system was modified to include a reference beam which can be measured simultaneously with the sample signal, permitting background correction of SFG spectra in real time. Sensitivity tests of this system yielded a signal-to-noise ratio of 100 at a surface coverage of 0.2 molecules/nm(2). Details on data analysis routines, pulse shaping methods of the visible beam, as well as the design of a purging chamber and sample stage setup are presented. These descriptions will be useful to those planning to set up a SFG spectrometer or seeking to optimize their own SFG systems for measurements of solid/liquid interfaces.