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Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain a matter of debate. Here, we examine the nature of anion–water interactions at the air/water interface using a combination of molecular dynamics simulations and quantum-mechanical energy decomposition analysis based on symmetry-adapted perturbation theory. Results are presented for a set of monovalent anions, including Cl−, Br−, I−, CN−, OCN−, SCN−, NO2−, NO3−, and ClOn− (n=1,2,3,4), several of which are archetypal examples of surface-active species. In all cases, we find that average anion–water interaction energies are systematically larger in bulk water although the difference (with respect to the same quantity computed in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br−(aq), I−(aq), ClO4−(aq), and SCN−(aq), and also for ClO−(aq), the charge-transfer (CT) energy is found to be larger at the interface than it is in bulk water, by an amount that is greater than the standard deviation of the fluctuations. The Cl−(aq) ion has a slightly larger CT energy at the interface, but NO3−(aq) does not, these two species are borderline cases where consensus is lacking regarding their surface activity. However, CT stabilization amounts to &lt, 20% of the total induction energy for each of the ions considered here, and CT-free polarization energies are systematically larger in bulk water in all cases. As such, the role of these effects in the surface activity of soft anions remains unclear. This analysis complements our recent work suggesting that the short-range solvation structure around these ions is scarcely different at the air/water interface from what it is in bulk water. Together, these observations suggest that changes in first-shell hydration structure around soft anions cannot explain observed surface activities.

Despite significant advances in all aspects of single particle cryo-electron microscopy (cryo-EM), specimen preparation still remains a challenge. During sample preparation, macromolecules interact with the air-water interface, which often leads to detrimental effects such as denaturation or adoption of preferred orientations, ultimately hindering structure determination. Randomly biotinylating the protein of interest (for example, at its primary amines) and then tethering it to a cryo-EM grid coated with two-dimensional crystals of streptavidin (acting as an affinity surface) can prevent the protein from interacting with the air-water interface. Recently, this approach was successfully used to solve a high-resolution structure of a test sample, a bacterial ribosome. However, whether this method can be used for samples where interaction with the air-water interface has been shown to be problematic remains to be determined. Here we report a 3.1 A structure of an RNA polymerase II elongation complex stalled at a cyclobutane pyrimidine dimer lesion (Pol II EC(CPD)) solved using streptavidin grids. Our previous attempt to solve this structure using conventional sample preparation methods resulted in a poor quality cryo-EM map due to Pol II EC(CPD)’s adopting a strong preferred orientation. Imaging the same sample on streptavidin grids improved the angular distribution of its view, resulting in a high-resolution structure. This work shows that streptavidin affinity grids can be used to address known challenges posed by the interaction with the air-water interface.

The formulation of a biodegradable carrier which effectively concentrates microorganisms on air-water interfaces is proposed. This avoids the dispersion of bacteria into the bulk liquid phase and at the same time prevents their sedimentation. This formulation can be used in biocontrol and bioremediation treatments where the target is at the position of the air-water interface, as in the case of the treatment of rice diseases caused by Sclerotium oryzae and Rhizoctonia complex. The carrier is an oil-in-water (O/W) emulsion which contains lecithin and chitosan in both phases at different proportions. In a stable formulation, bacteria that are adsorbed onto the surface of oil droplets are carried with them and flowed upward to the air-water interface, due to buoyancy forces. Fil: Morelli, Matias Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Ibañez, Manuel Vicente. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Leonardi, Rodrigo Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Santiago, Liliana G.. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Irazoqui, Horacio Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Heinrich, Josué Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina

Recently, experimental and theoretical works have reported evidence indicating that photochemical processes may significantly be accelerated at heterogeneous interfaces, although a complete understanding of the phenomenon is still lacking. We have carried out a theoretical study of interface and surface effects on the photochemistry of hydrogen peroxide (H2O2) using high-level ab initio methods and a variety of models. Hydrogen peroxide is an important oxidant that decomposes in the presence of light, forming two OH radicals. This elementary photochemical process has broad interest and is used in many practical applications. Our calculations show that it can drastically be affected by heterogeneous interfaces. Thus, compared to gas phase, the photochemistry of H2O2 appears to be slowed on the surface of apolar or low-polar surfaces and, in contrast, hugely accelerated on ionic surfaces or the surface of aqueous electrolytes. We give particular attention to the case of the neat air–water interface. The calculated photolysis rate is similar to the gas phase, which stems from the compensation of two opposite effects, the blue shift of the n→σ* absorption band and the increase of the absorption intensity. Nevertheless, due to the high affinity of H2O2 for the air–water interface, the predicted OH production rate is up to five to six orders of magnitude larger. Overall, our results show that the photochemistry of H2O2 in heterogeneous environments is greatly modulated by the nature of the surface, and this finding opens interesting new perspectives for technological and biomedical applications, and possibly in various atmospheres.

Understanding the air-water and oil-water interfacial behavior of plant proteins is crucial for developing stable emulsions and foams in food systems. Plant crops are often processed into protein extracts with high purity, which primarily consist of globulins. These globulins are often unable to form stiff interfacial layers owing to their compact and highly aggregated state and have inferior functionality compared with animal-derived proteins from milk or eggs. Much of the current focus is on modifying these proteins, whereas better interface stabilizing functionality can also be obtained by choosing more targeted protein extraction methods. This review will highlight the benefits and drawbacks of current and novel protein sources and protein extraction methods with respect to interfacial properties.

The phenomenon of amyloid polymorphism is a key feature of protein aggregation. Unravelling this phenomenon is of great significance for understanding the underlying molecular mechanisms associated with neurodegenerative diseases and for the development of amyloid-based functional biomaterials. However, the understanding of the molecular origins and the physicochemical factors modulating amyloid polymorphs remains challenging. Herein, we demonstrate an association between amyloid polymorphism and environmental stress in solution, induced by an air/water interface in motion. Our results reveal that low-stress environments produce heterogeneous amyloid polymorphs, including twisted, helical, and rod-like fibrils, whereas high-stress conditions generate only homogeneous rod-like fibrils. Moreover, high environmental stress converts twisted fibrils into rod-like fibrils both in-pathway and after the completion of mature amyloid formation. These results enrich our understanding of the environmental origin of polymorphism of pathological amyloids and shed light on the potential of environmentally controlled fabrication of homogeneous amyloid biomaterials for biotechnological applications.

Fabrication of anisotropic material is one of the important topics and we examined to introduce “anisotropic” nature by spreading polymer-grafted particle on the medium with polymer-reactive potential. Poly (tert-butyl methacrylate) (PtBMA) was polymerized from polystyrene latex (PSL) surface by ATRP to give PtBMA-grafted PSL (PSL-PtBMA). Particle monolayer was formed at air-water and air-acidic water interfaces and the monolayer characteristics were compared by π-A isotherm measurements, SEM observations, and contact angle measurements. π-A isotherms, in particular, indicates that the interaction between polymer chains become stronger by changing the subphase condition, which suggests that anisotropicparticle monolayer formation.

ispartof: ADVANCED MATERIALS INTERFACES vol:9 issue:1 status: published

Poly(N-isopropylacrylamide) (pNIPAM) microgels are soft and deformable particles, which can adsorb at liquid interfaces. In the present paper, we study the two-dimensional organization of charged and quasi-neutral microgels with different cross-linking densities, under compression at the air-water interface and the transfer of the microgel monolayer onto a solid substrate at different surface pressures. At low cross-linking densities, the microgels form highly ordered hexagonal lattices on the solid substrate over large areas, with a unique lattice parameter that decreases continuously as the surface pressure increases. We thus prove that the microgel conformation evolves at the air-water interface. The microgels undergo a continuous transition from a highly flattened state at low surface coverage, where the maximal polymer segments are adsorbed at the interface, to entangled flattened microgels, and finally the thickening of the layer up to a dense hydrogel layer of compacted microgels. Moreover, two batches of microgels, with and without charges, are compared. The contribution of electrostatic interactions is assessed via changing the charge density of the microgels or modulating the Debye length. In both cases, electrostatics does not change the lattice parameter, meaning that, despite the microgel different swelling ratio, charges do not affect neither interactions between particles at the interface nor microgels adsorption. Conversely, the cross-linking density has a strong impact on microgel packing at the interface: increasing the cross-linking density strongly decreases the extent of microgel flattening and promotes the occurrence of coexisting hexagonally ordered domains with different lattice parameters.

Hydrophobins are surface-active proteins that form a hydrophobic, water-repelling film around aerial fungal structures. They have a compact, particle-like structure, in which hydrophilic and hydrophobic regions are spatially, separated. This surface property renders them amphiphilic and is reminiscent of synthetic Janus particles. Here we report surface-specific chiral and nonchiral vibrational sum-frequency generation spectroscopy (VSFG) measurements of hydrophobins adsorbed to their natural place of action, the air-water interface. We observe that hydrophobin molecules undergo a reversible-change in orientation (tilt) at the interface when the pH is varied. We explain this local orientation toggle from the modification of the interprotein interactions and the interaction of hydrophobin with the water solvent, following the pH-induced change of the charge state of particular amino acids.

Cryo-EM samples prepared using the traditional methods often suffer from too few particles, poor particle distribution, or strongly biased orientation, or damage from the air-water interface. Here we report that functionalization of graphene oxide (GO) coated grids with amino groups concentrates samples on the grid with improved distribution and orientation. By introducing a PEG spacer, particles are kept away from both the GO surface and the air-water interface, protecting them from potential denaturation.

Adsorption of proteins onto liquid interfaces, such as the air-water interface, often leads to changes in the protein conformation. This can lead to changes in protein assembly behaviour, with aggregation and fibrillation often enhanced. To understand the relationship between protein conformation and aggregation, knowledge of protein structure at interfaces, on the single molecular level, is necessary. Using molecular dynamics simulations the effect of the air-water interface on conformation of the insulin B-chain is investigated. At the air-water interface the protein adopts an α-helical conformation, whereas in bulk solution it adopts disordered structures. The α-helical conformation is templated by the partitioning of hydrophobic side chains into the air, leading to the formation of an amphipathic helix. This structure presents a hydrophobic face which may lead to further aggregation, which helps explain the enhancement of insulin fibrillation at interfaces. This knowledge of the molecular conformation gives new insight into the contribution of protein structural change on the interfacial aggregation of proteins. Computational resources for this work were provided by the SFI/HEA funded Irish Centre for High End Computing. peer-reviewed

Atmospheric organic aerosols play a major role in climate, demanding a better understanding of their formation mechanisms by contributing multiphase chemical reactions with the participation of water. The sunlight driven aqueous photochemistry of small 2-oxocarboxylic acids is a potential major source of organic aerosol, which prompted the investigations into the mechanisms of glyoxylic acid and pyruvic acid photochemistry reviewed here. While 2-oxocarboxylic acids can be contained or directly created in the particles, the majorities of these abundant and available molecules are in the gas phase and must first undergo the surface uptake process to react in, and on the surface, of aqueous particles. Thus, the work also reviews the acid-base reaction that occurs when gaseous pyruvic acid meets the interface of aqueous microdroplets, which is contrasted with the same process for acetic acid. This work classifies relevant information needed to understand the photochemistry of aqueous pyruvic acid and glyoxylic acid and motivates future studies based on reports that use novel strategies and methodologies to advance this field.

The N-H stretching vibration is an important probe for investigating structural and functional properties of proteins but is often difficult to analyze as it overlaps with the O-H stretching vibration of water molecules. In this work we investigate the N-H signals of hydrophobins using conventional (VSFG) and heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSDG). Hydrophobins represent a group of surface active proteins that form highly-ordered protein films at the water-air interface and that give rise to prominent vibrational modes. We find that in conventional VSFG spectra N-H specific signals show significant changes in shape and intensity upon altering the pH values. These changes can easily be misinterpreted for conformational changes of the protein. Using HD-VSFG experiments, we demonstrate, that for hydrophobin films the change of the N-H response with pH can be well explained from the interference of the N-H response with the broad interfacial water O-H stretch band.

Journal article The conformational change exhibited by proteins at liquid interfaces, such as the air-water and oil-water interfaces, has long been of interest, both for understanding protein structure outside of native environments and for applications in areas including food technology and pharmaceuticals. Using molecular simulation this paper studies the conformations of two peptides derived from myoglobin, for which the emulsification behaviour has been studied. Both peptides were found to readily adsorb onto the air-water interface, with one of these (experimentally the more effective stabiliser) adopting a flat, extended conformation, with the other peptide remaining close its solution conformation. peer-reviewed 2017-04-14

It has long been recognized that liquid interfaces, such as the air water interface (AWI), can enhance the formation of protein fibrils. This makes liquid interfaces attractive templates for fibril formation but fully realizing this requires knowledge of protein behavior at interfaces, which is currently lacking. To address this, molecular dynamics simulation is used to investigate fragments of amyloid beta, a model fibril forming protein, at the air water interface. At the air water interface, the enrichment of aggregation-prone helical conformations provides a mechanism for the enhancement of fibrillation at interfaces. The conformational ensemble at the air water interface was also considerably reduced compared to bulk solution due to the tendency of hydrophobic side chains partitioning into the air restricting the range of conformations. Little overlap between the conformational ensembles at the AWI and in the bulk solution was found, suggesting that AWI induces the formation of a different set of structures compared to bulk solution. The smaller Aβ(16 22) and Aβ(25 35) fragments show an increase in the propensity for an ordered secondary structure at the air water interface but with a increased propensity for turn over other motifs, illustrating the importance of intra-protein interactions for stabilizing helical and extended conformations. Computational facilities for this work were provided by the SFI/HEA funded Irish Centre for High End Computing. SS was supported by a postgraduate research scholarship from the College of Science, NUI Galway. peer-reviewed

Polydopamine (PDA) films at the air - water interface can be easily obtained in slightly alkaline dopamine solutions using 02 dissolved in water as an oxidant but their insufficient stability limits their applications. In this work PDA films at the water surface were obtained by polymerization of dopamine hydrochloride (DA) by the enzyme laccase in a slightly acidic environment at a constant concentration of enzyme and DA concentrations in the range from 0.1 to 10 g/l. The dynamic surface elasticity, effective surface tension, film thickness and refractive index were measured as function of time to characterize the main steps of film formation. The obtained results show that laccase accelerates the polymerization process, decreases the required DA concentration for film formation, and insight from Brewster angle microscopy shows that laccase increases the resistance of the films to mechanical deformation. The dynamic surface elasticity of the obtained films is about seven times higher than the highest values for either pure PDA or pure laccase films. Laccase is inferred not only to provide crosslinking of the polymer but is also incorporated in the film resulting in its higher stability. The kinetic dependences of the surface properties allow the main steps of film formation to be distinguished: nucleation of polymer domains in a rapidly-formed protein film, coexistence of the protein and polymer domains, and their transformation into a highly dense composite film with encapsulated protein. Insight from atomic force microscopy images of films transferred to mica show that PDA fills in the gaps of spherical laccase aggregates when the DA concentration is increased and the film becomes less rough. The inclusion of laccase in PDA films significantly increases their stability.

Sea spray aerosol (SSA) is highly enriched in marine-derived organic compounds during seasons of high biological productivity, and saturated fatty acids comprise one of the most abundant classes of molecules. Fatty acids and other organic compounds form a film on SSA surfaces, and SSA particle surface-area-to-volume ratios are altered during aging in the marine boundary layer (MBL). To understand SSA surface organization and its role during dynamic atmospheric conditions, an SSA proxy fatty acid film and its individual components stearic acid (SA), palmitic acid (PA), and myristic acid (MA) are studied separately using surface pressure&ndash, area ( &Pi, &minus, A ) isotherms and Brewster angle microscopy (BAM). The films were spread on an aqueous NaCl subphase at pH 8.2, 5.6, and 2.0 to mimic nascent to aged SSA aqueous core composition in the MBL, respectively. We show that the individual fatty acid behavior differs from that of the SSA proxy film, and at nascent SSA pH the mixture yields a monolayer with intermediate rigidity that folds upon film compression to the collapse state. Acidification causes the SSA proxy film to become more rigid and form 3D nuclei. Our results reveal film morphology alterations, which are related to SSA reflectivity, throughout various stages of SSA aging and provide a better understanding of SSA impacts on climate.

Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) In this review we discuss about the immobilization of enzymes in Langmuir-Blodgett films in order to determine the catalytic properties of these biomacromolecules when adsorbed on solid supports. Usually, the conformation of enzymes depends on the environmental conditions imposed to them, including the chemical composition of the matrix, and the morphology and thickness of the film. In this review, we show an outline of manuscripts that report the immobilization of enzymes as LB films since the 1980's, and also some examples of how the surface properties of the floating monolayer prepared previously to the transfer to the solid support are important to determine the efficiency of the resulting device. Univ Fed Sao Paulo, Inst Environm Chem & Pharmaceut Sci, Rua Sao Nicolau 210, BR-09913030 Diadema, SP, Brazil Univ Fed Sao Paulo, Inst Environm Chem & Pharmaceut Sci, Rua Sao Nicolau 210, BR-09913030 Diadema, SP, Brazil FAPESP: 2015/10851-0 Web of Science

This thesis presents work on the heterogeneous chemistry of surfactant monolayers adsorbed at the air-water interface with gas phase hydroxyl radicals (OH). A novel hydroxyl radical source utilizing a low-temperature plasma known as the dielectric barrier discharge source (DBDS) is designed, characterized, and implemented as a high concentration OH generator (approximately 1000 fold higher than ambient conditions) in these experiments. Millimeter-sized water droplets containing a monolayer of surface-adsorbed species are exposed to this OH source until a designated level of oxidation. Field-induced droplet ionization mass spectrometry (FIDI-MS), which has been previously proven to be a surface-selective ionization technique, is used as an analytical method to determine the identity and relative quantity of oxidation products at the interface by sampling progeny droplets expelled upon the application of a pulsed electric field. Chapter 2 establishes the utility of the DBDS and FIDI-MS setup by oxidizing the 12-carbon n-alkyl surfactant dodecyltrimethylammonium (DTA+). Mechanistic details can be determined through the evolution of high mass oxidation products through a nearly complete conversion of the parent monolayer. Carbonyl, hydroxyl, peroxyl, and small amounts of fragmentation products can be discerned from the collision-induced dissociation mass spectra and through hydrogen-deuterium exchange experiments. Psuedo-first order kinetics can also be observed in this system, suggesting a Langmuir-Hinshelwood mechanism of OH-initiated oxidation. Chapter 3 extends the study of oxidation of hydrocarbon surfactants by looking at neighbor chain competition among a bis-quaternary ammonium gemini surfactants, a species that contains two long alkyl chains. Specifically, the experiment assesses the ability of two non-identical alkyl chains to compete with each other for the impinging gas phase hydroxyl radical. The increased reactivity of singly oxidized chains is evaluated by the oxidation of a gemini surfactant parent with identical alkyl chains. Chapter 4 investigates the oxidation of α,ω-surfactants, those that contain polar functional group separated by methylene spacers, discussing deviations of this class of surfactants with those that contain only one head group (i.e. DTA+). Chapter 5 demonstrates the DBDS and FIDI-MS system for use as a peptide footprinting technique for the amphiphilic species substance P. Localization of the site of oxidation can be done through FIDI-MS/MS analysis.

We describe a new interferometric technique to study gas-liquid and liquid-liquid interfaces. Bubbles and drops are subjected to an alternating electric field which excites capillary oscillations at the interface, if charged. Bubble or drop deformation is detected by the change of the internal optical path of a laser beam crossing perpendicular to the oscillation axis. Due to the closed geometry, a discrete spectrum of stationary oscillation frequencies (normal modes) is excited. The interferometric nature of the measurement and the resonant nature of the oscillation modes concur in allowing for high sensitivity, in the sub-nanometric region. We present a detailed description of the experimental setup and examples of applications of the technique to the study of both gas-liquid and liquid-liquid interfaces, either naked or with adsorbed surfactant monolayers, for bubbles and drops with diameter ~ 1 mm. In particular, the resonance frequencies and the width of the resonance peaks depend on the surface tension and the viscous dampening, respectively. We show that, by this new technique, properties of the interface can be accessed with confidence at the sub-nanometer scale, and surface phenomena, like the monolayer phase transition or the peculiarities of adsorption/desorption processes, can be unraveled in concentration regimes which are too low for existing methods.

The surface properties of a polydopamine layer at the air-water interface were studied by dilatational surface rheology, ellipsometry and Brewster angle microscopy (BAM). A significant increase of the dynamic surface elasticity was discovered when the concentration changed from 0.75 g/l to 2 g/l with the maximum value of about 60 mN/m at a concentration of 1 g/l. The obtained results indicate that the surface film consists of separate domains and the high surface elasticity is a consequence of the interactions between relatively rigid domains of the polymer film. This conclusion was confirmed by Brewster angle microscopy, which demonstrated different steps of the polydopamine film growth. Separate domains appeared at the first step while one can observe a continuous film close to equilibrium. An increase of the initial concentration led to faster polymerization and to the formation of a thicker film. The dynamic surface elasticity decreased in the concentration range from 2 g/l to 5 g/l when the thickness of the polymer film reached about 80 nm. In this case the film could be destroyed in the course of deformation. The cracks in the film resulted in a decrease of the dynamic surface elasticity.

Protein unfolding at an air–water interface is followed in real time by a recently developed simultaneous multiple-angle–wavelength-dispersive X-ray reflectometer with a time resolution of 1 s., Protein unfolding at an air–water interface has been demonstrated such that the X-ray reflectivity can be measured with an acquisition time of 1 s using a recently developed simultaneous multiple-angle–wavelength-dispersive X-ray reflectometer. This has enabled the electron density profile of the adsorbed protein molecules to be obtained in real time. A globular protein, lysozyme, adsorbed at the air–water interface is found to unfold into a flat shape within 1 s.

Investigating the mechanism of action of drugs whose pharmaceutical activity is associated with cell membranes is fundamental to comprehending the biochemical and biophysical processes that occur on membrane surfaces. In this work, we investigated the interaction of an ester-type derivative of uridine, 3′,4′,6′-trimyristoyl uridine, with models for cell membranes formed by lipid monolayers at the air-water interface. For that, selected lipids have been chosen in order to mimic tumorigenic and non-tumorigenic cells. For mixed monolayers with 2-dipalmitoyl- sn - glycero -3-phosphocholine (DPPC) or 1,2-dihexadecanoyl- sn - glycero -3-phospho- l -serine (DPPS), the surface pressure–area isotherms exhibited a noticeable shift to lower areas in relation to the areas predicted for ideal mixtures, indicating a condensation of the monolayer structure. Changes in the viscoelastic properties of the interfacial film could be inferred by analyzing the compressibility modulus of the monolayer. Structural and morphological changes were also evidenced by using vibrational spectroscopy and Brewster angle microscopy, respectively, with distinctive effects on DPPC and DPPS. As conclusion we can state that the lipid composition of the monolayer modulates the interaction with this lipophilic drug, which may have important implications in understanding how this drug acts on specific sites of the cellular membrane.

Preserving the enzyme structure in solid films is key for producing various bioelectronic devices, including biosensors, which has normally been performed with nanostructured films that allow for control of molecular architectures. In this paper, we investigate the adsorption of uricase onto Langmuir monolayers of stearic acid (SA), and their transfer to solid supports as Langmuir–Blodgett (LB) films. Structuring of the enzyme in β-sheets was preserved in the form of 1-layer LB film, which was corroborated with a higher catalytic activity than for other uricase-containing LB film architectures where the β-sheets structuring was not preserved. The optimized architecture was also used to detect uric acid within a range covering typical concentrations in the human blood. The approach presented here not only allows for an optimized catalytic activity toward uric acid but also permits one to explain why some film architectures exhibit a superior performance.

\Beta Carotene, positively recognized in literature [18, followed by subsequent papers] as regards its film-forming abilities at the water/air interface, was subjected to further Langmuir monolayer investigations. A wide variety of organic solvents and their mixtures have been tested in order to choose the best spreading solvents for monolayer experiments. A constant surface pressure rise, without characteristic for collapse spike or plateau, was observed upon compressing films dropped onto the air/water interface from all the solvents studied. However, a surface pressure increase alone is an insufficient criterion to evidence a true monolayer formation. The application of Brewster angle microscopy (BAM) indicated the presence of crystalline domains even at large molecular areas, contrary to homogeneous monolayer, which has been expected based on the above-mentioned reports. The addition of surface active substances (n-pentanol or iso-propanol) into \beta carotene solution does not improve whatsoever its spreading behaviour. Our experiments indicate that \beta carotene is not capable of forming monomolecular films at the free water surface and therefore is not suitable material for investigations with the Langmuir technique. Thus, this study shows the importance of using complementary - to classical surface manometry - techniques to confirm the true monolayer formation from molecules of interest.

We describe the first detailed experimental characterization of surface adsorption of an aqueous ion pair and quantify the unusual surface behavior of sodium nitrite, a ubiquitous component of natural waters. The onset of unusually strong adsorption at concentrations as low as ca. 0.1 M resembles the controversial 'Jones-Ray Effect,' wherein 13 salts exhibited surface tension minima in the millimolar region before resuming the normal linear increase with bulk concentrations. Given the compelling analogy recently found between adsorption of ions to the air-water interface, and the Hofmeister Effects of biochemistry, these results have important implications for the mechanism underlying these phenomena. Refereed/Peer-reviewed

The behavior of guanidinium chloride at the surface of aqueous solutions is investigated using classical molecular dynamics simulation. It is found that the population of guanidinium ions oriented parallel to the interface is greater in the surface region than in bulk. The opposite is true for ions in other orientations. Overall, guanidinium chloride is depleted in the surface region, in agreement with the fact that addition of guanidinium chloride increases the surface tension of water. The orientational dependence of the surface affinity of the guanidinium cation appears to be related to its anisotropic hydration. To bring the ion to the surface in the parallel orientation does not require hydrogen bonds to be broken, in contrast to other orientations. The surface enrichment of parallelly oriented guanidinium indicates that its solvation is more favorable near the surface than in bulk solution for this orientation. The dependence of the bulk and surface properties of guanidinium on the force field parameters, including polarizability, is also investigated. Despite significant quantitative differences between the force fields, the surface behavior is qualitatively robust. The implications of the above findings for the behaviorof guanidinium groups near biological interfaces are demonstrated on the example of arginine side chain orientations in lysozyme.

Milk whey proteins (MWP) and pectins (Ps) are biopolymer ingredients commonly used in the manufacture of colloidal food products. Therefore, knowledge of the interfacial characteristics of these biopolymers and their mixtures is very important for the design of food dispersion formulations (foams and/or emulsions). In this paper, we examine the adsorption and surface dilatational behavior of MWP/Ps systems under conditions in which biopolymers can saturate the air–water interface on their own. Experiments were performed at constant temperature (20 ºC), pH 7 and ionic strength 0.05M. Two MWP samples, beta-lactoglobulin ( beta-LG) and whey protein concentrate (WPC), and two Ps samples, low-methoxyl pectin (LMP) and high-methoxyl pectin (HMP) were evaluated. The contribution of biopolymers (MWP and Ps) to the interfacial properties of mixed systems was evaluated on the basis of their individual surface molecular characteristics. Biopolymer bulk concentration capable of saturating the air–water interface was estimated from surface pressure isotherms. Under conditions of interfacial saturation, dynamic adsorption behavior (surface pressure and dilatational rheological characteristics) of MWP/Ps systems was discussed from a kinetic point of view, in terms of molecular diffusion, penetration and configuration al rearrangement at the air–water interface. The main adsorption mechanism in MWP/LMP mixtures might be the MWP interfacial segregation due to the thermodynamic incompatibility between MWP and LMP (synergistic mechanism); while the interfacial adsorption in MWP/HMP mixtures could be characterized by a competitive mechanism between MWP and HMP at the air–water interface (antagonis- tic mechanism). The magnitude of these phenomena could be closely related to differences in molecular composition and/or aggregation state of MWP ( beta-LG and WPC). Fil: Perez, Adrián Alejandro. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Instituto de Tecnología de los Alimentos; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Carrera Sánchez, Cecilio. Universidad de Sevilla; España Fil: Rodríguez Patino, Juan. Universidad de Sevilla; España Fil: Rubiolo, Amelia Catalina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química (i); Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Santiago, Liliana. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Instituto de Tecnología de los Alimentos; Argentina

Copper(II) complexes spread on an aqueous solution surface were studied by a polarized total-reflection X-ray absorption fine structure (TR-XAFS) technique. The polarized TR-XAFS spectra at the Cu-K edge for copper(II) porphyrins and copper(II) chlorophyllin in a monolayer were measured in situ at the air-water interface. The polarization dependences of X-ray absorption near-edge structure (XANES) involving a 1s→4pz transition allowed us to estimate the molecular orientation and the local coordination structure around the copper(II) atom in the polarization plane selectively. The extended X-ray absorption fine structure (EXAFS) region of the polarized TR-XAFS spectra for the metal complexes present at the air-water interface was successfully analyzed for the first time. The relative coordination number for the copper center evaluated from the EXAFS analysis indicated larger values in the vertical polarization than in the horizontal one, in agreement with the standing-up molecular orientation at the air-water interface estimated from the XANES region., Analytical sciences, 25(4), pp.475-480; 2009

International audience; This study was undertaken to characterize the interfacial and foaming properties of different mixtures at the air-water interface under the same concentration range, and to put into light putative particular behaviors of mixtures. Different complementary techniques have been coupled such as null ellipsometry, surface tension and shear elastic constant measurements as well as polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Moreover, the foaming properties were also investigated by a bubbling method. Solutions of either ovalbumin or lysozyme exhibit different interfacial behaviors. Ovalbumin does not form multilayers, even at high concentration. Conversely, lysozyme forms films that are much thicker than a protein monolayer whereas the surface pressure is definitely smaller than that ovalbumin. It thus seems that ovalbumin is much more surface active than lysozyme. The ellipsometric angles for the binary mixtures are much higher than those of films formed by the pure proteins, independently of the molar ratio. These results suggest that there is a synergy in the interfacial adsorption between the two proteins, which is confirmed by PM-IRRAS measurements, most likely due to the electrostatic interactions between the oppositely charged lysozyme and ovalbumin. Moreover, shear elastic constant measurements suggest that lysozyme has a strong effect on the organization of the interfacial film. The foaming properties of the mixtures are, however, always close to those of the pure ovalbumin solution. Therefore, lysozyme does not foam alone, but is able to foam as well as pure ovalbumin after addition of a very small amount of ovalbumin, which suggests that the synergy between proteins has an impact on the foaming properties.

In this work the influence of pH and ionic strength on the stability of foams prepared with amaranthprotein isolate was analyzed. The behaviour observed was related to the physico-chemical and structuralchanges undergone by amaranth protein as a result of those treatments. The results obtained showthat foams prepared at acidic pH were more stable than the corresponding to alkaline pH. At pH 2.0the foams presented higher times and more volumes of drainage. This behaviour is consistent with thecharacteristics of the interfacial film, which showed a higher viscoelasticity and a greater flexibility atacidic pH than alkaline pH value, which in turn increased by increasing the concentration of proteins inthe foaming solution. It is also important to note that the presence of insoluble protein is not necessarilydetrimental to the properties of the foam. Detected changes in the characteristics of the interfacial filmas in the foam stability have been attributed to the increased unfolding, greater flexibility and net chargeof amaranth proteins at acidic conditions., Centro de Investigación y Desarrollo en Criotecnología de Alimentos

Saponins are natural surfactants with non-trivial surface and aggregation properties which find numerous important applications in several areas (food, pharma, cosmetic and others). In the current paper we study the surface properties of ten saponin extracts, having different molecular structure with respect to the type of their hydrophobic fragment (triterpenoid or steroid aglycone) and the number of sugar chains (1 to 3). We found that the triterpenoid saponins Escin, Tea Saponin and Ginsenosides have area per molecule in the range between 0.5 and 0.7 nm2, and the adsorbed molecules are orientated perpendicularly to the interface. The comparison of the experimentally measured surface elasticities with theoretically estimated ones shows that the saponins with very high dilatational and shear elasticities (up to 2000 mN/m) have molecular interaction parameter in the adsorption layers which is above the threshold value for two-dimensional phase transition. In other words, the highly elastic layers are in surface condensed state, due to strong attraction between the adsorbed molecules. Furthermore, these adsorption layers have non-linear rheological response upon expansion and contraction, even at relatively small deformation. Layers from the other studied saponins (steroids and crude mixtures of triterpenoid saponins), which are unable to form strong intermolecular bonds within the adsorption layer, have zero shear elasticity and viscosity and low dilatational elasticity and viscosity, comparable in magnitude to those reported in literature for protein adsorption layers. The comparison of the results, obtained by several independent experimental methods, allowed us to formulate the conditions under which the results from different interfacial rheology tests could be compared, despite the complex non-linear response of the saponin adsorption layers.

Hydroxypropyl cellulose (HPC) is a neutral polysaccharide derived from cellulose. It is, notably, added to non-dairy whipped topping. However, if the function of HPC is well recognized, its behavior is not well known. The interaction of HPC with each component (dispersed fat droplets, emulsifiers, proteins) should be studied step by step. In order to understand better the function of HPC, air–water interfacial rheology was used to characterize the behavior of HPC at this interface, with and without emulsifier. As HPC is a flexible polymer, it shows, at room temperature, a medium value of intrinsic viscosity, which is of the same order as of galactomannan ones. Addition of HPC in the aqueous phase leads to a decrease of the air–water surface tension (down to about 45–41 mN/m) where xanthan and guar gum have smaller or negligible effect. The time to reach the equilibrium of surface tension is reached after several hours. The smaller the concentration, the longer it is to reach the equilibrium. The competition between HPC and palmitic ester (E473) was studied. Kinetics of surface tension evolution of E473/HPC systems are different from that of HPC alone: mixed systems and HPC show an evolution of surface tension in three steps that could be explained through E473/HPC competition for the air/continuous phase interface.

Due to the combination of flexibility and amphiphilicity, in solution β -casein ( β -CN) acts as a surfactant molecule. It undergoes self-association and forms “micellar-like” aggregates in aqueous media. The aim of our study was to understand the role played by the charges borne by the N-terminal part of β -CN in its association behavior. β -CN polarity was modified in order to induce new aggregation properties and to understand better the main factors defining them. An unphosphorylated wild type recombinant form (WT A 2 ) of β -CN has been obtained by heterologous expression in the prokaryotic host Escherichia coli. A mutant (MU A 2 ) form of β -CN was engineered by duplicating the first 6 N-terminal amino acids residues (RELEEL) of WT A 2 , three additional negatively charged glutamic acids replacing some of the negative charges lost by the absence of phosphorylation in prokaryotic expression system. Dynamic light scattering and fluorimetry experiments showed that recombinant and native β -CN have different behaviors in temperature-dependent association. Micellisation was impaired by the absence of phosphate groups. Mutation conferred to the protein the ability to form aggregates whose size is inversely proportional to the concentration. These different behaviors during the association processes displayed by recombinant β -CN are correlated with modifications of the temperature-induced compaction of the proteins, what was revealed by circular dichroism measurements. WT A 2 was less condensed at the air-water interface than native β -CN; this was in part reversed by the additive mutation. However, MU A 2 compensating the lack of phosphate charges in recombinant caseins forms a more cohesive monolayer at the air–water interface than do the other β -CN, which means that the mutation reinforces monomers interaction at the surface.

The interfacial properties of the spread Langmuir films of C60Br24 and stearic acid have been studied at air–water interface by measuring their surface pressure–area isotherms. Monolayer studies on C60Br24 suggest a strong aggregation at air–water interface which corroborated the ab initio electron density calculations. The mixed films were found to be immiscible and non-ideal with an exception of X C 60 Br 24 = 0.5 which is the composition of greater stability. The interaction between C60Br24 and stearic acid results in the lowering of surface pressure withstood by stearic acid and hence the decrease in compressibility with the increasing X C 60 Br 24 , thereby indicating the domination of C60Br24 characteristics in the mixed monolayers.

Colloidal interfacial interactions in a capillary channel under different chemical and flow conditions were studied using confocal microscopy. Fluorescent latex microspheres (1.1 mu m) were employed as model colloids and the effects of ionic strength and flow conditions on colloidal retention at air-water interface (AWI) and contact line were examined in static and dynamic (flow) experiments. Colloids were preferentially attached to and accumulated at AWI, but their transport with bulk solution was non-negligible. Changing solution ionic strength in the range 1-100 mM had a marginal effect on colloidal accumulation, indicating forces other I tan electrostatic are involved. Flow through the open channel resembled Poiseuille flow with AWI acting as a non-stress-free boundary, which resulted in near stagnation of AWI and consequently promoted colloid accumulation. Retention on contact line was likely dominated by film-straining and was more significant in flow relative to static experiments due to hydrodynamic driving force. Modeling and dimensionless analysis of the flow behavior in the capillary channel clearly indicate the important role of apparent surface viscosity and surface tension in colloidal interfacial retention at the pore scale, providing insight that could improve understanding of colloid fate and transport in natural unsaturated porous media. (c) 2006 Elsevier Inc. All rights reserved.