Behavior of β-amyloid 1-16 at the air-water interface at varying pH by nonlinear spectroscopy and molecular dynamics simulations

The adsorption and aggregation of β-amyloid (1-16) fragment at the air-water interface was investigated by the combination of second harmonic generation (SHG) spectroscopy, Brewster angle microscopy (BAM), and molecular dynamics simulations (MD). The Gibbs free energy of surface adsorption was measured to be -10.3 kcal/mol for bulk pHs of 7.4 and 3, but no adsorption was observed for pH 10-11. The 1-16 fragment is believed not to be involved in fibril formation of the β-amyloid protein, but it exhibits interesting behavior at the air-water interface, as manifested in two time scales for the observed SHG response. The shorter time scale (minutes) reflects the surface adsorption, and the longer time scale (hours) reflects rearrangement and aggregation of the peptide at the air-water interface. Both of these processes are also evidenced by BAM measurements. MD simulations confirm the pH dependence of surface behavior of the β-amyloid, with largest surface affinity found at pH = 7. It also follows from the simulations that phenylalanine is the most surface exposed residue, followed by tyrosine and histidine in their neutral form.     

Molecular organization and aggregation of mucin at air–water interface in the presence of chromium(III) complexes

The interfacial organization of mucin (glycoprotein) in the presence of chromium(III) complexes has been assessed from the surface pressure–molecular area (π–A) isotherms in Langmuir films at air–water interface and the surface energy of their LB films through contact angle measurements. At pH 7.0, the electrostatic interaction of [Cr(en)₃]Cl₃ with mucin was found to bring about changes in the average surface area from 3.26 to 1.47 nm²; suggesting the possible formation of large aggregates of mucin. Adsorption experiments using surface potential measurements reveal that [Cr(en)₃]Cl₃ binds at a much faster rate to the available binding sites in mucin when compared to [Cr(salen)(H₂O)₂](ClO₄) which binds coordinatively to mucin.     

Molecular and colloidal self-assembly at the oil–water interface

Self-assembly is a versatile bottom-up approach for fabricating novel supramolecular materials with well-defined nano- or micro-structures associated with functionalities. The oil-water interface provides an ideal venue for molecular and colloidal self-assembly. This paper gives an overview of various self-assembled materials, including nanoparticles, polymers, proteins, and lipids, at the oil-water interface. Focus has been given to fundamental principles and strategies for engineering the self-assembly process, such as control of pH, ionic strength and use of external fields, to achieve complex soft materials with desired functionalities, such as nanoparticle surfactants, structured liquids, and proteinosomes. It has been shown that self-assembly at the oil-water interface holds great promise for developing well-structured complex materials useful for many research and industrial applications.     

How many phases and phase transitions do exist in Gibbs adsorption layers at the air-water interface?

Four different phases and four different first-order phase transitions have been shown to exist in Gibbs adsorption layers of mixtures containing n-hexadecyl dihydrogen phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2. These conclusions have been made from surface pressure-time (pi-t) adsorption isotherms measured with a film balance and from monolayer morphology observed with a Brewster angle microscopy (BAM). The observed four phases are gas (G), liquid expanded (LE), liquid condensed (LC) and LC' phases. Three first-order phase transitions are G-LE, LE-LC and LC-LC'. However, the thermodynamically allowed G-LC phase transition in a 1.2 x 10(-4) M mixture at 2 degrees C, which is below the so-called triple point, is kinetically separated into the G-LE and LE-LC phase transitions. The most interesting observation is that the homogeneous LC phase shows a new first-order phase transition named as LC-LC' at 2 or 5 degrees C. The LE and LC phases represent circular and fractal shaped domains, respectively, whereas the LC' phase shows very bright, anisotropic and characteristic shaped domains.     

Resistance of β-casein at the air-water interface to enzymatic cleavage

X-ray reflectivity from an air-buffer interfacial β-casein monomolecular film placed on a solution of chymosin (renin) showed unexpectedly slow proteolytic cleavage. To understand this, the separate structures of β-casein and chymosin, the presentation of each molecule to the other at the air/liquid interface, and that of their mixtures is reported. At the air/solution interface, the hydrophobicity of the protein molecules causes orientation and some deformation of the conformation. When β-casein was presented to a chymosin monomolecular interfacial film, the chymosin was largely displaced from the surface, which was accounted for by the different surfactancy of the two molecules at 25 °C. There was no observable proteolysis. In the reverse experiment, a significant enzymatic degradation and the signature of hydrophobic fragments was observed but only at and above an enzyme concentration of 0.015 mg/mL in the substrate. For comparison, the air/solution interface of premixed β-casein with chymosin in phosphate buffer showed that the film was composed of β-casein proteolytic fragments and chymosin.     

Sheet-like assemblies of charged amphiphilic α/β-peptides at the air-water interface

There is growing interest in the design of molecules that undergo predictable self-assembly. Bioinspired oligomers with well-defined conformational propensities are attractive from this perspective, since they can be constructed from diverse building blocks, and self-assembly can be directed by the identities and sequence of the subunits. Here we describe the structure of monolayers formed at the air-water interface by amphiphilic α/β-peptides with 1:1 alternation of α- and β-amino acid residues along the backbone. Two of the α/β-peptides, one a dianion and the other a dication, were used to determine differences between self-assemblies of the net negatively and positively charged oligomers. Two additional α/β-peptides, both zwitterionic, were designed to favor assembly in a 1:1 molar ratio mixture with parallel orientation of neighboring strands. Monolayers formed by these α/β-peptides at the air-water interface were characterized by surface pressure-area isotherms, grazing incidence X-ray diffraction (GIXD), atomic force microscopy and ATR-FTIR. GIXD data indicate that the α/β-peptide assemblies exhibited diffraction features similar to those of β-sheet-forming α-peptides. The diffraction data allowed the construction of a detailed model of an antiparallel α/β-peptide sheet with a unique pleated structure. One of the α/β-peptide assemblies displayed high stability, unparalleled among previously studied assemblies of α-peptides. ATR-FTIR data suggest that the 1:1 mixture of zwitterionic α/β-peptides assembled in a parallel arrangement resembling that of a typical parallel β-sheet secondary structure formed by α-peptides. This study establishes guidelines for design of amphiphilic α/β-peptides that assemble in a predictable manner at an air-water interface, with control of interstrand orientation through manipulation of Coulombic interactions along the backbone.     

Spectroscopic studies of molecular interaction at the liquid–liquid interface

The development of techniques to study the liquid–liquid interface is a major challenge. Spectroscopy in all its forms provides a powerful method of investigation, especially when combined with other optical techniques. Over the last 30 years, there have been significant developments in the methods for studying heterogeneous interfaces. As technology progresses, the sensitivity of existing techniques has been improved but there are major challenges still to be met, such as the measurement of interfacial dielectric constant and viscosity. This paper aims to summarise the use of spectroscopy to study molecular interactions at the liquid–liquid interface.     

Tilting operator for phospholipidic molecular domains at the liquid–gas interface

We derive a coordinate independent operator expression for the tilting operator of molecular domains at the liquid–gas interface. The domains are made up of phospholipidic molecules modeled as spherocylinders. The molecules of the domain are oriented parallel to each other. The centers of symmetry of the molecules form a lattice. The tilting operator keeps track of the deformations suffered by this lattice as the domain molecules are tilted relative to the normal to the interface. The results obtained are important for dynamic calculations of inclination dependent collective film characteristics, as in the simulation of surface density versus surface pressure curves in a Langmuir film. The tilting operation can be decomposed into three separate simple operations: a global rotation, a local oblique realignment, and a global vertical translation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aza-BODIPY molecular assembly at the liquid-solid interface driven by Br⋯FBF interactions

In this work, two aza-BODIPY derivatives, 3,5-diphenyl-1,7-di(p-dodecyloxyphenyl)-aza-BODIPY (CJF) and 3,5-di(p-bromophenyl-1,7-di(p-dodecyloxyphenyl)-aza-BODIPY (2Br-CJF) acted as model molecules to form the self-assembly monolayers on the solid-liquid interface. With the utilizing of scanning tunnelling microscope (STM), we demonstrated that intermolecular Br⋯FBF interactions existed in 2Br-CJF self-assembly structure and played an important role in strengthening the stability of 2Br-CJF self-assembly structure. This result is supported by density functional theory (DFT) calculation.     

Molecular structure of mixed adsorption layers surfactant—polymer at a liquid—liquid interface

Isotherms of the binding of dodecyl sulfate anions (DDS^–)and Na⁺ counterions during their coadsorption with a nonionic polymer. proxanol (PR). at the interface of dodecane-water emulsions have been measured by conductometry and Na-selective potentiometry. The adsorption of DDS^– and PR is concurrent. The affinity constant of PR to the interface determined by the Langmuir model decreases as the concentration of PR increases, and the surface concentration of DDS^– tends to a nonzero limiting value at high concentrations of PR. The surface (_o) and electrokinetic () potentials at the interface have been determined at various polymer concentrations by the spin probe and electrophoresis methods. The average dielectric permeability and density of polymer segments in We adsorption layer have been determined by ESR. The lower boundary of the hydrodynamic thickness of the polymer adsorption layer at the interface has been estimated from the dependences of _o and on the ionic strength.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1653–1661, July, 1996.     

Dynamics of water and oil at the solid–liquid interface in macroporous media and reservoir rocks

Nuclear magnetic relaxation dispersion experiments at different temperatures, using the magnetic field-cycling method, are reported. These experiments allow the obtaining of original information about water or oil molecular dynamics at solid–liquid interface in porous media, such as grain packing or reservoir rocks. These results on molecular dynamics at the pore surface are of real interest for oil-recovery. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones.     

Self-assembly of polystyrene-block-poly(ethylene oxide) copolymers at the air-water interface: is dewetting the genesis of surface aggregate formation?

Block copolymer self-assembly at the air-water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution and in the bulk; however, the present analysis of atomic force microscopy (AFM) and isotherm data at different spreading concentrations suggests a nonequilibrium mechanism for the formation of various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates (spaghetti, dots, rings, and chainlike aggregates) at the air-water interface starting with an initial dewetting of the copolymer spreading solution from the water surface. We show that different spreading concentrations provide kinetic snapshots of various stages of self-assembly at the air-water interface as a result of different degrees of PS chain entanglements in the spreading solution. Two block copolymers are investigated: MW = 141k (11.4 wt % PEO) and MW = 185k (18.9 wt % PEO). Langmuir compression isotherms for the 185k sample deposited from a range of spreading concentrations (0.1-2.0 mg/mL) indicate less dense packing of copolymer chains within aggregate cores formed at lower spreading concentrations due to a competition between the interfacial adsorption of PEO blocks and the kinetic restrictions of PS chain entanglements. From AFM analysis of the transferred Langmuir-Blodgett films, it is clear that PS chain entanglements in the spreading solution also affect the morphological evolution of surface aggregates for both samples, with earlier structures being trapped at higher concentrations. At the highest spreading concentration for the 141k copolymer, the coexistence of long spaghetti aggregates with cellular arrays of holes, along with various transition structures, indicates that various surface aggregates evolve from networks of rims formed as a result of dewetting of the evaporating spreading solution from the water surface.     

Immune recognition of cytochrome c I. — Molecular requirements for antibody recognition and immune response stimulation studied in vitro with synthetic peptides

The epitope specificity of antibodies to horse cytochrome c (cyt.c) in the primary and secondary immune response of C57BL mice was studied by means of the ELISA technique with synthetic peptides of cyt.c. It was found that, in the early primary response, N- and C-end fragments of cyt.c (peptides 2–13, 14–22 and 92–104) were preferentially recognized. In the secondary response, more antibodies to the epitopes of the central part of the molecule (peptides 61–69 and 46–56) were found. This was presumed to be due to the mode of cyt.c processing and presentation in the course of immune response: at first, cyt.c was recognized in the native form and then in the processed one. The capacity of cyt.c peptides to stimulate the formation of cyt.c-specific antibody-secreting cells (ASC) was studied in splenocyte culture of C57BL mice. Peptides stimulated more ASC than cyt.c did, but larger molar doses of peptides were required. Comparison of the capacity of related peptides (1–13 and 2–13, 61–69, 61–77 and 57–77) to be recognized by antibodies produced to native cyt.cin vivo and to stimulate anti-cyt.c ASC in vitro suggested certain molecular requirements for cyt.c epitope and agretope formation. These were partially confirmed by computer analysis.     

Hysteresis in the measurement of double-layer capacitance at the gold–ionic liquid interface

The capacitance of the polycrystalline gold electrode–ionic liquid BMIMTf [1-butyl-3- methyl imidazolium trifluoromethanesulfonate (triflate)] interface has been studied using ac voltammetry and electrochemical impedance spectroscopy (EIS). Slow potential scanning reveals a massive hysteresis between potential scans toward negative or positive directions and provides a distorted C(E) curve. Kinetic studies indicate that 10 min wait times are required after each potential step for system relaxation. EIS resolves the overall series capacitance into contributions from ion adsorption and double-layer capacitance, C_DL. Isolation of C_DL reveals that even 10 min of equilibration time is insufficient to completely remove the hysteresis. The shape of the negative-going C_DL(E) curve is camel-shaped whereas the shape of the positive-going curve is bell-shaped.     

Generalized two-dimensional correlation spectroscopy——Theory and applications in analytical field

After correlation analysis for general spectro- scopy, two-dimensional (2-D) correlation spectroscopy is obtained by extracting the information contained in the spectra in two dimensions, which is the function of two dependent spectral variables. 2-D correlation spectroscopy is initially regarded as an analytical data treatment method in the study of molecular interaction by using sinusoidal infrared sign[1]. In 1993, it was extended to generalized 2-D correlation spectroscopy, which used mo…     

Infrared and Raman spectroscopies of monolayers at the air–water interface

Infrared and Raman spectroscopies are now currently used to obtain molecular information (orientation, conformation, organization) on monolayers at the air–water interface. In the past year, several original studies were performed on peptides and proteins and their interaction with phospholipidic monolayers.     

Degradation of cellulose at the wet-dry interface: I—study of the depolymerization

The degradation of cellulose in paper due to the formation of a tideline at the wet-dry interface when paper is suspended in water was explored. SEC/MALS was used to assess the molar mass, while ICP/MS, SEM/EDS and CE/UV provided a qualitative and quantitative analysis of the elements and inorganic ions present in the paper. Immediately after the formation of the tideline, no significant depolymerization was observed at the wet-dry interface, despite the accumulation of water soluble brown and/or fluorescent degradation compounds and salts containing sodium, chlorine, sulfur and calcium. Various artificial aging configurations were applied to the paper with tidelines to evaluate the effect of the material accumulated at the wet-dry interface on the long-term stability of paper. The decrease in the molar mass of cellulose (above, at and below the tideline) differed depending on the type of aging, i.e. whether the entire sheet of paper was aged or whether small amounts of paper from the different areas were sampled and aged, individually or together, which evidences a complex degradation pathway. In the former aging configuration the material accumulated in the tideline affected the degradation of the tideline area to the same extent or more than the other areas. When the different areas of the paper sheet were sampled and aged together, it was found that the presence of the tideline clearly affected the degradation of the other paper areas. Conversely, in that case, cellulose within the tidelines was the least degraded. The area below the tideline, through which the water migrated, showed the most significant degradation.     

Admittance studies of hydrogen-induced states at the silicon—silicon dioxide interface

The admittance of Pd-thin SiO₂Si MOSCAP devices was studied as a function of the following variables: temperature, measurement frequency, oxide preparation conditions, applied gate voltage and ambient atmospheres of 100 ppm hydrogen in nitrogen and pure oxygen. Transient current, capacitance and annealing studies were also conducted for many of these variables. It is shown that hydrogen atoms produced by the catalytic action of the Pd on hydrogen molecules can be injected into the oxide—semiconductor interface where, depending on the choice of oxidation conditions for growing the oxide, they modify the density and capture cross-sections of the hydrogen-induced interfacial states. It is also demonstrated that below 125 °C, the injected hydrogen can be reversibly removed by changing the ambient gas from the H₂/N₂ mixture to pure oxygen.     

Accumulation of host—guest ion complexes with different counterions at the water—supercritical CO2 interface: a molecular dynamics study

The behavior of ion complexes at the water—supercritical carbon dioxide interface was considered by molecular dynamics simulations. The following complexes were studied: Cs⁺calix[4]crown-6, K⁺222 cryptate with chloride or dicarbollide (CCD^–) counterions, the Sr²⁺18C6 complex with the picrate (Pic^–) or perfluorooctanoate (PFO^–) counterions, and the Cl^–Tet ⁴⁺ complex with chloride counterions (Tet ⁴⁺ is a tetrahedral tetraammonium cation). The simulations demonstrate the analogy between aqueous interfaces with organic immiscible liquids and the CO₂ phase. Water and supercritical CO₂ are poorly miscible and form an interface. Most of the complexes are accumulated at the interface, instead of diffusing into the organic phase in which they should be more soluble. In addition, marked counterion effects are observed. The CCD^–, Pic^–, and PFO^– anions are surface active and are concentrated at the interface, but show different relationships with the complexes. The formation of ion pairs is precluded by the very hydrophobic CCD^– anions, which promote the extraction of cryptates as separated ion pairs to the CO₂ phase. Conversely, the extraction of the Sr²⁺ ions with 18C6 proceeds via a co-complexation mechanism, including the formation of the Sr18C6(PFO)₂, complex having a CO₂ affinity. The mechanism of assisted ion transfer to the CO₂ phase is discussed.