Orientation of peptides in aqueous monolayer films. Infrared reflection-absorption spectroscopy studies of a synthetic amphipathic beta-sheet

Infrared reflection-absorption spectroscopy (IRRAS) intensities of the Amide I vibration are used to develop a quantitative approach for determining the Euler angles that describe the orientation of protein beta-sheets in aqueous monolayer films. A synthetic amphipathic peptide, Val-Glu-Val-Orn-Val-Glu-Val-Orn-Val-Glu-Val-Orn-Val-OH is used as a test case. The pattern of Amide I frequencies suggests that the molecule is organized as an antiparallel beta-sheet at the air/water interface. The model used to simulate the Amide I intensities reveals that the beta-sheet has a slight preferential alignment parallel to the direction of compression; i.e., deviation from uniaxial symmetry is observed. In addition, the sheet is found to lie flat on the aqueous surface, with (presumably) the polar side chains interacting with the aqueous subphase. Limitations and advantages of the theoretical approach are discussed.     

Infrared spectra for a multi discipline spectroscopy system

Summary The infrared spectra quality requirements of the Spektrendatenbanken-Verbundsystem (SDVS) are reported with special reference to Expert System needs. Some of the potential benefits of the system are given. A Call for Spectra is made and the guidelines for submitting spectra to the database outlined.

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Infrarotspektren für ein multidisziplinäres Spektroskopie-System

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Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday     

Vibrational spectroscopy of individual doping centers in a monolayer organic crystal

A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C60 molecules supported on a thin Al2O3 film grown on the NiAl(110) surface. Vibronic states of the doping centers are observed with differential conductance (dIdV) spectroscopy. The double-barrier nature of the junction results in bipolar transport: same states participate in charge transport at both bias voltage polarities. Identification of the dIdV features corresponding to bipolar conduction enables a new mode of vibrational spectroscopy with STM.     

Absorption spectra of amorphous organic films

Using a low-temperature evaporation technique, amorphous-state films of naphthacence, perylene, coronene and 1,12-benzoperylene were prepared, and their absorption and fluorescence spectra were measured. The obtained results are discussed and compared with the corresponding crystal-state spectra.     

Infrared reflection-absorption spectroscopy and polarization-modulated infrared reflection-absorption spectroscopy studies of the aequorin langmuir monolayer

The Langmuir monolayer of aequorin and apoaequorin was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS techniques. The alpha-helices in the aequorin Langmuir monolayer were parallel to the air-water interface at zero surface pressure. When the surface pressure increased to 15 mN.(m-1), the alpha-helices became tilted and the turns became parallel to the air-water interface. As for apoaequorin, the alpha-helices were also parallel to the air-water interface at 0 mN.m(-1). However, the alpha-helix became tilted and the turns became parallel to the air-water interface quickly at 5 mN.m(-1). With further compression of the apoaequorin Langmuir monolayer, the orientation remained the same. The different behaviors of aequorin and apoaequorin at the air-water interface were explained by the fact that aequorin formed dimers at the air-water interface but apoaequorin was a monomer. It is more difficult for a dimer to be tilted by the compression of the Langmuir monolayer.     

Infrared reflection-absorption spectroscopy and polarization-modulated infrared reflection-absorption spectroscopy studies of the organophosphorus acid anhydrolase langmuir monolayer

The secondary structure of the organophosphorus acid anhydrolase (OPAA) Langmuir monolayer in the absence and presence of diisopropylfluorophosphate (DFP) in the subphase was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS (PM-IRRAS). The results of both the IRRAS and the PM-IRRAS indicated that the alpha-helix and the beta-sheet conformations in OPAA were parallel to the air-water interface at a surface pressure of 0 mN.m-1 in the absence of DFP in the subphase. When the surface pressure increased, the alpha-helix and the beta-sheet conformations became tilted. When DFP was added to the subphase at a concentration of 1.1 x 10(-5) M, the alpha-helix conformation of OPAA was still parallel to the air-water interface, whereas the beta-sheet conformation was perpendicular at 0 mN.m-1. The orientations of both the alpha-helix and the beta-sheet conformations did not change with the increase of surface pressure. The shape of OPAA molecules is supposed to be elliptic, and the long axis of OPAA was parallel to the air-water interface in the absence of DFP in the subphase, whereas the long axis became perpendicular in the presence of DFP. This result explains the decrease of the limiting molecular area of the OPAA Langmuir monolayer when DFP was dissolved in the subphase.     

Arrangement of trace metal contaminations in thin films of liquid crystals studied by X-ray standing wave technique

X-ray standing wave method has been applied to investigate the molecular organization in Langmuir–Blodgett films of liquid-crystalline lanthanide complex deposited on silicon substrates. The X-ray standing waves measurements were carried out at BESSY II on the beamline KMC-2. Energy spectra of characteristic fluorescence emitted from the samples have been recorded at each point in the X-ray reflectivity curve as the incident angle was scanned through the total external reflection region. The integrated intensity under selected peaks was analyzed as a function of the incident angle. Incorporation of metal ions (Fe, Zn, Cu, Ca) to the Langmuir monolayer from the triply distilled water subphase has been established. Owing to the element selectivity of X-ray standing wave technique the arrangement of several types of metal ions inside the Langmuir–Blodgett films was studied individually. The experimental data revealed that the shape of the fluorescence curves for the contamination metal ions is distinctly different indicating considerable differences in the lateral distribution of these ions in the film. These results may be attributed to the phase separation in Langmuir monolayer of liquid-crystalline lanthanide complex due to the incorporation of metal ions from the water subphase.     

Vibrational spectra of small silicon monoxide cluster cations measured by infrared multiple photon dissociation spectroscopy

The first gas-phase infrared spectra of silicon monoxide cations (SiO)(n)(+), n = 3-5, using multiple photon dissociation in the 550-1250 cm(-1) frequency range, are reported. All clusters studied here fragment via loss of a neutral SiO unit. The experimental spectra are compared to simulated linear absorption spectra from calculated low energy isomers for each cluster. This analysis indicates that a "ring" isomer is the primary contributor to the (SiO)(3)(+) spectrum, that the (SiO)(4)(+) spectrum results from two close-lying bicyclic ring isomers, and that the (SiO)(5)(+) spectrum is from a bicyclic ring with a central, fourfold-coordinated Si atom. Experiment and theory indicate that the energies and energetic orderings of (SiO)(n)(+) isomers differ from those for neutral (SiO)(n) clusters.     

IR reflection spectra of monolayer films sandwiched between two high refractive index materials

Monolayer films adsorbed on substrates with high refractive indices such as metals or semiconductors yield strongly enhanced infrared reflection spectra when they are contacted with a transparent, high refractive index ambient medium and are probed with p-polarized light at high incidence angles. The sensitivity increase arises from the enhancement of the perpendicular electric field within a thin, low refractive index layer sandwiched between two high refractive index materials and gives rise to signal intensity gains up to 2 orders of magnitude in combination with an essentially exclusive detection of only perpendicular surface vibrations. Experimental spectra of ordered monolayer films of octadecanethiol on gold and of octadecylsiloxane on silicon in this sandwich configuration yield enhancement factors between 15 (on Si) and 30 (on gold) compared to conventional grazing incidence external reflection spectra and are governed by a common, simple surface selection rule, which allows immediate quantitative evaluation and comparison of the film structures on different substrates.     

Infrared imaging of a solid phase surfactant monolayer

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.     

Infrared spectra of the CF3I dimer: a concurrent application of matrix-isolation spectroscopy and cavity ring-down spectroscopy

We have observed infrared spectra of the CF(3)I dimer produced in a supersonic jet by matrix-isolation Fourier transform infrared spectroscopy and infrared cavity ring-down (IR-CRD) spectroscopy. In the matrix-isolation experiments, the dimer was isolated in an Ar matrix by the pulse-deposition method. The recorded spectral range covers the symmetric (nu(1)) and doubly degenerate (nu(4)) C-F stretching regions. From the concentration dependence of the matrix-isolation spectra we have assigned one dimer band for each fundamental region. It was not easy to identify the dimer band for the nu(4) band because of the multiplet feature of the monomeric nu(4) band caused by the site symmetry breaking. The spectra of (CF(3)I)(2) in the nu(4) band region were thus also measured in the gas phase by IR-CRD spectroscopy, where we detected two dimer bands. Comparing the observed band positions with the results of quantum chemical calculations, we have assigned the observed dimer bands to the head-to-head isomer. The structure of (CF(3)I)(2) and its photochemical implications are discussed, in comparison with methyl iodide dimer reported previously [Ito et al., Chem. Phys. Lett. 343, 185 (2001)].     

Infrared analysis of thin inorganic and organic films on metals

Infrared spectroscopy is shown to be an adequate method for qualitative and quantitative characterization of technical surface layers for corrosion protection. The measuring conditions for each type of sample have to be selected carefully. For the sake of reliability a certain redundancy in the evaluated data might be useful. It is the aim of this work to gather experience for routine quality control of steel and aluminium surfaces.Duridine, Granodine and Alodine used in this work are registered trademarks of Collardin GmbH     

High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.     

Study of molecular aggregation of artificial amyloid in a langmuir monolayer by infrared spectroscopy

A synthesized peptidolipid (C18IIGLM-NH2) comprised of a single C18-saturated hydrocarbon chain connected to the amino acid sequence IIGLM terminated with the NH2 group was spread on water, which formed a stable Langmuir monolayer. The Langmuir and Langmuir-Blodgett (LB) films have been characterized by measurements of surface pressure-area (pi-A) and surface potential-area (DeltaV-A) isotherms and infrared multiple-angle incidence resolution spectrometry (MAIRS). The Langmuir monolayer had a significantly larger limiting molecular area than that of a similar molecule of C18IIGLM-OH, which was reported in our previous study. The surface dipole moment analysis coupled with the pi-A isotherm suggested that the C18IIGLM-NH2 monolayer was extraordinarily stiff and the fundamental structure of the monolayer was brought about before the monolayer compression. The infrared MAIRS analysis of the C18IIGLM-NH2 LB film revealed that the backbone structure of the monolayer was the 'antiparallel' beta sheet aligned parallel to the substrate. Since the C18IIGLM-OH LB film was made of 'parallel' beta sheet with a random orientation, it has been found that the present C18IIGLM-NH2 Langmuir monolayer has a largely different monolayer structure, although the chemical structures are slightly different from each other by the terminal group only.     

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces

Investigation into nanostructured organic films has served many purposes, including the design of functionalized surfaces that may be applied in biomedical devices and tissue engineering and for studying physiological processes depending on the interaction with cell membranes. Of particular relevance are Langmuir monolayers, Langmuir–Blodgett (LB) and layer-by-layer (LbL) films used to simulate biological interfaces. In this review, we shall focus on the use of vibrational spectroscopy methods to probe molecular-level interactions at biomimetic interfaces, with special emphasis on three surface-specific techniques, namely sum frequency generation (SFG), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and surface-enhanced Raman scattering (SERS). The two types of systems selected for exemplifying the potential of the methods are the cell membrane models and the functionalized surfaces with biomolecules. Examples will be given on how SFG and PM-IRRAS can be combined to determine the effects from biomolecules on cell membrane models, which include determination of the orientation and preservation of secondary structure. Crucial information for the action of biomolecules on model membranes has also been obtained with PM-IRRAS, as is the case of chitosan removing proteins from the membrane. SERS will be shown as promising for enabling detection limits down to the single-molecule level. The strengths and limitations of these methods will also be discussed, in addition to the prospects for the near future.     

Comparative X-ray standing wave analysis of metal-phosphonate multilayer films of dodecane and porphyrin molecular square

The nanoscale structures of multilayer metal-phosphonate thin films prepared via a layer-by-layer assembly process using Zr(4+) and 1,12-dodecanediylbis(phosphonic acid) (DDBPA) or porphyrin square bis(phosphonic acid) (PSBPA) were studied using specular X-ray reflectivity (XRR), X-ray fluorescence, and long-period X-ray standing wave (XSW) analysis. The films were prepared in 1, 2, 3, 4, 6, and 8 layer series on both Si(001) substrates for XRR and on 18.6 nm period Si/Mo layered-synthetic microstructure X-ray mirrors for XSW. After functionalizing the SiO(2) substrate surfaces with a monolayer film terminated with phosphonate groups, the organic multilayer films were assembled by alternating immersions in (a) aqueous solutions containing Zr(4+)or Hf(4+) (final metal layer only) cations and then (b) organic solvent solutions of PO(3)-R-PO(3)(4-), where R was DDBPA or PSBPA spacer molecule. The Hf(4+) cation served as the marker for the top surface of the films, whereas the Zr(4+) cation was present in all other layers. The PSBPA also contained Zn and Re atoms at its midline which served as heavy-atom markers for each layer. The long-period XSW generated by the 0th- (total external reflection) through 4th-order Bragg diffraction conditions made it possible to examine the Fourier transforms of the fluorescent atom distributions over a much larger q(z) range in reciprocal space which permitted simultaneous analysis of Hf, Zn/Re, and Zr atomic distributions.