Infrared Reflection Adsorption Spectroscopy Research Articles

Recent advances in organic molecule reactions on metal surfaces.

Chemical reactions of organic molecules on metal surfaces have been intensively investigated in the past decades, where metals play the role of catalysts in many cases. In this review, first, we summarize recent works on spatial molecules, small H2O, O2, CO, CO2 molecules, and the molecules carrying silicon groups as the new trends of molecular candidates for on-surface chemistry applications. Then, we introduce spectroscopy and DFT study advances in on-surface reactions. Especially, in situ spectroscopy technologies, such as electron spectroscopy, force spectroscopy, X-ray photoemission spectroscopy, STM-induced luminescence, tip-enhanced Raman spectroscopy, temperature-programmed desorption spectroscopy, and infrared reflection adsorption spectroscopy, are important to confirm the occurrence of organic reactions and analyze the products. To understand the underlying mechanism, the DFT study provides detailed information about reaction pathways, conformational evolution, and organometallic intermediates. Usually, STM/nc-AFM topological images, in situ spectroscopy data, and DFT studies are combined to describe the mechanism behind on-surface organic reactions.

(Invited) On the Inhibition Mechanism of an Alkylammonium Nitrate Ionic Liquid Family on Al Alloys: Electrochemical and XPS/in Situ PM-IRRAS Studies

AA2024 are high strength Al alloys widely used in the aerospace industry due to their excellent strength to weight ratio. However, their high susceptibility to localised corrosion, caused by the presence of intermetallic particles embedded in the Al matrix, requires the use of chromate-based surface treatment formulations1. Several alternatives have been explored in the last decades, such as vanadates, chromium (III) based conversion coatings, rare-earth inhibitors or more recently, lithium containing conversion coatings2. However, these alternatives are often not versatile (cannot be transposed to all alloy systems) or are not as cost effective as chromates. Ionic liquids (ILs), usually defined as salts with a melting point below 100°C, can be considered as promising corrosion inhibitors due to their low vapour pressure, chemical and thermal stability3. In addition, they can exhibit versatile inhibition properties associated with the quasi-infinite possible anion-cation combinations3–5. In previous work, the effect of an alkylammonium nitrate IL family, namely ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and butylammonium nitrate (BAN) as potential corrosion inhibitors for an AA2024-T6 Al alloy was investigated6. Electrochemical tests such as potentiodynamic polarisation and electrochemical impedance spectroscopy were performed to identify the effect of the anion and cation on the anodic and cathodic kinetics. Herein, surface analysis was performed to rationalise the electrochemical results, particularly on the reactivity of NO3 - and R-NH3 + on the Cu-rich intermetallic particles. To this end, electrospray ionisation of EAN, BAN and NaNO3 was realised on Cu. The effect of the IL concentration and cation chain length, using PM-IRRAS (polarisation modulation -infrared reflection adsorption spectroscopy) was used to investigate molecule orientation and surface-molecule bonding as a function of surface coverage; The results provided new insights on the inhibition mechanism of this IL family. O. Gharbi, S. Thomas, C. Smith, and N. Birbilis, Npj Mater Degrad, 2, 12 (2018) http://www.nature.com/articles/s41529-018-0034-5.A. Kosari et al., Corros Sci, 190, 109651 (2021) https://doi.org/10.1016/j.corsci.2021.109651.C. Verma, E. E. Ebenso, and M. A. Quraishi, J Mol Liq, 233, 403–414 (2017) http://dx.doi.org/10.1016/j.molliq.2017.02.111.J. Sun, P. C. Howlett, D. R. MacFarlane, J. Lin, and M. Forsyth, Electrochim Acta, 54, 254–260 (2008).N. V. Likhanova et al., Corros Sci, 52, 2088–2097 (2010) http://dx.doi.org/10.1016/j.corsci.2010.02.030.A. Z. Benbouzid et al., Corrosion Communications, 9, 57–64 (2023).

Characterization of Molecular Interactions in the Bondline of Composites from Plasma-Treated Aluminum and Wood.

Wood and aluminum composites are becoming increasingly attractive due to their ability to combine the advantages of both materials: the lightweight nature of wood and the strength of aluminum. However, using conventional wood adhesives like polyvinyl acetate (PVAc) to bond these dissimilar materials is challenging and requires special surface treatments. Prior studies have demonstrated that applying a dielectric barrier discharge plasma treatment significantly enhances shear and bending strengths in beech wood/aluminum bonds. This study focuses on the molecular interactions between PVAc and aluminum or beech wood influenced by plasma surface modification. Surface-sensitive methods, including X-ray photoelectron spectroscopy, infrared reflection adsorption spectroscopy and atomic force microscopy, were employed to characterize the PVAc films on the corresponding surfaces and to identify possible interactions. The ultrathin PVAc films required for this purpose were deposited by spin coating on untreated and plasma-treated aluminum. The aluminum surface was cleaned and oxidized by plasma. Additionally, hydroxyl species could be detected on the surface. This can lead to the formation of hydrogen bonds between the aluminum and the carbonyl oxygen of PVAc after plasma treatment, presumably resulting in increased bond strength. Furthermore, the beech wood surface is activated with polar oxygen species.

SPECTRAL-LUMINESCENT AND ELECTROPHYSICAL STUDY OF RADIATION OXIDATION OF BERYLLIUM IN WATER

The radiation oxidation of Be in water at room temperature in the absorbed dose range Dγ = 0.5…180 kGy was studied by radiothermoluminescence (RTL), infrared reflection-adsorption spectroscopy (IRRAS), and electrical conductivity. The participation and role of surface relaxing intermediate-active particles in the dynamics of changes in the oxidation process are considered. Using the RTL method, the role of surface oxygen hole centers generated by γ-irradiation and chemisorbed oxygen in the formation of nanooxide films was experimentally established. The formation of nanooxide films on the surface of Be in water was traced in the IR reflection spectra. The kinetics of radiation oxidation of beryllium has been studied and its radiation passivation has been established. According to the logarithmic dose dependence of the surface resistivity Be, two stages of the oxidation process were revealed. It is shown that the formation of nano oxide films leads to a decrease in the surface electrical conductivity of beryllium by 2 orders of magnitude and an increase in the thickness of oxide films by 1.6 times.

Interaction of Cyanoacrylate with Metal Oxide Surfaces (Cu, Al).

Cyanoacrylates are an extremely reactive class of adhesives. Despite their commercial use as instant adhesives, the adhesion mechanism, especially to technically relevant oxidized metal surfaces, has not yet been sufficiently investigated. In the present work, ultra-thin ethyl cyanoacrylate films are deposited on copper oxide and aluminum oxide by spin coating and cured there. Various surface sensitive spectroscopy methods are used to identify possible interactions. X-Ray photoelectron spectroscopy (XPS) indicates, among other information, hydrogen bonding of the carbonyl group to the oxidized surfaces. Metastable induced electron spectroscopy (MIES) measurements support the theory of this preferential molecular orientation. In addition, XPS shows the presence of an ionic carboxylate (COO- ) species at the interface. Infrared reflection adsorption spectroscopy (IRRAS) measurements confirm this ionic interaction and furthermore allow to investigate the influence of water on the reaction. A possible interaction mechanism of cyanoacrylates with metal oxides could be proposed. The formation of a carboxylate species probably occurs by hydrolysis of the ethyl group via the intermediate of a carboxyl (COOH) species.

Enhanced Electrochemical CO2 Reduction to Formate on Poly(4-vinylpyridine)-Modified Copper and Gold Electrodes.

Developing active and selective catalysts that convert CO2 into valuable products remains a critical challenge for further application of the electrochemical CO2 reduction reaction (CO2RR). Catalytic tuning with organic additives/films has emerged as a promising strategy to tune CO2RR activity and selectivity. Herein, we report a facile method to significantly change CO2RR selectivity and activity of copper and gold electrodes. We found improved selectivity toward HCOOH at low overpotentials on both polycrystalline Cu and Au electrodes after chemical modification with a poly(4-vinylpyridine) (P4VP) layer. In situ attenuated total reflection surface-enhanced infrared reflection-adsorption spectroscopy and contact angle measurements indicate that the hydrophobic nature of the P4VP layer limits mass transport of HCO3– and H2O, whereas it has little influence on CO2 mass transport. Moreover, the early onset of HCOOH formation and the enhanced formation of HCOOH over CO suggest that P4VP modification promotes a surface hydride mechanism for HCOOH formation on both electrodes.

The Role of Cholesterol in the Interaction of the Lipid Monolayer with the Endocrine Disruptor Bisphenol-A.

Among pollutants of emerging concern, endocrine disruptors (ED) have been shown to cause side effects in humans and animals. Bisphenol-A (BPA) is an ED by-product of the plastic industry and one of the chemicals with the highest volume produced yearly. Here, we studied the role of cholesterol in the BPA exposure effects over membrane models. We used Langmuir films of both neat lipid DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and cholesterol (Chol) and a binary mixture containing DPPC/Chol, exposing it to BPA. We evaluate changes in the π-A isotherms and the PM–IRRAS (polarization modulation–infrared reflection adsorption spectroscopy) spectra. BPA exposure induced changes in the DPPC and Chol neat monolayers, causing mean molecular area expansion and altering profiles. However, at high surface pressure, the BPA was expelled from the air–water interface. For the DPPC/Chol mixture, BPA caused expansion throughout the whole compression, indicating that BPA is present at the monolayer interface. The PM–IRRAS analysis showed that BPA interacted with the phosphate group of DPPC through hydrogen bonding, which caused the area’s expansion. Such evidence might be biologically relevant to better understand the mechanism of action of BPA in cell membranes once phosphatidylcholines and Chol are found in mammalian membranes.

Parity Effects in the Physicochemical Properties of Self-Assembled Monolayers

Large-area metal/molecule/metal junctions based on self-assembled monolayers (SAMs) of organothiolates are of interest for investigations of charge transport across ultrathin organic films and the fabrication of molecular electronic devices. Several recent reports demonstrate an odd-even dependency in the rate of charge transport through SAMs of the insulating n-alkanethiolates (CH3C n S)1,2 or the redox-active ferrocenylalkanethiolates (FcC n S)3. This parity effect has been attributed to differences in the orientation of the terminal functional group and alkyl chain packing in SAMs consisting of an odd versus even number of methylene repeat units.We have sought experimental evidence for odd-even distinctions in the molecular organization of these SAMs using infrared reflection-adsorption spectroscopy (IRRAS), electrochemistry, and contact angle goniometry. The findings of the investigations will be presented in this talk and the molecular origin of the parity effect of SAM macroscopic properties will be discussed.

Morphology of Vapor-Deposited Acetonitrile Films

Crystalline acetonitrile has two polymorphs, a high-temperature (HT) phase that is stable between 217 K and its melting point at 229 K and a low-temperature (LT) phase that is stable below 217 K. Solid acetonitrile films can be prepared by vapor deposition in an ultrahigh vacuum chamber. To prevent sublimation of the film, temperatures are often kept below 150 K. While the LT phase is thermodynamically favored at these low temperatures, such preparation usually results in the formation of the metastable HT polymorph. In this work we use reflection adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) experiments to investigate the effects of the deposition temperature and underlying substrate on the morphology of acetonitrile films prepared with molecular beam deposition. We obtained the elusive LT phase when dosing at 120 K on a graphene substrate and on a crystalline decane layer. Dosing acetonitrile on other surfaces produced the HT phase, as did annealing of amorphous films. We used TPD experiments to determine the Gibbs energy difference between the HT and the LT phases. Our ΔG values agree with extrapolation of equilibrium calorimetry data. We also observed that acetonitrile films were amorphous when dosed at temperatures ≤ 60 K and porous for temperatures ≤ 50 K.

The prion fragment PrP106-127 adopts a secondary structure typical of aggregated fibrils in langmuir monolayers of brain lipid extract

Understanding protein aggregation is essential to unveil molecular mechanisms associated with neurodegenerative diseases such as Alzheimer’s, Huntington’s and spongiform encephalopathy, particularly to determine the role of interaction with cell membranes. In this study, we employ Langmuir monolayers as cell membrane models to mimic interaction with the peptide KTNMHKHMAGAAAAGAVVGGLG−OH, a fragment from the human prion protein including residues 106−127, believed to be involved in protein aggregation. Using in situ polarization-modulated infrared reflection adsorption spectroscopy (PM-IRRAS) for Langmuir monolayers and FTIR for solid films, we found that PrP106−127 adopts mainly β-sheets, random coils and β-turns in Langmuir monolayers and in Langmuir-Blodgett (LB) and cast films. This also applies to monolayers and solid films made with PrP106−127 and a brain total lipid extract (BTLE). In contrast, some α-helices are observed in the secondary structure of PrP106−127 in monolayers, and especially in solid films, of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). In summary, in a model representing brain cells (BTLE), the secondary structure of PrP106−127 is typical of fiber aggregates, while aggregation is unlikely if PrP106−127 interacts with a membrane model (DOPC) characteristic of mammalian cells.

Platinum and Palladium Monolayer Electrocatalysts for Formic Acid Oxidation

The direct formic acid fuel cell holds great promise as a next generation portable power source. Here we report an experimental study on Pt and Pd monolayer (Pt* and Pd*) atop the close-packed facet of transition metal substrates (denoted Pt*/M or Pd*/M, where M = Pt, Pd, Ir, Au, Rh and Ru) model catalysts for formic acid oxidation (FAO) and demonstrate a promotional effect on Pt*/Au(111) and Pd*/Pt(111) surfaces. Contrary to findings on monometallic surfaces, in situ infrared reflection adsorption spectroscopy collected during formic acid oxidation at potentials anodic of 375 mV reveal that the poisoning reaction intermediate CO is not formed on the monolayer surfaces, suggesting a promotion of the direct mechanisms of FAO at the expense of the indirect mechanism through CO. Our density functional theory calculations on model surfaces with 1:1 stoichiometry for overlayer to support metal atoms for the thermochemistry of the elementary steps defined by adsorbed CO, OH, COOH, and HCOO on the (111) facet of Pt, PtML/Au, Pd, and PdML/Pt largely attribute the promotional effect observed for the monolayer catalysts to easier removal of CO brought about by easier activation of water, relative to the monometallic catalysts. This synergy between experiments and theory brings us closer to a fundamental understanding of FAO electrocatalysts under reaction conditions, paving the way to achieving more active electrocatalysts possibly with less Pt or Pd; a pre-requisite for the mass commercialization of this technology.

Experimental methods in chemical engineering: Fourier transform infrared spectroscopy—FTIR

AbstractWhen molecules absorb infrared radiation (IR), their vibrational mode—stretching and bending of the electric dipole—changes to an excited state. Functional groups in organic molecules absorb IR related to their characteristic vibrational modes. A Fourier transform infrared absorption (FTIR) analyzer measures the absorbed IR to identify molecular composition of surfaces, structural and geometric isomers, orientation in polymers and solutions, and quantify impurities. We describe the power of FTIR instruments and their basic operating principles, including the main experimental setups available: transmission, diffuse reflectance (DRIFTS), reflection adsorption infrared spectroscopy (RAIRS), and attenuated total reflection (ATR), including the recent advances related to time‐resolved and operando applications. In catalytic studies, FTIR spectroscopy has demonstrated its versatility over the last several decades to understand reaction mechanisms, measure gas phase composition, and identify active sites. Over 3000 articles include catalysis and FTIR as keywords but 50 000 articles per year mention IR. We generated a bibliometric map of keywords in articles that Web of Science indexed in 2016 and 2017. The map identified four broad clusters of research related to or applying FTIR: nano‐composites, composites, and mechanical properties; nano‐particles, degradation, graphene oxide, and photo‐catalysis; adsorption, aqueous solutions, and waste water; and drug delivery, silver and gold nano‐particles, green synthesis, and antibacterial activity. Together with a synopsis of the principals of IR spectroscopy and a review of the applications, we discuss uncertainties and limitations of the technique.

Two/three-dimensional interfacial properties of the novel peptide as a selective destroyer of biomembrane

Peptides are widely used in biomedicine field for their good biocompatibility and variable secondary structures. In this study, a novel amphiphilic peptide (amPE) was designed for antimicrobial agent. Its thermal responsiveness in solution and in liposome membrane was studied by circular dichroism (CD). Then, amPE’s adsorption and further insertion behaviors at lipid monolayer were investigated by Langmuir-Blodgett (LB) and polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS). Results showed that amPE could selectively insert into DPPG monolayer due to the electrostatic interaction and cause an increase in content of α-helix. Adsorption kinetics under different initial surface pressures showed higher surface pressure prevented peptide’s insertion. Then, experiments of dye leakage from liposomes were carried for discussing how amPE disturbed the liposome membrane. Serious dye leakage from DPPG liposome was observed and the leakage was further improved when temperature increased from 37 °C to 45 °C. The designed amPE showed its good potential as antimicrobial/antitumor agent and great superiority of thermal responsiveness.

Surface-Driven Keto-Enol Tautomerization: Atomistic Insights into Enol Formation and Stabilization Mechanisms.

Tautomerisation of simple carbonyl compounds to their enol counterparts on metal surfaces is envisaged to enable an easier route for hydrogenation of the C=O bond in heterogeneously catalyzed reactions. To understand the mechanisms of enol formation and stabilization over catalytically active metal surfaces, we performed a mechanistic study on keto-enol tautomerization of a monocarbonyl compound acetophenon over Pt(111) surface. By employing infrared reflection adsorption spectroscopy in combination with scanning tunneling microscopy, we found that enol can be formed by building a ketone-enol dimer, in which one molecule in the enol form is stabilized through hydrogen bonding to the carbonyl group of the second ketone molecule. Based on the investigations of the co-adsorption behavior of acetophenone and hydrogen, we conclude that keto-enol tautomerization occurs in the intramolecular process and does not involve hydrogen transfer through the surface hypothesized previously.

Surface‐Driven Keto–Enol Tautomerization: Atomistic Insights into Enol Formation and Stabilization Mechanisms

AbstractTautomerisation of simple carbonyl compounds to their enol counterparts on metal surfaces is envisaged to enable an easier route for hydrogenation of the C=O bond in heterogeneously catalyzed reactions. To understand the mechanisms of enol formation and stabilization over catalytically active metal surfaces, we performed a mechanistic study on keto–enol tautomerization of a monocarbonyl compound acetophenon over Pt(111) surface. By employing infrared reflection adsorption spectroscopy in combination with scanning tunneling microscopy, we found that enol can be formed by building a ketone–enol dimer, in which one molecule in the enol form is stabilized through hydrogen bonding to the carbonyl group of the second ketone molecule. Based on the investigations of the co‐adsorption behavior of acetophenone and hydrogen, we conclude that keto–enol tautomerization occurs in the intramolecular process and does not involve hydrogen transfer through the surface hypothesized previously.

CO2 Adsorption on Magnetite Fe3O4(111)

We monitored the adsorption of carbon dioxide (CO2) on well-ordered Fe3O4(111) films using infrared reflection-adsorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The result...

Disruption of Tumor Cells Using a pH-Activated and Thermosensitive Antitumor Lipopeptide Containing a Leucine Zipper Structure.

Antitumor peptides may potentially alleviate the problem of chemoresistance but do not yet target tumor cells and would be cytotoxic to normal cells. Here, we designed a pH-activated and thermosensitive lipopeptide (C6-Pep) containing a leucine zipper and an alkyl chain and assessed the ability of C6-Pep to kill cancer cells. Pep, the same sequence without the N-terminal hexanoic acid moiety, was generated as a less hydrophobic control. First, lipopeptide adsorption into lipid monolayers was studied using Langmuir-Blodgett and polarization modulation infrared reflection adsorption spectroscopy. Under weakly acid conditions, electrostatic interactions between C6-Pep and negatively charged phospholipids increased the adsorption/insertion of C6-Pep (vs Pep) into lipid monolayers. Cargo leakage from liposomes was assayed to model lipopeptide-induced lipid membrane disruption. The ability of C6-Pep to disrupt liposomes depended on the peptide molecular structure/hydrophobicity, solution pH, and temperature-induced uncoiling of the zipper structure; the greatest cargo leakage from the liposome with negative charge was observed for C6-Pep at pH 5.5 under mildly hyperthermic conditions (45 °C). In vitro, C6-Pep was significantly more cytotoxic toward HeLa cells at pH 5.5 under hyperthermic conditions than at pH 7.4 and/or 37 °C. Overall, this study demonstrates that amphipathic C6-Pep can insert into cell membranes in the low-pH tumor microenvironment, whereas the application of heat promotes the uncoiling of the zipper structure, leading to the disruption of tumor cell membranes and cell death. pH-activated and thermosensitive C6-Pep represents a promising tool to kill cancer cells via a strategy that does not invoke chemoresistance and may have low side effects.

Ternary Pt-Rh-SnO2 Catalyst Synthesized from Vapor Phase for Ethanol Oxidation

Ethanol is an ideal fuel for direct oxidation fuel cells for portable and mobile application as it offers multiple advantages over hydrogen and methanol. The most important of these are ease of transportation, storage and handling, and high energy density [1]. Platinum and palladium based catalyst are commonly used for ethanol oxidation. The major issue with current catalysts is their inability to cleave the C-C bond in ethanol [2] resulting in incomplete oxidation of ethanol [3]. As a result the fuel cell operates at low efficiency. Hence, bimetallic or trimetallic catalyst are developed to both promote the C-C bond cleavage and to stabilize the hydroxyl group for reducing CO poisoning [4]. In particular, the Pt-Rh-SnO2 catalyst showed predominant oxidation of ethanol to CO2. SnO2 provides hydroxyl species to oxidize the dissociated CO at Rh sites and Pt facilitate ethanol dehydrogenation. The electronic structure of Rh is modified to afford moderate bonding to ethanol, intermediates and products, promoting the C-C bond cleavage.[5,6] Preliminary results of Pt-Rh-SnO2 (3:1:3) catalyst synthesis via reactive spray deposition technology (RSDT) shows promising activity for ethanol oxidation (Figure 1a) and good fuel cell performance (Figure 1b). This can be attributed to the porous electrode morphology that provide high surface area for the gas diffusion electrode (Figure 1c). However, in-situ infrared reflection-adsorption spectroscopy (IRRAS) in the electrochemical cell suggests mixed products of carbon dioxide, acetic acid, and acetaldehyde, as shown in Figure 1d. This implies that the mixing between Pt, Rh, and SnO2 could be inhomogeneous and the interplay between the three components could be hindered.[6] This work focus on the optimization of the interface between three components by varying the relative atomic ratio of Pt, Rh, and Sn. Furthermore, the effect of RSDT process parameters, such as fuel and gas flow rate, precursor solubility, on the precursor mixing are investigated. The surface elemental components and chemical environment are characterized using X-ray photoelectron spectroscopy (XPS). In addition, high-resolution transmission electron microscopy (HRTEM) is employed to study the elemental distribution of the catalyst particles and investigate the phase separation of Pt, Rh, and SnO2. Figure 1. (a) Electrochemical measurement of ethanol oxidation with rotating disk electrode; (b) fuel cell performance Anode: RSDT deposited gas diffusion electrode of Pt-Rh-SnO2, Cathode 45 wt% Pt/C, 2 mg/cm2. Membrane: Nafion 115; (c) Surface morphology of Pt-Rh-SnO2 on gas diffusion electrode; (d) In situ IRRAS of ethanol oxidation at various potentials form -0.2 V to 0.55 V vs. Ag/AgCl. Acknowledgment This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Monitoring the Interaction of CO with Graphene Supported Ir Clusters by Vibrational Spectroscopy and Density Functional Theory Calculations

The interaction of carbon monoxide (CO) with graphene supported Ir cluster (Ir/graphene/Ir(111)) and a Ir(111) single crystal surface was studied by infrared reflection–adsorption spectroscopy (IRRAS). The cluster morphology was characterized by scanning tunneling microscopy and density functional theory (DFT) calculations predicted the adsorption frequencies of CO molecules on the Ir single crystal surface and clusters. After exposing the clean Ir(111) surface to CO at 195 K, one intense vibrational band is observed at 2043 cm–1, which is assigned to on top CO species. This band shifts to a much higher frequency at 2082 cm–1 at higher CO exposure. After exposing clean graphene/Ir(111) to CO at 195 K, no CO band was observed in the IR spectra, which confirms a full graphene layer over the Ir(111) surface. However, CO molecules adsorb on Ir clusters supported on graphene/Ir(111) at 195 K. For the 0.05, 0.1, 0.15, and 0.2 ML Ir clusters, two IR bands were observed at 2060 and 2088 cm–1, 2050 and 2070 cm–1, ...

Cospreading of Anionic Phospholipids with Peptides of the Structure (KX)4K at the Air-Water Interface: Influence of Lipid Headgroup Structure and Hydrophobicity of the Peptide on Monolayer Behavior.

Mixtures of anionic phospholipids (PG, PA, PS, and CL) with cationic peptides were cospread from a common organic solvent at the air-water interface. The compression of the mixed film was combined with epifluorescence microscopy or infrared reflection adsorption spectroscopy (IRRAS) to gain information on the interactions of the peptide with the different lipids. To evaluate the influence of the amino acid X of peptides with the sequence (KX)4K on the binding, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) was mixed with different peptides with increasing hydrophobicity of the uncharged amino acid X. The monolayer isotherms of DPPG/(KX)4K mixtures show an increased area for the lift-off due to incorporation of the peptide into the liquid-expanded (LE) state of the lipid. The surface pressure for the transition from LE to the liquid-condensed (LC) state is slightly increased for peptides with amino acids X with moderate hydrophobicity. For the most hydrophobic peptide (KL)4K two plateaus are seen at a charge ratio PG to K of 5:1, and a strongly increased transition pressure is observed for a charge ratio of 1:1. Epifluorescence microscopy images and infrared spectroscopy show that the lower plateau corresponds to the LE-LC phase transition of the lipid. The upper plateau is connected with a squeeze-out of the peptide into the subphase. To test the influence of the lipid headgroup structure on peptide binding (KL)4K was cospread with different anionic phospholipids. The shift of the isotherm to larger areas for lift-off and to higher surface pressure for the LE-LC phase transition was observed for all tested anionic lipids. Epifluorescence microscopy reveals the formation of LC domains with extended filaments indicating a decrease in line tension due to accumulation of the peptides at the LC-domain boundaries. This effect depends on the size of the headgroup of the anionic phospholipid.