Air-buffer Interface Research Articles

Effect of cationic dendrimer on membrane mimetic systems in the form of monolayer and bilayer

Interactions between a zwitterionic phospholipid, 1, 2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and four anionic phospholipids dihexadecyl phosphate (DHP), 1, 2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG), 1, 2-dipalmitoyl-sn-glycero-3-phosphate (DPP) and 1, 2-dipalmitoyl-sn-glycero-3-phospho ethanol (DPPEth) in combination with an additional amount of 30 mol% cholesterol were separately investigated at air-buffer interface through surface pressure (π) - area (A) measurements. π-A isotherm derived parameters revealed maximum negative deviation from ideality for the mixtures comprising 30 mol% anionic lipids. Besides the film functionality, structural changes of the monomolecular films at different surface pressures in the absence and presence of polyamidoamine (PAMAM, generation 4), a cationic dendrimer, were visualised through Brewster angle microscopy and fluorescence microscopic studies. Fluidity/rigidity of monolayers were assessed by surface dilatational rheology studies. Effect of PAMAM on the formation of adsorbed monolayer, due to bilayer disintegration of liposomes (DPPC:anionic lipids= 7:3 M/M, and 30 mol% cholesterol) were monitored by surface pressure (π) - time (t) isotherms. Bilayer disintegration kinetics were dependent on lipid head group and chain length, besides dendrimer concentration. Such studies are considered to be an in vitro cell membrane model where the alteration of molecular orientation play important roles in understanding the nature of interaction between the dendrimer and cell membrane. Liposome-dendrimer aggregates were nontoxic to breast cancer cell line as well as in doxorubicin treated MDA-MB-468 cell line suggesting their potential as drug delivery systems.

The effect of the gold nanoparticles and gold nano-clusters on the behavior of natural lung surfactant

One of the initial barriers encountered by the airborne nanoparticles (NPs) upon entry into the body is the lung surfactant (LS), which plays a vital role in maintaining the normal respiratory cycle of the lungs. Interaction between gold nanoparticles (AuNPs), which are commonly used in many areas, especially in pharmaceutical applications due to their functional features and LS has been evaluated by combining Langmuir Trough (2D) and Pendant-Drop tensiometer (3D) approaches. AuNPs were synthesized via Turkevich method and AuNP clusters were prepared by using dithiothreitol (DTT) with the different concentrations. Model LS was extracted from a bovine lung and brought into vesicular form after purification. Our results showed that AuNP-DTT clusters are able to penetrate into the LS monolayer and vesicles, change the normal behavior of air-buffer interface, organization of the molecules and phase behavior of the system. In 2D studies, it is seen that the interaction of AuNPs and AuNP-DTT clusters decreases the surface pressure of phase transition region which indicates decreasing the elasticity with a similar manner for first 3 h. In the longer periods, the monolayer presented an increased resistance to compression, indicating the low compressibility of monolayer. After 7 h interaction with AuNP-DTT clusters, at the same surface area (48 cm2), the π of LS films can just reach ∼30 mN·m−1 instead of ∼45 mN·m−1 which proves the induction of the isotherm to form a more disordered structure with lower cohesion between the molecules. In 3D studies, it is found that there becomes a competitive adsorption between LS vesicles and AuNP-DTT clusters in the long-term interaction which may enhance the pulmonary toxicity of the inhaled nanostructure. The ultimate insight provided by this study is to evaluate the toxicity of the interaction of asymmetrical NPs on biological barriers with a realistic approach. As a result of our investigations, it is proved that long-term exposure of anisotropic clusters can cause potential adverse effects on the healthy environment and breathing cycle of lungs more than smaller individual counterparts with the same concentration.

The interaction between BSA and DOTAP at the air-buffer interface

In this article, the interaction between bovine serum albumin (BSA) and the cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) at the air-buffer interface was investigated at different subphase’s pH values (pH = 3, 5 and 10). Surface pressure measurements (π − A) and penetration kinetics process (π − t) were carried out to reveal the interaction mechanism and the dynamical behavior. The data showed that π − A isotherms moved towards larger mean molecular area when the concentration of BSA ([BSA]) increased, the amount of BSA adsorbed onto DOTAP monolayer reached a threshold value at a [BSA] of 5 × 10−8 M, and BSA desorbed from the lipid monolayer as time goes by. The results revealed that the association of BSA with DOTAP at the air-buffer interface was affected by the subphase’s pH value. When pH = 10, the interaction mechanism between them was a combination of hydrophobic interaction and electrostatic attraction, so BSA molecules could be well separated and purified from complex mixtures. AFM images demonstrated that pH value and [BSA] could affect the morphology feature of DOTAP monolayer and the adsorption and desorption processes of BSA. So the study provides an important experimental basis and theoretical support for learning the interaction mechanism among biomolecules in separation and purification of biomolecules and biosensor.

Mechanical oscillation of dynamic microtubule rings

Microtubules driven by kinesins self-assemble into ring-shaped structures which exhibit dynamic reorganization at an air-buffer interface.

The behavior of the adsorption of cytochrome C on lipid monolayers: A study by the Langmuir–Blodgett technique and theoretical analysis

Cytochrome c (Cyt c) is an essential component of the inner mitochondrial respiratory chain because of its function of transferring electrons. The feature is closely related to the interaction between Cyt c and membrane lipids. We used Langmuir–Blodgett monolayer technique combined with AFM to study the interaction of Cyt c with lipid monolayers at air–buffer interface. In our work, by comparing the mixed Cyt c-anionic (DPPS) and Cyt c-zwitterionic (DPPC/DPPE) monolayers, the adsorption capacity of Cyt c on lipid monolayers is DPPS>DPPE>DPPC, which is attributed to their different headgroup structures. π–A isothermal data show that Cyt c (v=2.5μL) molecules are at maximum adsorption quantity on lipid monolayer. Moreover, Cyt c molecules would form aggregations and drag some lipids with them into subphase if the protein exceeds the maximum adsorption quantity. π–T curve indicates that it takes more time for Cyt c molecular conformation to rearrange on DPPE monolayer than on DPPC. The compressibility study reveals that the adsorption or intermolecular aggregation of Cyt c molecules on lipid monolayer will change the membrane fluidization. In order to quantitatively estimate Cyt c molecular adsorption properties on lipid monolayers, we fit the experimental isotherm with a simple surface state equation. A theoretical model is also introduced to analyze the liquid expanded (LE) to liquid condensed (LC) phase transition of DPPC monolayer. The results of theoretical analysis are in good agreement with the experiment.

Interface-controlled calcium phosphate mineralization: effect of oligo(aspartic acid)-rich interfaces

The phase behavior of an amphiphilic block copolymer based on a poly(aspartic acid) hydrophilic block and a poly(n-butyl acrylate) hydrophobic block was investigated at the air–water and air–buffer interface.

Mixed alkaline phosphatase/sphingomyelin monolayer at the air-buffer interface: phase behavior and morphology

The glycosylphosphatidyl inositol (GPI)-anchored proteins are localized on the outer of the plasma membrane and clustered in membrane microdomain known as lipid rafts. Among them, mammalian alkaline phosphatase (AP) is an enzyme widely distributed. So, it has important biological significance to study the combination of AP with lipid monolayer. In our work, the interaction between AP and sphingomyelin has been studied at the air-buffer interface as a biomimetic membrane system by the Langmuir film technique and atomic force microscopy. The surface pressure-area isotherm for the mixed alkaline phosphatase/sphingomyelin monolayer shown the presence of a transition from a liquid-expanded phase to the liquid-expanded/liquid-condensed coexist phase. And the surface compressional modulus suggested the mixed alkaline phosphatase/sphingomyelin monolayer has larger compressibility compared with the pure sphingomyelin monolayer. Besides, according to the micrographs, we inferred when combined with lipid monolayer at the air-buffer interface, the AP molecules formed polymer not multilayer or micelle. And, according to the limiting molecules area of AP, we inferred that 12 AP molecules formed a hexagon polymer unit.

Determining Surface Activity and Membrane Interactions of Ranaspumin-2 and an Engineered Derivative, Surfactant Resisting Foam Formation

Ranaspumin-2 (Rsn-2) is a surfactant protein identified as one of the major components of tungara frog foam nests which protects fertilized eggs from dehydration, temperature changes, and potential pathogens. Unlike chemical detergents, the protective protein does not disrupt cell membranes making it a strong candidate for agricultural applications, save for its foaming upon agitation which can result in wind resistance and reduced coverage. Surfactant Resisting Foam formation (SRFN) protein was engineered to retain the biological compatibility of Rsn2, ultimately the same surface tension reduction, but contains a more flexible hinge point in the clamshell-like structure which reduces foaming. Both Rsn-2 and SRFN exhibited appreciable surface activity down to sub-μg/ml concentrations without a lag time, measured as surface pressure at the air-buffer interface. Mixing and agitation, to model environmental conditions, has limited effect on the surfactant qualities of Rsn-2 and SRFN. Rsn-2 formed a significant amount of foam that dissipated slowly, but mixing did not affect the resultant surface pressure or equilibration time. For SRFN, a protein of equal size, the main effect of agitation was to accelerate the equilibration time of protein adsorbing to the interface; direct agitation led to little foaming. To confirm that the site-direction mutations in the hinge region of SRFN do not enhance the protein's ability to insert into a lipid membrane, we modeled the outer leaflet of the cell membrane using an egg phosphatidylcholine lipid monolayer at the air-buffer interface and saw no insertion as physiologically relevant pressures.

Chitosan as a subphase disturbant of membrane lipid monolayers. The effect of temperature at varying pH: I. DPPG

Temperature-mediated effects of chitosan dissolved in acetate buffer at different pHs, on the structural and thermodynamic characteristics of dipalmitoylphosphatidylglycerol (DPPG) monolayers, employed here as a model of the bacterial cell membrane, were investigated. The investigation was done to improve the understanding if in its antibacterial activity chitosan disturbs the bacterial cell membrane. The Langmuir film technique was employed, where the compression surface pressure–area (π–A) isotherms of DPPG monolayer formed at the air-buffer interface were measured at five temperatures in the range 15–37°C for three pHs in the range 3.5–6.0, with the chitosan concentration in the subphase varied between 0.002 and 1mg/mL. Illustrating the extent of DPPG–chitosan interactions, the characteristics of the monolayers on the chitosan-containing solutions were assessed by monitoring differences in the course of the π–A isotherms relative to pure DPPG films. The characteristics revealed that chitosan interacted with the lipid film not only superficially but also inserted to a certain degree into the film. Increasing temperature enhanced these effects, the more strongly, the lower was the pH. Furthermore, the transition of the monolayer from the liquid-expanded to liquid-condensed phase was found to be an endothermic process accompanied by an increase in disorder. The effects were most pronounced at pH 3.5, and were markedly enhanced by chitosan. Most interestingly, the analysis of the critical temperatures provided evidence that the interaction of chitosan with DPPG monolayers is not only due to electrostatic but also to non-electrostatic contributions. The most effective disturbing effects of chitosan on DPPG monolayer were observed at the highest temperature 37°C applied at pH 3.5.

Adsorption of the antimicrobial peptide arenicin and its linear derivative to model membranes – A maximum insertion pressure study

Antimicrobial peptides (AMPs) are widely distributed in nature representing an ancient tool of host defence against bacterial invasion. The ability of the cyclic cationic antimicrobial peptide arenicin, Ar-1, and its novel and highly efficient linear derivative, C/S-Ar-1, to interact with 2D model membranes (POPC, DPPC, POPG, and DPPG) has been studied. Different phospholipid monolayers at the air–buffer interface, as simple and easy to manipulate models of the membrane surface, were used for these studies. The equilibrium adsorption pressure of both peptides depends on the charge of the lipid head group as well as on the lipid packing density (the largest value is observed for POPG). Only in the case of DPPG, the original Ar-1 penetrates much faster into the DPPG monolayer compared to the linear derivative C/S-Ar-1. Additionally, C/S-Ar-1 has been injected under lipid layers compressed to different starting pressures. In this way, the maximum insertion pressure (MIP) has been determined. Only for the negatively charged POPG, the MIP is clearly above the commonly accepted membrane pressure values, whereas they are between 30 and 34mN/m for the zwitterionic lipids and the condensed DPPG. The results point to the importance of the membrane charge and fluidity. Electrostatic interactions can be clearly enhanced by hydrophobic ones.

Effect of bovine serum albumin on the functionality and structure of catanionic surfactant at air–buffer interface

Interaction of bovine serum albumin (BSA) with the solvent spread monolayer of a catanionic surfactant, octadecyltrimethylammonium dodecylsulfate, (C18TA+DS−) at the air–buffer interface was investigated by measuring the surface pressure with time and change in surface area. Dipalmitoylphosphatidylcholine (DPPC) was used as reference. Kinetics of BSA desorption from the interface to the buffer subphase, that of C18TA+DS− and DPPC through their interaction with BSA, were also studied at different BSA concentrations (in the subphase) and surface pressures. Surface pressure (π)–area (A) isotherms (at pH=5.4, μ=0.01, T=298K) revealed that the coacervate/DPPC monolayer becomes expanded in the presence of BSA at low π while their protein bound species are released into the subphase at high π. Film morphology, studied by epifluorescence microscopy (EFM) and atomic force microscopy (AFM), reveals that the sizes of the domains of both DPPC and coacervate decrease in the presence of BSA. Presence of BSA in the coacervate and DPPC monolayer was supported from AFM data analysis.

Formation of ring-shaped assembly of microtubules with a narrow size distribution at an air–buffer interface

Biopolymers such as actin, microtubules and DNA are well known for their fascinating in vivo self-organization phenomena. Considerable efforts have been devoted to mimicking their organization process in vitro that produced ring-shaped or toroid structures in an irreversible manner. However, understanding the factors that lead to formation of such assembled structures deserves more investigation to achieve a unified insight into the assembly process, particularly of the microtubules. Here, we report an active assembly process of microtubules (MTs) at an air–buffer interface that resulted in ring-shaped microtubule structures with a narrow size distribution and a high yield. Using an “air–buffer interface control system” combined with the newly developed “inert chamber system (ICS)” we have also successfully observed the reversible conformational transition between ring- and linear-shaped microtubules at the air–buffer interface. This is the first ever direct in situ observation of a reversible assembly process of MTs and probably provides us with valuable discernment to understand the in vivo organizational behavior of biopolymers.

Mechanical reorganization of cross-linked F-actin networks at the air-buffer interface

The response of an F-actin network at the air-buffer interface to nonlinear compression is studied. We observe two distinct classes of behavior: a mechanically induced structural reorganization of the monolayer and a subsequent reversible, nonlinear mechanical response. Measurements of pressure area isotherms, bulk modulus, and the epifluorescence microscopy of tracer beads provide evidence that an initial structural reorganization occurs that corresponds to the mechanically induced formation of bundles of actin. Once a steady state structure is formed, subsequent compressions exhibit four distinct regimes as a function of decreasing total area: a low density fragile network, a relatively uniform film of cross-linked bundles, a region characterized by a buckling instability, and finally, a close packed region. Evidence for the distinct regions comes from the epifluorescence microscopy of tracer beads and measurements of the bulk modulus. Key features of the dynamics include domain rearrangements in the fragile network and an irregular, jerk-like motion during the buckling transition.

Conformational Properties of Arenicins: From the Bulk to the Air–Water Interface

The structures of two antimicrobial peptides (arenicin Ar-1 and its linear derivative C/S-Ar-1) are studied in different solutions and at the air-water interface using spectroscopic methods such as circular dichroism (CD) and infrared reflection absorption spectroscopy (IRRAS) as well as grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity (XR). Both peptides exhibit similar structures in solution. In the buffer used for most of the experiments the main secondary structure elements are 22 % β-turn, 38 % β-sheet and 38 % random coil. The amphiphilic peptides are surface-active and form a Gibbs monolayer at the air-buffer interface. The surface activity is drastically increased by increasing the ionic strength of the subphase. The β-sheet layer is quite stable and can be compressed to higher surface pressures. This adsorption layer is very crystalline. Bragg peaks corresponding to an interstrand distance of 4.78 Å and to an end-to-end distance have been observed. This end-to-end distance can be connected with the observed differences in the layer thickness leading to the assumption that the peptides form a hairpin which is bended depending on the interactions with the counterions.

Physicochemical studies on the interaction of serum albumin with pulmonary surfactant extract in films and bulk bilayer phase

Functionality, structure and composition of the adsorbed films of bovine lipid extract surfactant (BLES), in the absence and presence of bovine serum albumin (BSA), at the air-buffer interface was characterized through surface tension, atomic force microscopy and time of flight secondary ion mass spectrometric methods. Gel and fluid domains of BLES films were found to be altered significantly in the presence of BSA. Differential scanning calorimetric studies on BLES dispersions in presence of BSA revealed that the perturbations of the lipid bilayer structures were significant only at higher amount of BSA. FTIR studies on the BLES dispersions in buffer solution revealed that BSA could affect the lipid head-group hydrations in bilayer as well as the methylene and methyl vibration modes of fatty acyl chains of the phospholipids present in BLES. Serum albumin could perturb the film structure at pathophysiological concentration while higher amount of BSA was required in perturbing the bilayer structures. The studies suggest a connected perturbed bilayer to monolayer transition model for surfactant inactivation at the alveolar-air interface in dysfunctional surfactants.

Assembly and structure of alpha-helical peptide films on hydrophobic fluorocarbon surfaces.

The structure, orientation, and formation of amphiphilic alpha-helix model peptide films on fluorocarbon surfaces has been monitored with sum frequency generation (SFG) vibrational spectroscopy, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, and x-ray photoelectron spectroscopy (XPS). The alpha-helix peptide is a 14-mer of hydrophilic lysine and hydrophobic leucine residues with a hydrophobic periodicity of 3.5. This periodicity yields a rigid amphiphilic peptide with leucine and lysine side chains located on opposite sides. XPS composition analysis confirms the formation of a peptide film that covers about 75% of the surface. NEXAFS data are consistent with chemically intact adsorption of the peptides. A weak linear dichroism of the amide pi( *) is likely due to the broad distribution of amide bond orientations inherent to the alpha-helical secondary structure. SFG spectra exhibit strong peaks near 2865 and 2935 cm(-1) related to aligned leucine side chains interacting with the hydrophobic surface. Water modes near 3200 and 3400 cm(-1) indicate ordering of water molecules in the adsorbed-peptide fluorocarbon surface interfacial region. Amide I peaks observed near 1655 cm(-1) confirm that the secondary structure is preserved in the adsorbed peptide. A kinetic study of the film formation process using XPS and SFG showed rapid adsorption of the peptides followed by a longer assembly process. Peptide SFG spectra taken at the air-buffer interface showed features related to well-ordered peptide films. Moving samples through the buffer surface led to the transfer of ordered peptide films onto the substrates.

Adsorption of GST-PI3K γ at the Air-Buffer Interface and at Substrate and Nonsubstrate Phospholipid Monolayers

The recruitment of phosphoinositide 3-kinase γ (PI3K γ) to the cell membrane is a crucial requirement for the initiation of inflammation cascades by second-messenger production. In addition to identifying other regulation pathways, it has been found that PI3K γ is able to bind phospholipids directly. In this study, the adsorption behavior of glutathione S-transferase (GST)-PI3K γ to nonsubstrate model phospholipids, as well as to commercially available substrate inositol phospholipids (phosphoinositides), was investigated by use of infrared reflection-absorption spectroscopy (IRRAS). The nonsubstrate phospholipid monolayers also yielded important information about structural requirements for protein adsorption. The enzyme did not interact with condensed zwitterionic or anionic monolayers; however, it could penetrate into uncompressed fluid monolayers. Compression to values above its equilibrium pressure led to a squeezing out and desorption of the protein. Protein affinity for the monolayer surface increased considerably when the lipid had an anionic headgroup and contained an arachidonoyl fatty acyl chain in sn-2 position. Similar results on a much higher level were observed with substrate phosphoinositides. No structural response of GST-PI3K γ to lipid interaction was detected by IRRAS. On the other hand, protein adsorption caused a condensing effect in phosphoinositide monolayers. In addition, the protein reduced the charge density at the interface probably by shifting the pK values of the phosphate groups attached to the inositol headgroups. Because of their strongly polar headgroups, an interaction of the inositides with the water molecules of the subphase can be expected. This interaction is disturbed by protein adsorption, causing the ionization state of the phosphates to change.

Expression and Localization of StarD7 in Trophoblast Cells

The StAR-related lipid transfer (START) domain is defined as a motif of around 200 amino acids implicated in lipid/sterol binding. In a previous study, we identified the StarD7 transcript encoding one of the 15 family members with START domain present in the human genome. This transcript was found to be overexpressed in choriocarcinoma JEG-3 cells. In addition, we demonstrated that the recombinant StarD7 protein forms stable Gibbs and Langmuir monolayers at the air-buffer interface, showing marked surface activity and interaction with phospholipid monolayers, mainly with phosphatidylserine, cholesterol and phosphatidylglycerol. This study was undertaken to evaluate the expression and localization of StarD7 protein in trophoblastic samples. Here, we show for the first time the presence of StarD7 protein in human trophoblast cells. Western blot assays revealed a unique specific 34 kDa protein in JEG-3 cell line, choriocarcinoma tissue, complete hydatidiform mole, early and normal term placenta. Immunohistochemical data from early and normal term placentas and complete hydatidiform moles showed that this protein is abundant in the syncytiotrophoblasts, mainly at the apical side of the syncytium, with a weak and focal reaction in the cytotrophoblast cells. Furthermore, an increased StarD7 mRNA and protein expression, as well as a change in its sub-cellular localization was observed in in vitro differentiating cytotrophoblast isolated from normal term placenta. Taken together, these findings support and allow future studies to explore the possibility that StarD7 protein mediates transplacental lipid transport and/or is involved in syncytialization.

Lipid membrane templates the ordering and induces the fibrillogenesis of Alzheimer's disease amyloid‐β peptide

The lipid membrane has been shown to mediate the fibrillogenesis and toxicity of Alzheimer's disease (AD) amyloid-beta (Abeta) peptide. Electrostatic interactions between Abeta40 and the phospholipid headgroup have been found to control the association and insertion of monomeric Abeta into lipid monolayers, where Abeta exhibited enhanced interactions with charged lipids compared with zwitterionic lipids. To elucidate the molecular-scale structural details of Abeta-membrane association, we have used complementary X-ray and neutron scattering techniques (grazing-incidence X-ray diffraction, X-ray reflectivity, and neutron reflectivity) in this study to investigate in situ the association of Abeta with lipid monolayers composed of either the anionic lipid 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DPPG), the zwitterionic lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), or the cationic lipid 1,2-dipalmitoyl 3-trimethylammonium propane (DPTAP) at the air-buffer interface. We found that the anionic lipid DPPG uniquely induced crystalline ordering of Abeta at the membrane surface that closely mimicked the beta-sheet structure in fibrils, revealing an intriguing templated ordering effect of DPPG on Abeta. Furthermore, incubating Abeta with lipid vesicles containing the anionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) induced the formation of amyloid fibrils, confirming that the templated ordering of Abeta at the membrane surface seeded fibril formation. This study provides a detailed molecular-scale characterization of the early structural fluctuation and assembly events that may trigger the misfolding and aggregation of Abeta in vivo. Our results implicate that the adsorption of Abeta to anionic lipids, which could become exposed to the outer membrane leaflet by cell injury, may serve as an in vivo mechanism of templated-aggregation and drive the pathogenesis of AD.

Interfacial properties and structural analysis of the antimicrobial peptide NK‐2

The structure of the antimicrobial peptide NK-2 has been studied at the air-water interface and in different solutions using spectroscopic methods such as circular dichroism (CD) and infrared reflection absorption spectroscopy (IRRAS) as well as specular X-ray reflectivity (XR). NK-2 adopts an unordered structure in water, buffer, and in the presence of monomeric cationic and noncharged amphiphiles. However, it forms a stable alpha-helix in 2,2,2-trifluoroethanol (TFE) and in micellar solutions of anionic, cationic as well as nonionic amphiphiles, whereas only in sodium dodecyl sulfonate solutions the alpha-helical structure can also be found below the critical micellar concentration (cmc). The amphiphilic molecule NK-2 is surface active and forms a Gibbs monolayer at the air-buffer interface. In contrast, no adsorption was observed if NK-2 is dissolved in water. During the adsorption process in buffer solutions, NK-2 undergoes a conformational transition from random coil in bulk to alpha-helix at the interface. This change of the peptide's secondary structure is known to be associated with its antimicrobial activity. A comparison of the experimental IRRA spectra with the simulated spectra indicates that the adsorbed NK-2 alpha-helix lies flat at the interface. This is confirmed by XR measurements which show that the thickness of the NK-2 layer is approximately 17 A, which is the average diameter of a alpha-helix, indicating that only a monomolecular adsorption layer is formed.