Low Surface Pressure Research Articles

Kinetic analysis of the interaction between poly(amidoamine) dendrimers and model lipid membranes

We used fluorescence spectroscopy and surface tensiometry to study the interaction between low-generation (G1 and G4) poly(amidoamine) (PAMAM) dendrimers, potential vehicles for intracellular drug delivery, and model lipid bilayers. Membrane association of fluorescently labeled dendrimers, measured by fluorescence anisotropy, increased with increasing size of the dendrimer and with increasing negative charge density in the membrane, indicating the electrostatic nature of the interaction. When the membrane was doped with pyrene-labeled phosphatidyl glycerol (pyrene-PG), pyrene excimer fluorescence demonstrated a dendrimer-induced selective aggregation of negatively charged lipids when the membrane was in the liquid crystalline state. A nonlinear Stern–Volmer quenching of dendrimer fluorescence with cobalt bromide suggested a dendrimer-induced aggregation of lipid vesicles, which increased with the dendrimer's generation number. Surface tensiometry measurements showed that dendrimers penetrated into the lipid monolayer only at subphysiologic surface pressures (<30mN/m). We conclude that the low-generation PAMAM dendrimers associate with lipid membranes predominantly electrostatically, without significantly compromising the bilayer integrity. They bind stronger to membranes with higher fluidity and lower surface pressure, which are characteristic of rapidly dividing cells.

Equivalent Aqueous Phase Modulation of Domain Segregation in Myelin Monolayers and Bilayer Vesicles

Purified myelin can be spread as monomolecular films at the air/aqueous interface. These films were visualized by fluorescence and Brewster angle microscopy, showing phase coexistence at low and medium surface pressures (<20–30 mN/m). Beyond this threshold, the film becomes homogeneous or not, depending on the aqueous subphase composition. Pure water as well as sucrose, glycerol, dimethylsulfoxide, and dimethylformamide solutions (20% in water) produced monolayers that become homogeneous at high surface pressures; on the other hand, the presence of salts (NaCl, CaCl2) in Ringer's and physiological solution leads to phase domain microheterogeneity over the whole compression isotherm. These results show that surface heterogeneity is favored by the ionic milieu. The modulation of the phase-mixing behavior in monolayers is paralleled by the behavior of multilamellar vesicles as determined by small-angle and wide-angle x-ray scattering. The correspondence of the behavior of monolayers and multilayers is achieved only at high surface pressures near the equilibrium adsorption surface pressure; at lower surface pressures, the correspondence breaks down. The equilibrium surface tension on all subphases corresponds to that of the air/alkane interface (27 mN/m), independently on the surface tension of the clean subphase.

Phage Langmuir monolayers and Langmuir–Blodgett films

Stable, insoluble Langmuir monolayer films composed of Staphylococcus aureus-specific lytic bacteriophage were formed at an air–water interface and characterized. The phage monolayer was very strong, withstanding a surface pressure of ∼40 mN/m at 20 °C. The surface pressure–area ( Π– A) isotherm possessed a shoulder at ∼7 × 10 4 nm 2/phage particle, attributed to a change in phage orientation at the air–water interface from horizontal to vertical capsid-down/tail-up orientation as surface pressure was increased. The Π– A-dependence was accurately described using the Volmer equation of state, assuming horizontal orientation to an air–water interface at low surface pressures with an excluded area per phage particle of 4.6 × 10 4 nm 2. At high pressures phage particles followed the space-filling densely packed disks model with a specific area of 8.5 × 10 3 nm 2/phage particle. Lytic phage monolayers were transferred onto gold-coated silica substrates from the air–water interface at a constant surface pressure of 18 mN/m by Langmuir–Blodgett method, then dried and analyzed by scanning electron microscopy (SEM) and ellipsometry. Phage specific adsorption ( Γ) in Langmuir–Blodgett (LB) films measured by SEM was consistent with that calculated independently from Π–A isotherms at the transfer surface pressure of 18 mN/m ( Γ = 23 phage particles/μm 2). The 50 nm-thickness of phage monolayer measured by ellipsometer agreed well with the horizontal phage average size estimated by SEM. Surface properties of phage Langmuir monolayer compare well with other monolayers formed from nano- and micro-particles at the air–water interface and similar to that of classic amphiphiles 1,2-diphytanoyl-sn-glycero-3-phosphocholine (phospholipid) and stearic acid.

Cross-Sectional Area Increase at Phase Transition on Compression: An Unexpected Phenomenon Observed in an Amide Monolayer

At low temperatures (T ≤ 10 °C), the surface pressure−area (π−A) isotherms of some amphiphilic amides reveal a striking second critical point indicating the existence of a second phase transition between two condensed phases. Studies of 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) monolayers have shown that this phase transition between two condensed phases is accompanied by an abrupt change of important 2D lattice parameters. Thorough grazing incidence X-ray diffraction (GIXD) studies have revealed a new phenomenon. The cross-sectional area (A0) of the alkyl chain of HTPA jumps from a smaller value in the phase at the lower surface pressure to a larger value in the phase existing at higher pressure. The opposite behavior should be expected and is usually noted in the lattice structures of Langmuir monolayers. The phase diagram of the HTPA monolayers is constructed on the basis of the equilibrium π−A isotherms. The fact that two condensed monolayer phases exist and the phase observed above the fluid/c...

Langmuir−Schaefer (LS) Macroinitiator Film Control on the Grafting of a Thermosensitive Polymer Brush via Surface Initiated-ATRP

A novel surface modification based on the Langmuir−Schaefer (LS) monolayer technique was utilized for controlled surface-initiated atom transfer radical polymerization (SI-ATRP) of polymer brushes. The fabrication of these films was prepared by spreading an insoluble monolayer of an amphiphilic macroinitiator, poly(styrene-copolymer-hydroxyethyl methacrylate-2-bromoisobutyryl bromide) (p(St-co-HeBiB)), capable of initiating the ATRP polymerization of n-isopropylacrylamide (NIPAM). During the LS monolayer deposition, the surface pressure was varied in order to probe its effect on the initiator distribution and subsequent grafting density and thickness. The surface-bound macroinitiator ranged from a loosely (low surface pressure) to a densely (high surface pressure) crowded initiation sites on a monolayer. Increasing LS macroinitiator film thicknesses were observed with deposition at increasing surface pressures, i.e. 5, 15, 20, 30, and 50 mN/m using ellipsometry. However, based on the XPS data, there was a...

Dynamic surface properties of polyethylenimine and sodium dodecylsulfate complex solutions

Measurements of the concentration and adsorption time dependencies of the dynamic surface elasticity of mixed polyethylenimine/sodium dodecyl sulfate (PEI/SDS) solutions show transitions between a few states of different surface elasticity. In the range of low surface pressures the elasticity is governed by the branched structure of the macromolecules, while at high surface pressures the elasticity exhibits a behaviour typical for other polyelectrolyte/surfactant surface layers.

Low-Reynolds-Number Airfoil Investigation of Lower-Surface Leading-Edge Flaps

A low-speed wind-tunnel investigation was undertaken to evaluate the ability of a small leading-edge flap attached to the pressure surface of a Clark Y airfoil to improve low-Reynolds-number performance. Different flap geometries and designs were evaluated. Testing encompassed the measurement of lift and drag, surface pressures, and surface-flow topology rendered using titanium dioxide. Tests were undertaken at Reynolds numbers of 75,000 and 150,000. The results show that the flaps shift the zero-lift angle of attack in the positive direction, with the lift decrement stemming from reduced lower-surface pressure. The flaps proved to be beneficial only at high angles of attack, where they cause a sustained lift plateau and eliminate hysteresis.

Polymers at the Water/Air Interface, Surface Pressure Isotherms, and Molecularly Detailed Modeling

Surface pressure isotherms at the air/water interface are reproduced for four different polymers, poly-L-lactic acid (PLLA), poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), and poly(isobutylene) (PiB). The polymers have the common property that they do not dissolve in water. The four isotherms differ strongly. To unravel the underlying details that are causing these differences, we have performed molecularly detailed self-consistent field (SCF) modeling. We describe the polymers on a united atom level, taking the side groups on the monomer level into account. In line with experiments, we find that PiB spreads in a monolayer which smoothly thickens already at a very low surface pressure. PMMA has an autophobic behavior: a PMMA liquid does not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only forms after the collapse of the film at a relatively high pressure. The isotherm of PDMS has regions with extreme compressibility which are linked to a layering transition. PLLA wets the water surface and spreads homogeneously at larger areas per monomer. The classical SCF approach features only short-range nearest-neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, we account for a power-law van der Waals contribution in the model. Two-gradient SCF computations are performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact is obtained, together with the associated line tension value.

Effect of nano-clay platelets on the J-aggregation of thiacyanine dye organized in Langmuir–Blodgett films: A spectroscopic investigation

In this paper we report the effect of the incorporation of nano-dimensional clay platelets, laponite, on the J-aggregation of a thiacyanine dye N, N′-dioctadecyl thiacyanine perchlorate (NK) assembled into Langmuir–Blodgett (LB) monolayers. π– A isotherms and atomic force microscopic studies confirm the successful incorporation of clay platelets into the Langmuir monolayer of NK. J-aggregates of NK remain present in LB films lifted at lower as well as higher surface pressures in the absence of laponite clay platelets. However, with the incorporation of clay platelets, J-aggregates are formed only in LB films lifted at higher surface pressure of 30 mN/m and totally absent in the films lifted at lower surface pressures of 10 and 15 mN/m. This may be due to the formation of nano-trapping level by overlapping of clay platelets at higher surface pressure. NK molecules may get squeezed to these nano-trapping to form J-aggregates.

Phase state and surface topography of palmitoyl-ceramide monolayers

In cell biology (and in many biophysical) studies there is a natural tendency to consider ceramide as a highly condensed, solid-type lipid conferring rigidity and close packing to biomembranes. In the present work we advanced the understanding of the phase behavior of palmitoyl-ceramide restricted to a planar interface using Langmuir monolayers under strictly controlled and known surface packing conditions. Surface pressure–molecular area isotherms were complemented with molecular area–temperature isobars and with observations of the surface topography by Brewster Angle Microscopy. The results described herein indicate that palmitoyl-ceramide can exhibit expanded, as well as condensed phase states. Formation of three phases was found, depending on the surface pressure and temperature: a solid (1.80 nm thick), a liquid-condensed (1.73 nm thick, likely tilted) and a liquid-expanded (1.54 nm thick) phase over the temperature range 5–62 °C. A large hysteretic behavior is observed for the S phase monolayer that may indicate high resistance to domain boundary deformation. A second (or higher) order S → LC phase transition is observed at about room temperature while a first order LC → LE transition occurs in a range of temperature encompassing the physiological one (observed above 30 °C at low surface pressure). This phase behavior broadens the view of ceramide as a type of lipid not-always-rigid but able to exhibit polymorphic properties.

Formation of Calcium Carbonate Crystal Using Phospholipid Monolayer Template Under Ambient Condition

Formation and assembly of monodispersed calcite (CaCO3) dots of diameters 6−10 nm using a zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer as template at a two-dimensional interface is reported here. We utilized penetration of Ca2+ at lower surface pressure (π) into the DPPC monolayer, governed by a single exponential association method. In situ surface pressure (π)-area (A) isotherms as well as compressibility (β) analysis reveal a positive association of Ca2+ with DPPC. The natural exposure of the resultant monolayer to air results in formation of single crystalline CaCO3 nanodots. HRTEM and SAED techniques have been used to study the crystal structure of the resultant CaCO3 nanodots incorporated within the lipid monolayer.

Specific anchoring modes of two distinct dystrophin rod sub-domains interacting in phospholipid Langmuir films studied by atomic force microscopy and PM-IRRAS

Dystrophin rod repeats 1–3 sub-domain binds to acidic phosphatidylserine in a small vesicle binding assay, while the repeats 20–24 sub-domain does not. In the present work, we studied the adsorption behaviour of both sub-domains at the air/liquid interface and at the air/lipid interface in a Langmuir trough in order to highlight differences in interfacial properties. The adsorption behaviour of the two proteins at the air/liquid interface shows that they display surface activity while maintaining their alpha-helical secondary structure as shown by PM-IRRAS. Strikingly, R20–24 needs to be highly hydrated even at the interface, while this is not the case for R1–3, indicating that the surface activity is dramatically higher for R1–3 than R20–24. Surface-pressure measurements, atomic force microscopy and PM-IRRAS are used in a Langmuir experiment with DOPC–DOPS monolayers at two different surface pressures, 20 mN/m and 30 mN/m. At the lower surface pressure, the proteins are adsorbed at the lipid film interface while maintaining its alpha-helical structure. After an increase of the surface pressure, R1–3 subsequently produces a stable film, while R20–24 induces a reorganization of the lipid film with a subsequent decrease of the surface pressure close to the initial value. AFM and PM-IRRAS show that R1–3 is present in high amounts at the interface, being arranged in clusters representing 3.3% of the surface at low pressure. By contrast, R20–24 is present at the interface in small amounts bound only by a few electrostatic residues to the lipid film while the major part of the molecule remains floating in the sub-phase. Then for R1–3, the electrostatic interaction between the proteins and the film is enhanced by hydrophobic interactions. At higher surface pressure, the number of protein clusters increases and becomes closer in both cases implying the electrostatic character of the binding. These results indicate that even if the repeats exhibit large structural similarities, their interfacial properties are highly contrasted by their differential anchor mode in the membrane. Our work provides strong support for distinct physiological roles for the spectrin-like repeats and may partly explain the effects of therapeutic replacement of dystrophin deficiency by minidystrophins.

Study of the Interaction of Lactoferricin B with Phospholipid Monolayers and Bilayers

Bovine lactoferricin (LfcinB) is an antimicrobial peptide obtained from the pepsin cleavage of lactoferrin. The activity of LfcinB has been extensively studied on diverse pathogens, but its mechanism of action still has to be elucidated. Because of its nonspecificity, its mode of action is assumed to be related to interactions with membranes. In this study, the interaction of LfcinB with a negatively charged monolayer of dipalmitoylphosphatidylglycerol has been investigated as a function of the surface pressure of the lipid film using in situ Brewster angle and polarization modulation infrared reflection absorption spectroscopy and on transferred monolayers by atomic force microscopy and polarized attenuated total reflection infrared spectroscopy. The data show clearly that LfcinB forms stable films at the air-water interface. They also reveal that the interaction of LfcinB with the lipid monolayer is modulated by the surface pressure. At low surface pressure, LfcinB inserts within the lipid film with its long molecular axis oriented mainly parallel to the acyl chains, while at high surface pressure, LfcinB is adsorbed under the lipid film, the hairpin being preferentially aligned parallel to the plane of the interface. The threshold for which the behavior changes is 20 mN/m. At this critical surface pressure, LfcinB interacts with the monolayer to form discoidal lipid-peptide assemblies. This structure may actually represent the mechanism of action of this peptide. The results obtained on monolayers are correlated by fluorescent probe release measurements of dye-containing vesicles made of lipids in different phases and support the important role of the lipid fluidity and packing on the activity of LfcinB.

In vitro gastric digestion of interfacial protein structures: visualisation by AFM

The rational design of healthier foods to control fat uptake in the diet is an emerging area of research. In order to address these dietary issues effectively one requires an improved understanding of the influence of food structure on the digestion of lipids. Despite the crucial roles played by interfaces in determining the stability of emulsions and the breakdown of emulsion structure on digestion, there are few systematic basic studies dealing directly with the effects of digestion conditions on interfacial structures. A major concern in the attempted manipulation of protein-based interfacial structures is the potential enzymatic breakdown of these structures during digestion. This study has been designed to investigate the effects of the proteolytic enzyme pepsin on β-lactoglobulin surface layers under in vitro gastric digestion. Interfacial dilatational rheology has been used to measure the evolution of the viscoelastic properties of the pre-formed interfacial protein networks. Furthermore, Atomic Force Microscopy (AFM) has been used to visualise directly the effects of degradation on model interfacial protein layers (β-lactoglobulin). The results obtained show that pepsin partially hydrolyses the surface-adsorbed β-lactoglobulin molecules under gastric conditions, but does not break the interconnected surface network. In order to investigate the influence of the presence of surfactants on the enzymatic digestion of emulsions, the effect of addition of the model surfactant Tween 20 on the pepsin-induced changes in interfacial structure under simulated gastric conditions was studied. Modification of the surface conformation of the proteins by surfactant, during ‘orogenic’ displacement, revealed an unexpected synergism which leads to enhanced hydrolysis of the proteins; hence, weakening the network and allowing rupture to occur at lower surface pressures. Such observations are important new generic features of the digestion process which could be potentially manipulated to control the enzyme hydrolysis, allowing rational design of food structures and manipulation of properties such as fat digestion.

Tailor made synthesis of amphiphilic azoaromatics via regioselective C–N bond fusion. Comparative studies of surface properties of the two positional isomers and cobalt complexes

Tailor made synthesis of the isomeric azoaromatics, HL(1)-HL(4) [HL = (arylamino)phenylazopyridine] containing a single hydrophobic tail (C(n) = C(10) and C(12)) is described. The coordination induced C-N bond fusion synthetic protocol has been successfully used for the synthesis of the compounds, which are subsequently characterized using various spectroscopic techniques. The single crystal X-ray structure of compound HL(4) has revealed that the hydrophobic chain in it orients itself with all-trans conformation of alkyl groups. Studies of their surface properties clearly demonstrate that these behave as surfactants. Amphiphilic properties of the compounds are followed by the studies of compression isotherms and their surface morphologies are studied with the use of high resolution field emission scanning electron microscopy (FE-SEM) as well as atomic force microscopy (AFM). Distinct differences in surface properties in the two HL isomers are observed and disposition of the hydrophobic tail with respect to the head group is shown to play a significant role in the organization process of the molecules at the air-water interface. Transferred monolayers of the above two isomeric compounds show agglomerated nano-domain structures. This phenomenon has been explained considering hydrophobic tail-tail repulsive interaction within the adjacent molecules. Surface properties of the double tail complex, [Co(L(1))(2)]ClO(4) (1) along with that of the single tail complex, [Co(L(1))(L(5))]ClO(4) (3) are also reported. Amphiphilic behavior of the above azoaromatics are distinctly different than those in their metal free state. Notably, the double tail complex (1) favors bi-layer formation even at low surface pressure region (approximately 10 mN m(-1)). The single tail cobalt complex (3), on the other hand, forms a monolayer at high surface pressure region leading finally to the collapse at a very high pressure approximately 60 mN m(-1).

Numerical study of pleated fabric cartridges during pulse-jet cleaning

We established a numerical model and used computational fluid dynamics (CFD) analysis to observe transient flow behavior across pleated filter cartridges in a dust collector during pulse-jet cleaning. The numerical results were in good agreement with the filter-testing data during important periods including during pressure ramp-up and valve opening. Larger errors for predicting overpressure occurred during the pressure ramp-down period likely due to the uniformity of the filters' permeability. This confirmed that the numerical model demonstrated the cleaning efficiency and the local cleaning quality of three different filter cartridges with different filter dimensions and pleat ratios. Data calculated from the average static pressure on the filters' surfaces were more closely correlated to cleaning efficiency than overpressure. The surface static pressure distribution along all filter cartridges showed that the top area of the filter cartridge is difficult to clean because of the lower surface pressure generated by the pulse jet. Filter cartridges with higher pleat ratios were found to have greater instances of incomplete cleaning due to the large variation of static pressure distribution along the filter cartridges. Our results showed that although information such as average pressure and pressure distribution on the filter surface is difficult to obtain by physical measurement, this data is tractable using CFD analysis and is useful for filter design and system optimization.

Effects of Fullerenes on Phospholipid Membranes: A Langmuir Monolayer Study

We investigate two-component Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC)/C(60) by recording surface pressure/area (pi/A) and surface potential/area (DeltaV/A) isotherms and by direct Brewster angle microscopy (BAM) imaging. Atomic force microscopy (AFM) is employed to study morphologies of the mixed monolayers transferred to a solid substrate by the Langmuir-Blodgett technique. C(60) is shown to have little influence on isotherms of the DPPC/C(60) monolayers even at a molar fraction as high as X(C60)=0.3. The elastic modulus (Cs(-1) versus pi curves of the DPPC/C(60) monolayers almost overlay each other, as well as that of pure DPPC, that is, the elasticities of pure DPPC monolayers and DPPC/C(60) monolayers are remarkably similar. AFM studies reveal that fullerene flocs form at low surface pressures (pi<or=15 mN m(-1)), are gradually disaggregated and dispersed in the DPPC monolayer with increasing surface pressure up to 35 mN m(-1), and are then progressively squeezed out to form protruded islands as the surface pressure increases up to 65 mN m(-1). Our work provides experimental support to the computational result that C(60) can dissolve in lipid bilayers without significantly compromising their mechanical properties, a finding which has important implications for the toxicity and development of drug vehicles from fullerene materials.

X-Ray Diffraction and Reflectivity Validation of the Depletion Attraction in the Competitive Adsorption of Lung Surfactant and Albumin

Lung surfactant (LS) and albumin compete for the air-water interface when both are present in solution. Equilibrium favors LS because it has a lower equilibrium surface pressure, but the smaller albumin is kinetically favored by faster diffusion. Albumin at the interface creates an energy barrier to subsequent LS adsorption that can be overcome by the depletion attraction induced by polyethylene glycol (PEG) in solution. A combination of grazing incidence x-ray diffraction (GIXD), x-ray reflectivity (XR), and pressure-area isotherms provides molecular-resolution information on the location and configuration of LS, albumin, and polymer. XR shows an average electron density similar to that of albumin at low surface pressures, whereas GIXD shows a heterogeneous interface with coexisting LS and albumin domains at higher surface pressures. Albumin induces a slightly larger lattice spacing and greater molecular tilt, similar in effect to a small decrease in the surface pressure. XR shows that adding PEG to the LS-albumin subphase restores the characteristic LS electron density profile at the interface, and confirms that PEG is depleted near the interface. GIXD shows the same LS Bragg peaks and Bragg rods as on a pristine interface, but with a more compact lattice corresponding to a small increase in the surface pressure. These results confirm that albumin adsorption creates a physical barrier that inhibits LS adsorption, and that PEG in the subphase generates a depletion attraction between the LS aggregates and the interface that enhances LS adsorption without substantially altering the structure or properties of the LS monolayer.

Interactions of antimicrobial peptide from C-terminus of myotoxin II with phospholipid mono- and bilayers

Comparative studies of the effect of a short synthetic cationic peptide, pEM-2 (KKWRWWLKALAKK), derived from the C-terminus of myotoxin II from the venom of the snake Bothrops asper on phospholipid mono- and bilayers were performed by means of Langmuir Blodgett (LB) monolayer technique, atomic force microscopy and calcein leakage assay. Phospholipid mono- and bilayers composed of single zwitterionic or anionic phospholipids as well as lipid mixtures mimicking bacterial cell membrane were used. LB measurements indicate that the peptide binds to both anionic and zwitterionic phospholipid monolayers at low surface pressure but only to anionic at high surface pressure. Preferential interaction of the peptide with anionic phospholipid monolayer is also supported by a more pronounced change of the monolayer pressure/area isotherms induced by the peptide. AFM imaging reveals the presence of nanoscale aggregates in lipid/peptide mixture monolayers. At the same time, calcein leakage experiment demonstrated that pEM-2 induces stronger disruption of zwitterionic than anionic bilayers. Results of the study indicate that electrostatic interactions play a significant role in the initial recognition and binding of pEM-2 to the cell membrane. However, membrane rupturing activity of the peptide depends on interactions other than simple ionic attraction.

Interaction of Poly(l-arginine) with Negatively Charged DPPG Membranes: Calorimetric and Monolayer Studies

The interaction of poly(L-arginine) (PLA) with dipalmitoyl-phosphatidylglycerol (DPPG) bilayer membranes and monolayers was studied by differential scanning calorimetry (DSC), isothermal titration calorimetry (ITC), and monolayer experiments. The binding of PLA affected the main transition temperature of lipid bilayers (T(m)) only marginally. Depending on the PLA chain length, T(m) was slightly increased or decreased. This finding was attributed to the superposition of two counteracting effects on the transition after PLA binding. The main transition enthalpy (DeltaH(m)) was decreased upon PLA binding and the formation of a ripple phase (P(beta)') was suppressed. ITC experiments showed that two distinct processes are involved in binding of PLA to gel phase (L(beta)') membranes. At low peptide content the binding reaction is endothermic, and at high peptide concentration the binding becomes exothermic. However, the enthalpy of binding to fluid (L(alpha)) membranes was exothermic for all peptide-to-lipid ratios. The temperature dependence of PLA binding to fluid palmitoyl-oleoyl-phosphatidylglycerol (POPG) membranes showed a decrease in binding enthalpy with increasing temperature (Delta(R)C(p) < 0), indicating hydrophobic contributions to the free energy of binding. For longer PLA chains, the binding enthalpy for L(alpha) membranes was more exothermic than for shorter chains. Monolayer adsorption experiments showed two consecutive binding processes. At low initial surface pressures (pi(0)) a condensation of the lipid film (Deltapi < 0) is first observed after PLA injection into the subphase, followed by an increase in film pressure (Deltapi > 0) due to insertion of peptide side chains into the monolayer. At higher pi(0) only an increase in film pressure can be observed due to the insertion of the side chains. Deltapi increases with increasing pi(0). The insertion of the peptide into the monolayer is corroborated by the observed shift of pi-A isotherms to higher molecular areas. All presented experiments show that the binding of PLA to DPPG membranes has not only electrostatic but also nonelectrostatic contributions.