Platelet compatibility improvement by proper choice of acidic terminal functionality for mixed-charge self-assembled monolayers

In this study two different series of mixed-charge self-assembled monolayers (SAMs) prepared with -N(+)(CH(3))(3)-terminated alkanethiol and strong dissociated monovalent -SO(3)H acid-terminated or weaker dissociated divalent -PO(3)H(2) acid-terminated alkanethiol in pure ethanol were characterized. The influence of the acidity of the anionic functionality in the mixed-charge SAMs on the surface characteristics and platelet compatibility was investigated. X-ray photoelectron spectroscopy indicated that a nearly equivalent amount of countercharged terminal groups was noted on the surface of -SO(3)H/-N(+)(CH(3))(3) mixed SAMs, while "-N(+)(CH(3))(3) thiol poor" phenomena were found on -PO(3)H(2)/-N(+)(CH(3))(3) mixed SAMs instead. This was caused by the distinct differences in solvation capability between the acidic anionic functional groups and solvent molecules and/or the interactions among the terminal ends of the thiols. This acidity difference also affected other interfacial properties and the platelet compatibility. The mixed SAMs formed from the mixture of -SO(3)H- and -N(+)(CH(3))(3)-terminated thiols showed higher surface hydrophilicity and exhibited the least amount of platelets adhered, but these two mixed SAMs were all fairly negatively surface charged. The structure of the hydration layer near the surfaces was likely affected by the acidity of the anionic functionality, and this would cause such a distinct behavior in platelet compatibility. It was concluded that the hydrophilic surfaces with nearly equal amounts of surface positively and negatively charged components could exhibit better platelet compatibility. This work demonstrated that the nature of the acidic terminal ends of alkanethiol is also a key factor for preparing mixed-charge SAMs with good platelet compatibility.     

Molecular gradients: an efficient approach for optimizing the surface properties of biomaterials and biochips

A variety of molecular gradients of alkanethiols with the structure HS-(CH(2))(m)-X (m = 15; X = COOH, CH(2)NH(2), or CH(3)) and oligo(ethylene glycol)-terminated alkanethiols with the structures HS-(CH(2))(15)-CO-NH-Eg(n) (n = 2, 4, or 6), HS-(CH(2))(15)-CO-NH-Eg(2)-(CH(2))(2)-NH-CO-(CH(2))(4)-biotin, and HS-(CH(2))(15)-CO-NH-Eg(6)-CH(2)-COOH were prepared on polycrystalline gold films. These gradients were designed to serve as model surfaces for fundamental studies of protein adsorption and immobilization phenomena. Ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy, operating in scanning mode, were used to monitor the layer composition, gradient profiles, tail group conformation, and overall structural quality of the gradient assemblies. The gradient profiles were found to be 4-10 mm wide, and they increased in width with increasing difference in molecular complexity between the thiols used to form the gradient. The oligo(ethylene glycol) thiols are particularly interesting because they can be used to prepare so-called conformational gradients, that is, gradients that display a variation in oligo(ethylene glycol) chain conformation from all trans on the extreme Eg(2,4) sides, via an amorphous-like phase in the mixing regimes, to helical at the extreme Eg(6) sides. We demonstrate herein a series of experiments where the above gradients are used to evaluate nonspecific binding of the plasma protein fibrinogen, and in agreement with previous studies, the highest amounts of nonspecifically bound fibrinogen were observed on all-trans monolayers, that is, on the extreme Eg(2,4) sides. Moreover, gradients between Eg(2) and a biotinylated analogue have been prepared to optimize the conditions for the immobilization of streptavidin. Ellipsometry and infrared spectroscopy reveal high levels of immobilization over a fairly broad range of compositions in the gradient regime, with a maximum between 50 and 60% of the biotinylated analogue in the monolayer. A pI gradient composed of (NH(3)(+)/COO(-))-terminated thiols was also prepared and evaluated with respect to its ability to separate differently charged proteins, pepsin, and lysozyme, on a solid surface.     

Designing highly specific biosensing surfaces using aptamer monolayers on gold

To build highly specific surfaces using aptamer affinity reagents, the effects of linker and coadsorbents were investigated for maximizing target binding and specificity for aptamer-based self-assembled monolayers (SAMs) supported on gold. An aptamer that binds the protein thrombin was utilized as a model system to compare different mixed monolayer systems toward maximizing binding and selectivity to the immobilized aptamer. Important factors used to optimize binding characteristics of thrombin to the aptamer-based monolayer films include changes in design elements of the linker and different coadsorbent thiols. Binding events measured by surface plasmon resonance (SPR) and ellipsometry showed that the binding performance of the aptamer SAMs depends principally on the linker and to a lesser extent on the coadsorbent. SAMs formed with HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-aptamer exhibited a 4-fold increase in binding capacity versus SAMs made using HS-(CH2)6-TTTTT-aptamer. Furthermore, SAMs made using HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-aptamer showed nearly complete specificity for thrombin versus bovine serum albumin (BSA, less than 2% bound), while a SAM incorporating a random DNA fragment (HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-RANDOM) showed little binding of thrombin. Irrespective of the aptamer-linker system, use of HS-(CH2)11(OCH2CH2)3OH, referred to as EG(3), as a coadsorbent enhanced binding of thrombin by approximately 2.5-fold compared to that of HS-(CH2)6-OH (mercaptohexanol, MCH).     

Formation of alkanethiol and alkanedithiol monolayers on GaAs(001)

The formation of alkanethiol (H-(CH2)n-SH, n = 8-18) and 1,8-octanedithiol (HS-(CH2)8-SH) monolayer films on n-type GaAs(001) has been systematically studied. We observed a nonlinear dependence of the film thickness on molecular length, which is drastically different from monolayer films of the same molecules on metals. For 8 < or = n < or = 14, the films are only 3-4.5 A thick, significantly smaller than the corresponding molecular length. For n = 16 and 18, the measured film thicknesses were 9 and 11 A, respectively, consistent with molecules orienting with a tilt angle of approximately 60 degrees from the surface normal. Unlike the alkanethiols, the thickness of the 1,8-octanedithiol monolayer is almost the same as its molecular length, indicating that dithiol molecules orient vertically with only one thiol end group bound to the GaAs surface. Additional support for this conclusion comes from the fact that X-ray photoelectron spectroscopy of the 1,8-octanedithiol monolayer clearly resolves two types of S atoms in the monolayer: those bound to the GaAs surface and those existing as free thiols. A suggestion was made on the mechanisms for alkanethiol and alkanedithiol monolayer formation.     

Specific adsorption of cytochrome C on cardiolipin-glycerophospholipid monolayers and bilayers

In this study, we examined the adsorption of cytochrome c (cyt c) on monolayers and liposomes formed from (i) pure 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), or cardiolipin (CL) and on (ii) the more thermodynamically stable binary mixtures of POPE/CL (0.8:0.2 mol/mol) and POPC/CL (0.6:0.4 mol/mol). Constant surface pressure experiments showed that the maximum and minimum interactions occurred in the pure CL (anionic phospholipid) and the pure POPE (zwitterion) monolayers, respectively. Observation by atomic force microscopy (AFM) of the images of Langmuir-Blodgett (LB) films extracted at 30 mN m-1 suggests that the different interactions of cyt c with POPE/CL and the POPC/CL monolayers could be due to lateral phase separation occurring in the POPE/CL mixture. The competition between 8-anilino-1-naphthalene sulfonate (ANS) and cyt c for the same binding sites in liposomes that have identical nominal compositions with respect to those of the monolayers was used to obtain binding parameters. In agreement with the monolayer experiments, the most binding was observed in POPE/CL liposomes. All of our observations strongly support the existence of selective adsorption of cyt c on CL, which is modulated differently by different neutral phospholipids (POPE and POPC).     

Formation of alkanethiol monolayer on Ge(111).

Alkanethiols, CH3(CH2)(n-1)SH, are shown to react readily with HF-treated Ge(111) surface at room temperature to form a high-quality monolayer. The resulting films are characterized by using contact angle analysis (CAA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and high-resolution electron energy loss spectroscopy (HREELS). The octanethiol and octadecanethiol films on Ge(111) both exhibit 101 degrees and 40 degrees water and hexadecane contact angles, respectively. These values indicate that the thiol surface coverage is relatively high, and that the films possess a high degree of orientational ordering. The angle-resolved XPS analysis supports that thiols are bound to the Ge surface by Ge-S bonds at the monolayer/Ge interface. The film thickness values obtained by XPS and SE agree well with the earlier reported values on alkyl monolayers on Ge(111) prepared by Grignard reaction. On the basis of HREEL spectra taken after thermal annealing steps, the monolayers are found to be thermally stable up to 450 K. The thermal stability provides further evidence that thiols are covalently bonded to Ge(111).     

Gas-surface scattering dynamics of CO2, NO2, and O3 in collisions with model organic surfaces

High-energy (70 kJ/mol) molecular beams of CO(2), NO(2), and O(3) were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro (CF(3)(CF(2))(8)-, or F-) ω-functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation of atmospherically important triatomic molecules on model organic surfaces. Overall, the extent of energy transfer in gas collisions with all of the surfaces studied was substantial. Specifically, the triatomics scatter from each surface only after dissipating greater than 80% of their incident energy. Furthermore, although the OH-SAM is a more rigid surface, the extent of energy transfer and accommodation of these molecules to the CH(3)- and OH-SAMs were approximately the same. The similar scattering dynamics are likely due to significant gas-surface attractive forces between the triatomics and the OH terminal groups, which compensate for the rigidity of this monolayer. In contrast to the OH- and CH(3)-SAMs, the dominant pathway in collisions of the gases with the F-SAM was impulsive scattering. The portion of molecules that accommodated (<40%) to the F-SAM was about half of the amount that accommodated (～70%) to the CH(3)- and OH-SAMs. Although differences in the surface properties had a significant effect on the dynamics, variances in the chemical and physical properties of the three gases, CO(2), NO(2), and O(3), were found to have little effect on the extent of energy transfer and accommodation for collisions with any one surface.     

Mixed alkanethiol monolayers on gold surfaces: Wetting and stability studies

Studies of wetting and stability of mixed monolayers containing hydrophobie and hydrophilic components are discussed. We are reporting the observation of an apparent concentration-driven transition in the cosine of the contact angles of liquids on mixed monolayers. It is suggested that this phenomenon is due to a possible (true or rounded) surface phase transition, resulting in the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields. The variation of the surface free-energy, both polar and dispersive parts, has been determined as a function of surface OH-concentration. The surface free-energy of the 100% OH surface is close to that found for water, as might be expected for a surface coated with several monolayers of water. Zisman plots obtained for several of the surfaces using polar and nonpolar liquids give γ_c values which follow the observed dispersive contribution to the total surface free energy, and thus do not present a good approximation to the surface free energy (i.e., γ_c < γ_sv).Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH₃ groups at their air-monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (%OH). The effect of different storage environments and temperature on the changing contact angles are discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize over time, depending on the chain length, by surface reorganization and the adsorption of contaminants.     

Investigation into pH-responsive self-assembled monolayers of acylated anthranilate-terminated alkanethiol on a gold surface

This article describes the preparation of pH-responsive self-assembled monolayers (SAMs) of acylated anthranilate-terminated alkanethiol. These monolayers are formed by chemisorption of the alkanethiol molecules onto a gold surface, resulting in different wetting properties of the surfaces depending upon the pH. By using various characterization techniques (e.g., infrared spectroscopy, cyclic voltammetry, contact angle measurements, and surface energy analysis), we have found that the changes in the wetting properties originate from the different surface structures of the monolayers in different pH environments. From surface energy analysis, we found that the disperse components of the surface energy on such SAMs predominate after treatment with pH 1 water, whereas the polar components of the surface energy on such SAMs predominate after treatment with pH 13 water. It is greatly anticipated that this line of research will provide new insight into the mechanism behind pH-responsive properties, facilitating the design and synthesis of new surface-active molecules for the fabrication of pH-responsive functional surfaces.     

Dependence of the rate of an interfacial Diels-Alder reaction on the steric environment of the immobilized dienophile: an example of enthalpy-entropy compensation

This paper describes a physical organic study of the relationship between the rate for an interfacial Diels-Alder reaction and the steric environment around the reacting molecules. The study used as a model reaction the cycloaddition of cyclopentadiene with a self-assembled monolayer (SAM) presenting benzoquinone groups surrounded by hydroxyl-terminated alkanethiolates. The accessibility of the quinone was varied by preparing monolayers from hydroquinone-terminated alkanethiols of different lengths [HS(CH(2))(n)-HQ, n = 6-14] and a hydroxyl-terminated alkanethiol [HS(CH(2))(11)-OH] of constant length. Cyclic voltammetry was used to measure the rate of the reaction by monitoring the decay of the redox-active quinone. The second-order rate constant showed a modest change as the position of quinone was varied relative to the hydroxyl groups of the monolayer. For monolayers wherein the quinone groups were extended from the interface, the rate constants oscillated near 0.20 M(-1) s(-1) with an even-odd dependence on the length of the alkanethiol. For monolayers that positioned the quinone groups below the surrounding hydroxyl groups, the rate constants decreased by approximately 8-fold. Examination of the activation parameters revealed that the quinone groups that were positioned below the interface (and in a crowded environment) reacted with an enthalpy of activation that was 4 kcal/mol greater than did the quinones that were accessible at the interface. The reaction of the buried quinone, however, proceeded with an entropy of activation that was more favorable by 13 eu, and therefore with a similar free energy of activation. The combination of SAMs for preparing model interfaces and cyclic voltammetry for measuring rates provides a new opportunity for physical organic studies of interfacial reactions.     

Construction of mixed mercaptopropionic acid/alkanethiol monolayers of controlled composition by structural control of a gold substrate with underpotentially deposited lead atoms.

Mixed monolayers of 3-mercaptopropionic acid (MPA) and alkanethiols of various chain lengths have been constructed on Au based on a novel concept, namely, control of the composition of the component thiols in mixed monolayers by controlling the surface structure of the substrate. The Au substrate surface was first modified with underpotentially deposited Pb (UPD Pb) atoms, followed by the formation of a self-assembled monolayer (SAM) of alkanethiol. The UPD Pb atoms were then oxidatively stripped from the surface to create vacant site, on which MPA was adsorbed to finally form the mixed monolayers. The surface coverages of Pb, alkanethiol and MPA, and the total numbers of thiols were determined using an electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and reductive desorption voltammetry. These results demonstrate that the surface coverage of MPA in the mixed monolayers is determined by the initial coverage of UPD Pb. Fourier transform infrared spectra also support this conclusion. The observed single peak in the cyclic voltammogram for the reductive desorption shows that MPA and alkanethiol do not form their single-component domains. Scanning tunneling microscopy revealed the single-row pinstripe structure for all the thiol adlayers formed during each step of the preparation. This shows that the surface structure of the mixed monolayers is determined by the structure of the initially formed SAM on Au partially covered with UPD Pb.     

Asymmetric or symmetric bilayer formation during oblique drop impact depends on rheological properties of saturated and unsaturated lipid monolayers

Bilayer structures are formed by approaching two liquid surfaces with phospholipid monolayers, which are brought into contact by oblique drop impact on a liquid surface. Asymmetric bilayers can be produced by the coupling of drop and target monolayers. In contrast, symmetric bilayers or multilayers are formed by collapse of the compressed target monolayer. We show that under all studied conditions bilayer/multilayer synthesis takes place. The experimental conditions for the synthesis of asymmetric or symmetric bilayers are described quantitatively in terms of the surface rheological (surface elasticity and dilational viscosity) and the hydrodynamical parameters (Weber number and impact angle). The composition and mechanical properties of the phospholipid monolayers strongly influences the patterns of drop impact and the bilayer/multilayer formation. Cholesterol stiffens unsaturated phospholipid monolayers and fluidifies saturated monolayers. All monolayers form asymmetric vesicle-like structures, which are stable in the aqueous medium. Additionally, unsaturated phospholipid monolayers without cholesterol form symmetric vesicles by folding parts of the target monolayer. Sufficient presence of cholesterol in unsaturated phospholipid monolayers inhibits the folding of the target monolayer and the subsequent formation of symmetric bilayers. The rheological properties of saturated and unsaturated phospholipid monolayers and their mixtures with cholesterol are discussed. Based on drop impact results it is shown that the state of a so far undefined region in the DPPC/cholesterol phase diagram is a fluid phase.     

Interface and molecular electronic structure vs tunneling characteristics of CH3- and CF3-terminated thiol monolayers on Au(111)

By means of density functional theory calculations, we investigate work functions, energy level alignments, charge transfers, and tunneling characteristics of CH3- and CF3-terminated alkane- and diphenylthiol monolayers on Au(111). While the alignments of the energy levels and the charge transfers at the metal-molecule interface are found to be determined by the value of the clean Au surface work function relative to the HOMO ionization potential (IP) at the thiolate end of the monolayer, the change of work function for the modified Au(111) surface is dominated by the properties of the thiolate monolayer, including the character, saturated or conjugated, of the molecule and the chemical nature and orientation of the terminal group. The tunneling currents through the adsorbed molecular monolayers are calculated using the Tersoff-Hamann approach. The computed difference between the I-V characteristics for the CH3- and CF3-terminated alkanethiol monolayers agree well with available experimental data. The energy barrier at the metal-molecule interface, the molecular electronic structure, and the IP of the terminal group are the key parameters which determine the tunneling properties.     

Preferential adsorption of amino-terminated silane in a binary mixed self-assembled monolayer

The composition and structure of a binary mixed self-assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APS, NH(2)(CH(2))(3)Si(OCH(2)CH(3))(3)) and octadecyltrimethoxysilane (ODS, CH(3)(CH(2))(17)Si(OCH(3))(3)) on a silicon oxide surface have been characterized by water contact-angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. XPS demonstrated that APS in the mixed SAM is significantly enriched in comparison to that in solution, indicating the preferential adsorption of APS during the SAM formation. AFM observations showed that the mixed SAM becomes rougher. SFG revealed that the coadsorption of APS induced a conformation disordering in the ODS molecules present in the mixed SAM. The surface enrichment of APS has been explained in terms of differences in the surface adsorption rates of the two components as well as in the self-congregation states of APS molecules in the bulk solution. Furthermore, the structure of the water molecules on the mixed SAM surface in contact with the aqueous solutions at different pH's has also been studied. The results indicate that the mixed-SAM modified surface is positively charged at pH < 5 and negatively charged at pH > 7.     

Experimental study of the interplay between long-range electron transfer and redox probe permeation at self-assembled monolayers: evidence for potential-induced ion gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.     

Mechanical stabilization effect of water on a membrane-like system

The penetration resistance of a prototypical model-membrane system (HS-(CH2)11-OH self-assembled monolayer (SAM) on Au(111)) to the tip of an atomic force microscope (AFM) is investigated in the presence of different solvents. The compressibility (i.e., height vs tip load) of the HS-(CH2)11-OH SAM is studied differentially, with respect to a reference structure. The reference consists of hydrophobic alkylthiol molecules (HS-(CH2)17-CH3) embedded as nanosized patches into the hydrophilic SAM by nanografting, an AFM-assisted nanolithography technique. We find that the penetration resistance of the hydrophilic SAM depends on the nature of the solvent and is much higher in the presence of water than in 2-butanol. In contrast, no solvent-dependent effect is observed in the case of hydrophobic SAMs. We argue that the mechanical resistance of the hydroxyl-terminated SAM is a consequence of the structural order of the solvent-SAM interface, as suggested by our molecular dynamics simulations. The simulations show that in the presence of 2-butanol the polar head groups of the HS-(CH2)11-OH SAM, which bind only weakly to the solvent molecules, try to bind to each other, disrupting the local order at the interface. On the contrary, in the presence of water the polar head groups bind preferentially to the solvent that, in turn, mediates the release of the surface strain, leading to a more ordered interface. We suggest that the mechanical stabilization effect induced by water may be responsible for the stability of even more complex, real membrane systems.     

Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents

Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent. Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results. Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude. W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs. Copyright 1999 Academic Press.     

Monolayer behavior of binary systems of betulinic acid and cardiolipin: thermodynamic analyses of Langmuir monolayers and AFM study of Langmuir-Blodgett monolayers

Betulinic acid (BA, a natural pentacyclic triterpene) can induce mitochondrial membrane damage and trigger the mitochondrial pathway of apoptosis in tumor cells. The monolayer behavior of binary systems of BA and cardiolipin (CL, a unique phospholipid found only in mitochondria membrane in animals) was studied by surface pressure-area (π-A) measurements and analyses and Atomic force microscopy (AFM) observation. The miscibility analysis presents that in mixed monolayers BA takes both tilted and nearly perpendicular orientations at surface pressure below 30 mN/m but only nearly perpendicular orientation at 30 mN/m. The thermodynamic stability analysis indicates that phase separation and repulsion occur in mixed BA/CL monolayers. The compressibility analysis shows that at 30 mN/m, 20% addition of BA does markedly translate the liquid-condensed CL monolayer to mixed BA/CL monolayer with the coexistence of liquid-condensed and liquid-expanded phases. The AFM images of supported monolayers give direct evidence of the conclusions obtained from the analyses of π-A isotherms. These results confirm that at high surface pressure near to real biologic situations, BA orients nearly perpendicularly with hydroxyl group toward water, causes phase separation and changes the permeability of CL film, which correlates with the mitochondrial membrane damage induced by BA.     

Surface-induced aggregation of beta amyloid peptide by co-substituted alkanethiol monolayers supported on gold

The primary pathological characteristic of Alzheimer's disease is the presence in the brain of self-assembled beta amyloid (Abeta) protein fibrils, consisting of 35-43 amino acid residues. The toxicity of the aggregated protein structures has previously been proposed to be related to the interaction of Abeta fibrils with neuronal membranes (phospholipid bilayers). Here, surfaces consisting of self-assembled alkanethiol monolayers with different end groups--supported on Au--are used to test the effect of surface chemistry on the structure and morphology of aggregates formed from an active fragment (Abeta10-35) of the Abeta peptide. The influence of monolayer nature (end group) on the aggregation of Abeta10-35 was examined using reflection-absorption infrared spectroscopy (RAIRS) and scanning force microscopy (SFM). Evaluation of the SFM and RAIRS data reveals the presence of Abeta10-35 protein on the various monolayer surfaces, with the surface protein possessing predominantly beta-sheet and random-coil conformations. Time-dependent studies of the extent of Abeta10-35 aggregation and deposition on the various surfaces and the effect of the monolayers on seeding of Abeta10-35 aggregates in solution are also discussed.     

Penetration of surfactin into phospholipid monolayers: nanoscale interfacial organization

Atomic force microscopy (AFM) combined with surface pressure-area isotherms were used to probe the interfacial behavior of phospholipid monolayers following penetration of surfactin, a cyclic lipopeptide produced by Bacillus subtilis strains. Prior to penetration experiments, interfacial behavior of different surfactin molecules (cyclic surfactins with three different aliphatic chain lengths--S13, S14, and S15--and a linear surfactin obtained by chemical cleavage of the cycle of the surfactin S15) has been investigated. A more hydrophobic aliphatic chain induces greater surface-active properties of the lipopeptide. The opening of the peptide ring reduces the surface activity. The effect of phospholipid acyl chain length (dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine- (DPPC), and distearoylphosphatidylcholine) and phospholipid polar head (DPPC, dipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylserine) on monolayer penetration properties of the surfactin S15 has been explored. Results showed that while the lipid monolayer thickness and the presence of electrostatic repulsions from the interfacial film do not significantly influence surfactin insertion, these parameters strongly modulate the ability of the surfactin to alter the nanoscale organization of the lipid films. We also probed the effect of surfactin structure (influence of the aliphatic chain length and of the cyclic structure of the peptide ring) on the behavior of DPPC monolayers. AFM images and isotherms showed that surfactin penetration is promoted by longer lipopeptide chain length and a cyclic polar head. This indicates that hydrophobic interactions are of main importance for the penetration power of surfactin molecules.