Polymerization of a cysteinyl peptidolipid Langmuir film

The surface pressure-area isotherm of a cysteinyl peptidolipid on a pure water subphase (pH 5.8) was compared with that on a water subphase saturated with oxygen and buffered with ammonium bicarbonate (pH 7.8). A reduction of the limiting molecular area was observed for the isotherm measured on the subphase saturated with oxygen. Hysteresis in the compression-decompression cycles of the Langmuir film was also observed. Taking into consideration the chemical structure of the peptidolipid, we rationalized that the free sulfhydryl groups of the peptidolipid were oxidized in the presence of oxygen in the alkaline subphase to form intermolecular disulfide bonds at the air-water interface. The surface topography of the peptidolipid Langmuir film was observed by epi-fluorescence microscopy and the Langmuir-Blodgett film by environmental scanning electron microscopy (ESEM). The micrographs showed evidence of the polymerization of the cysteinyl peptidolipid at the air-water interface. Furthermore, the XPS spectra of the Langmuir-Blodgett films also proved the existence of disulfide bonds. The control peptidolipid C(18)-Ser-Gly-Ser-OH showed identical surface pressure-area isotherms in the presence or absence of an oxygen-saturated subphase.     

Synthesis and supramolecular self-assembly study of a novel porphyrin molecule in Langmuir and Langmuir-Blodgett films

The self-assembly and supramolecular engineering of porphyrins into ordered arrays have recently attracted much interest because of their promising application potential in molecular and electronic devices, spintronics, energy harvesting and storage, catalysis, and sensor development. We herein report the synthesis and supramolecular self-assembly study of a novel porphyrin molecule, 2Por-TAZ, in Langmuir and Langmuir-Blodgett films. The 2Por-TAZ molecule contains two porphyrin macrocycles attached to a triaminotriazine headgroup. Triaminotriazines are known to form a highly ordered linear supramolecular self-assembly through complementary hydrogen bonding with barbituric acid molecules at the air-water interface. Surface pressure-area isotherm measurements and polarized UV-vis absorption spectroscopic studies indicate that the 2Por-TAZ molecules adopted an edge-on orientation at the air-water interface. Polarized UV-vis absorption study also revealed that the 2Por-TAZ molecules formed linear supramolecular networks on pure water and barbituric acid subphase with porphyrin flat planes facing toward the compression direction. The binding of barbituric acid with 2Por-TAZ molecules was observed from the expansion of the Langmuir monolayer film. Compared to the transferred LB film from pure water subphase, both the UV-vis absorbance and fluorescence emission intensity of the LB film transferred from barbituric acid subphase increased significantly.     

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

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

Monolayer behavior of a pyridyl head-group-containing amphiphile and its miscibility with poly(D,L-lactide-co-glycolide) on different pH subphase

In this paper, we investigated the Langmuir film and Langmuir-Blodgett (LB) monolayer film of a nonionic amphiphilic molecule, 4-(6-p-pyridyloxyl)hexyloxyl-4'-dodecyloxylazobenzene (C(12)AzoC(6)Py) and its mixture with poly(D,L-lactide-co-glycolide) (PLG) at different subphase pH values (2.0, 2.6, 3.3, 4.4, and 6.5, respectively) by surface pressure-area (pi-A) isotherms, in situ interface Brewster angle microscopy (BAM), and ex situ atomic force microscopy (AFM). For pure C(12)AzoC(6)Py, its pi-A isotherms display a plateau when the subphase pH value is lower than 3.0. The pressure of the plateau increases with the decrease of pH until 2.0. Over the plateau, the pi-A isotherms become almost identical to the one under neutral conditions. The appearance of such a plateau can be explained as the coexistence of protonation and unprotonation of pyridyl head groups of the employed amphiphile. In contrast to the homogeneous surface morphology of pure C(12)AzoC(6)Py near the plateau by BAM observation, the surface in the case of its mixing with PLG exhibits a dendritic crystalline state under low surface pressure at subphase pH lower than 3.0. The crystalline state becomes soft and gradually melts into homogeneous aggregates with surface pressure increasing to a higher value than that of the plateau. Meanwhile, the hydrolysis of PLG in the mixture system at the interface has been affirmed to be restrained to a very large extent. And the PLG was believed to be compelled to the up layer of the LB film due to the phase separation, which is examined by AFM. Based on the experimental results, the corresponding discussion was also performed.     

Study of adsorption and penetration of E2(279-298) peptide into Langmuir phospholipid monolayers

The peptide corresponding to the sequence (279-298) of the Hepatitis G virus (HGV/GBV-C) E2 protein was synthesized, and surface activity measurements, pi-A compression isotherms, and penetration of E2(279-298) into phospholipid monolayers spread at the air-water interface were carried out on water and phosphate buffer subphases. The results obtained indicated that the pure E2(279-298) Langmuir monolayer exhibited a looser packing on saline-buffered than on pure water subphase and suggest that the increase in subphase ionic strength stabilizes the peptide monolayer. To better understand the topography of the monolayer, Brewster angle microscopy (BAM) images of pure peptide monolayers were obtained. Penetration of the peptide into the pure lipid monolayers of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) and into mixtures of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) at various initial surface pressures was investigated to determine the ability of these lipid monolayers to host the peptide. The higher penetration of peptide into phospholipids is attained when the monolayers are in the liquid expanded state, and the greater interaction is observed with DMPC. Furthermore, the penetration of the peptide dissolved in the subphase into these various lipid monolayers was investigated to understand the interactions between the peptide and the lipid at the air-water interface. The results obtained showed that the lipid acyl chain length is an important parameter to be taken into consideration in the study of peptide-lipid interactions.     

Role of microenvironment in the mixed Langmuir-Blodgett films

This paper reports the pi-A isotherms and spectroscopic characteristics of mixed Langmuir and Langmuir-Blodgett (LB) films of nonamphiphilic carbazole (CA) molecules mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). pi-A isotherm studies of mixed monolayer as well as the remarkable change in collapse pressure of the mixed monolayer isotherms definitely show that CA is incorporated into PMMA and SA matrices. However, CA is stacked in the PMMA/SA chains and forms microcrystalline aggregates, as is evidenced from the scanning electron micrograph picture. The nature of these aggregated species in the mixed LB films has been revealed by UV-vis absorption and fluorescence spectroscopic studies. The presence of two different kinds of band systems in the fluorescence spectra of the mixed LB films have been observed. This may be due to the formation of low-dimensional aggregates in the mixed LB films. Intensity distribution of different band systems is highly sensitive to the microenvironment of two different matrices as well as also on the film thickness.     

Study of molecular interaction in the mixed film of arachidic acid and metal beta-diketonate complexes by the "surface ions" method

Molecular interaction is very important for the mechanical properties and application of Langmuir films. In general, fatty acid film is stabilized by certain "subphase ions." In this work, two metal beta-diketonate complexes (M(tmhd)n, tmhd=2,2,6,6-tetramethyl-3,5-heptanedionate) were used as "surface ions" to form stable condensed films with different ratios at the air/water interface. The pi-A isotherms of the mixed films had been measured. The smaller molecular area of the metal beta-diketonate complexes indicated that the metal beta-diketonate complexes form multilayer condensed structures at high pressure at the air/water interface. However, arachidic acid (AA) retained a monolayer structure at high pressure in the mixed system. No considerable phase separations appeared during the compression of the mixed films, which indicated that the mixed films of metal beta-diketonate complexes and AA were miscible and stable. The molecular interaction of the two components in the mixed films was investigated in detail. Mixed systems with the mixing ratio of M(tmhd)n:AA=1:2 were chosen to study the effects of the interaction on the mechanical properties of the mixed films. The molecular interaction between AA and Ce(tmhd)4 is proved to be more significant than that between AA and Sr(tmhd)2, and the pi-A isotherms of the mixed films differ a lot from that of pure AA monolayer. Due to the strong intermolecular interaction, the liquid region disappears in the Ce(tmhd)4/AA mixed films, and dynamic elasticity is improved especially at high surface pressure. On the other hand, the interaction between the AA and the Sr(tmhd)2 is much weaker and the effects of the interaction on the properties (pi-A isotherm and dynamic elasticity) of the mixed films are not so significant, especially at low surface pressure. These results are in accordant with that of the UV spectra analyses.     

Inhibition of aggregation of a biomimic peptidolipid Langmuir monolayer by Congo red studied by UV-vis and infrared spectroscopies

A synthetic peptidolipid consisted of a hydrocarbon chain with a chain length of C18 and a peptide moiety of IIGLM terminated with an amine group, designated as C18IIGLM-NH2, has been employed as a biomimic model compound of amyloid peptide for exploring molecular interaction and orientation with the use of the Langmuir monolayer and Langmuir-Blodgett film techniques. Inspired by a well-known fact that a stain reagent, Congo red (CR), binds well to the amyloid-mimic part (IIGLM), inhibition of molecular aggregation of C18IIGLM-NH2 by interaction with CR was expected, and it has been investigated by use of surface pressure-area isotherm, surface dipole moment-area isotherm, Brewster-angle microscopy, and UV-vis/infrared spectroscopies. It has been revealed that monomeric CR molecules whose long axis is parallel to the Langmuir monolayer surface are penetrating the C18IIGLM-NH2 Langmuir monolayer, which plays a role of inhibition of molecular aggregation via hydrogen bonding.     

Use of UV-vis reflection spectroscopy for determining the organization of viologen and viologen tetracyanoquinodimethanide monolayers

UV-vis reflection spectroscopy has been used for proving in situ the organization of pure viologen and hybrid viologen tetracyanoquinodimethanide monolayers at the air-water interface. Other more classical measurements concerning Langmuir monolayers, including surface pressure-area and surface potential-area isotherms, are also provided. The organization of the viologen in the Langmuir monolayer was investigated upon the different states of compression, and the tilt angle of the viologen moieties with respect to the water surface was determined. A gradual transition of the viologen molecules from a flat orientation in the gas phase to a more tilted position with respect to the water surface in the condensed phases occurs. The addition of a tetracyanoquinodimethane (TCNQ) salt in the subphase leads to the penetration of TCNQ anions into the positively charged viologen monolayer forming a hybrid viologen tetracyanoquinodimethanide film where a charge-transfer interaction between the two moieties is observed. From a quantitative analysis of the reflection spectra, an organization model of these hybrid monolayers at the air-water interface is proposed, suggesting a parallel arrangement of viologen and TCNQ units with a 1:2 stoichiometry.     

Molecular organization in protein-lipid film on the water surface studied by x-ray standing wave measurements under total external reflection

The x-ray standing wave method has been applied to study self-assembling processes in a protein-lipid film formed by injecting the protein-lipid mixture of alkaline phosphatase and phosphatidylinositol under the phospholipid monolayer preliminarily deposited on the water subphase by Langmuir method. X-ray standing wave measurements allowed to determine the composition of the protein-lipid film and to locate ions position in the direction normal to the film surface. The presence of trace Ni contamination incorporated in the protein-lipid film from the water subphase has been established. Numerical analysis of the X-ray standing wave fluorescence data revealed that after injection under the phospholipid monolayer, the protein-lipid mixture separated in a self-assembled manner to layered structure, molecules of alkaline phosphatase arranged themselves into a pure protein layer containing no phospholipid molecules.     

Interactions of L-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.     

Film structures of poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers on water or Ag nanoparticle suspension

Newly designed poly(amido amine) dendrimers, which have an azacrown core, hexyl spacers, and methyl ester terminals (aza-C6-PAMAM dendrimer), were spread at the air-water and air-silver nanoparticle suspension interfaces, and their film structures were examined by surface pressure-area (pi-A) and surface potential-area (DeltaV-A) isotherms and epifluorescence microscopy. It was revealed that generation (G) 1.5 aza-C6-PAMAM dendrimer on a water subphase formed homogeneous film with face-on configuration, and this configuration was maintained during compression. On the other hand, a G2.5 dendrimer film on the air-water interface took initially homogeneous and face-on configuration that was followed by the conformational change during compression. Using a silver nanoparticle suspension as subphase, G1.5 film was significantly reinforced, and the partial collapse (cracks) in the film appeared as network texture. For a G2.5 dendrimer film, the pi-A and DeltaV-A isotherm properties were similar to that on the water subphase except for the collapsed film; small spots instead of cracks were formed under the film after collapse. These effects of the silver nanoparticle may be due to the formation of a dendrimer/silver nanoparticle composite. The formation process of the nanocomposite film was verified by UV-vis spectroscopy. For the G1.5 dendrimer, silver clusters and nanoparticles adsorbed to the dendrimer film after spreading and formed a small amount of aggregates. During compression, the aggregation proceeded even at low surface pressure. For the G2.5 dendrimer, a dendrimer/nanoparticle composite was also formed after spreading. However, with the initial compression, the absorption bands of clusters, nanoparticles, and aggregate increased together. Upon further compression, while the bands of cluster and nanoparticles decreased, the bands of aggregate still increased. These results suggest that the G2.5 dendrimer covered the cluster and nanoparticles more efficiently than the G1.5 dendrimer did because of the larger molecular size.     

Effects of a central metal on the organization of 5,10,15,20-tetra-(p-chlorophenyl)-rare earth porphyrin hydroxyl compound at the air/water interface and in Langmuir-Blodgett films

Langmuir monolayers and Langmuir-Blodgett (LB) films of 5,10,15,20-tetra-(p-chlorophenyl) terbium/gadolinium porphyrin hydroxyl compound (TbOH and GdOH) and their mixtures with stearic acid (SA) in a molar ratio of 1:1 were investigated by Brewster angle microscopy (BAM), ultraviolet-visible (UV-vis), and infrared (IR) spectroscopy and atomic force microscopy (AFM). pi-A isotherms showed that well-defined Langmuir monolayers were formed at an air/water interface for the porphyrins and their mixture with SA. The BAM observations suggest that the pi-pi interaction between the GdOH molecules is stronger than that between the TbOH molecules. This result can be further confirmed by the AFM measurements. After the introduction of SA, the pi-pi interaction between the TbOH molecules is broken and thus two phases formed in the mixed LB film. However, it cannot break the stronger pi-pi interaction between the GdOH molecules. Therefore, no phase separation is observed in the GdOH/SA LB film. IR reflection-absorption (RA) spectra showed that the COOH groups of SA are partly converted to COO(-) groups, suggesting that there is an interaction between MOH and SA in the films. This interaction leads the benzene rings of TbOH to rotate toward parallel to the substrate and those of GdOH to rotate toward perpendicular to the substrate. All these results have demonstrated that the central metal ions have great effects on the organization and formation of the films.     

Surface chemistry and in situ spectroscopy of a lysozyme langmuir monolayer

Surface pressure and surface potential-area isotherms were used to characterize a lysozyme Langmuir monolayer. The compression-decompression cycles and stability measurements showed a homogeneous and stable monolayer at the air-water interface. Salt concentration in the subphase and pH of the subphase were parameters controlling the homogeneity and stability of the Langmuir monolayer. In situ UV-vis and fluorescence spectroscopies were used to verify the homogeneity of the lysozyme monolayer and to identify the chromophore residues in the lysozyme. Optimal experimental conditions were determined to prepare a homogeneous and stable lysozyme Langmuir monolayer.     

Miltefosine-cholesterol interactions: a monolayer study

Mixed Langmuir monolayers of miltefosine (hexadecylphosphocholine) and cholesterol have been investigated by recording surface pressure-area (pi-A) isotherms at different subphase pHs (2, 6, and 10) and temperatures (10, 20, 25, and 30 degrees C). The change of both pH and temperature within the investigated range does not modify significantly the behavior of mixed films. The most pronounced effect involves condensation of the miltefosine monolayer by cholesterol, which diminishes in the following order: pH 6 > pH 2 > pH 10. The analyses of pi-A and compressibility modulus dependencies indicate the existence of interactions in the investigated system; at pH 2 and 6, the strongest were found to occur for the mixed film of miltefosine molar fraction (XM) between 0.6 and 0.7 (mean value, 0.66). Such a composition corresponds to the stable complex formation wherein 2 miltefosine molecules and 1 molecule of cholesterol are strongly bound together, mainly by attractive hydrophobic forces between their apolar tails. However, at pH 10 the highest stability occurs for mixtures containing a smaller proportion of miltefosine (XM = 0.5), which means that on alkaline subphases the ability to condense the miltefosine monolayer by cholesterol is less efficient as it requires a higher proportion of cholesterol (1:1 as compared to 1:2 at pH 2 and 6) to attain the maximum stability of the mixed film. The attractive forces between miltefosine and cholesterol are also weaker at pH 10 due to a greater solvatation of the miltefosine polar group. A similar trend is observed on increasing subphase temperature, when monolayers are more expanded.     

Sequence-specific control of azobenzene assemblies by molecular recognition of DNA

We investigated the molecular recognition between the amphiphile AzoAde, which is composed of azobenzene in the hydrophobic and adenine in the hydrophilic portion of the molecule, and oligonucleotides having a homogeneous base (dA30, dT30, dG30, and dC30) at the air-water interface. On the basis of the complementary base-pairing of DNA in the duplex, orderly arrangement of AzoAde on templated dT30 was examined using pi-A isotherm, UV-vis RAS, FT-IR RAS, and XPS measurements. Although there was little interaction between AzoAde and mismatched oligonucleotides (dA30, dG30, and dC30), AzoAde prepared on a dT30 subphase stoichiometrically assembled and interacted with dT30, subsequently forming a J-form assembly at the air-water interface. AFM observation of the LB films revealed the nanostructure of the J-formed AzoAde monolayer on the dT30 subphase as well as the domain structures of the H-formed monolayers on the other oligonucleotide subphases. Therefore, dT30 has a potential application as a template for assembling AzoAde at the air-water interface.     

The miscibility of poly(D,L-lactide-co-glycolide) with amphiphilic molecules and the interaction of their mixtures with DNA at air/water interface

The miscibility of poly(D,L-lactide-co-glycolide) (PLG) with three amphiphilic molecules and the interaction of the PLG/surfactant mixtures with DNA at air/water interface are investigated by pi-A isotherms, Brewster angle microscopy (BAM) and atomic force microscopy (AFM) techniques. The pi-A isotherms of the PLG mixtures with cationic C(12)AzoC(6)PyBr, and C(12)AzoC(6)N(CH(3))(3)Br, are quite different from the pi-A isotherm of pure PLG on water subphase. In contrast to the case, the pi-A isotherm of PLG mixed with nonionic C(12)AzoC(6)OPy is almost identical to the pure PLG except some increasing of molecular area. Similar phenomena are observed on DNA subphase. The in situ BAM and ex situ AFM observations demonstrate that the dispersion of PLG at air/water interface becomes good when it mixes with the two cationic surfactants, whereas quite poor due to the phase separation when it mixes with the nonionic amphiphilic molecule. Based on these results we conclude that the cationic surfactants can affect the conformation change of PLG at air/water interface and figure a well miscibility with polymer whereas the nonionic amphiphilic molecule presents poor miscibility. In addition, the even mixing of the PLG and the cationic surfactants is favorable for the adsorption to DNA more effectively.     

Study of the aggregation of human insulin Langmuir monolayer

The human insulin (HI) Langmuir monolayer at the air-water interface was systematically investigated in the presence and absence of Zn(II) ions in the subphase. HI samples were dissolved in acidic (pH 2) and basic (pH 9) aqueous solutions and then spread at the air-water interface. Spectroscopic data of aqueous solutions of HI show a difference in HI conformation at different pH values. Moreover, the dynamics of the insulin protein showed a dependence on the concentration of Zn(II) ions. In the absence of Zn(II) ions in the subphase, the acidic and basic solutions showed similar behavior at the air-water interface. In the presence of Zn(II) ions in the subphase, the surface pressure-area and surface potential-area isotherms suggest that HI may aggregate at the air-water interface. It was observed that increasing the concentration of Zn(II) ions in the acidic (pH 2) aqueous solution of HI led to an increase of the area at a specific surface pressure. It was also seen that the conformation of HI in the basic (pH 9) medium had a reverse effect (decrease in the surface area) with the increase of the concentration of Zn(II) ions in solution. From the compression-decompression cycles we can conclude that the aggregated HI film at air-water interface is not stable and tends to restore a monolayer of monomers. These results were confirmed from UV-vis and fluorescence spectroscopy analysis. Infrared reflection-absorption and circular dichroism spectroscopy techniques were used to determine the secondary structure and orientation changes of HI by zinc ions. Generally, the aggregation process leads to a conformation change from α-helix to β-strand and β-turn, and at the air-water interface, the aggregation process was likewise seen to induce specific orientations for HI in the acidic and basic media. A proposed surface orientation model is presented here as an explanation to the experimental data, shedding light for further research on the behavior of insulin as a Langmuir monolayer.     

Spectroscopic studies on molecular recognition capabilities of a nucleolipid bearing thymine headgroup to adenosine

The Langmuir-Blodgett (LB) films of octadecanoyl ester of 1-(2-carboxyethyl) thymine deposited from pure water and aqueous adenosine subphases were investigated by ultraviolet-visible (UV-vis), Fourier transform infrared-attenuated total reflection (FTIR-ATR), and Fourier transform surface-enhanced Raman scattering (FT-SERS) spectroscopy. The obtained spectral results indicate that the adenosine molecules in the subphase can be transferred onto solid substrates by LB techniques as a result of the formation of base pairs at the air/water interface. UV-vis spectra alternations indicated that, with increasing adenosine concentration in subphase, more adenosine molecules were recognized by nucleolipid monolayer and were transferred onto the quartz substrates. The closed-packing of the constituent molecules facilitates the photodimerization of the thymine moieties in the headgroup under ultraviolet irradiation. FTIR-ATR results suggest that the hydrocarbon chains of nucleolipid in the LB films deposited from pure water and aqueous adenosine take on a close-packed all trans conformation. By analyzing the FT-SERS spectra results, it can be deduced that the orientation of nucleobase in the headgroup is different before and after the recognition effect occurred. For LB film deposited from pure water, the nucleobases are lying flat on the silver substrates; whereas for LB film deposited from aqueous adenosine, the base pairs take an end-on adsorption on silver substrate.     

Effect of molecular surface packing on the enzymatic activity modulation of an anchored protein on phospholipid Langmuir monolayers

The catalytic activity of a glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase has been studied in Langmuir phospholipid monolayers at different surface pressures. The enzyme substrate, p-nitrophenyl phosphate, was injected into the subphase of mixed enzyme/lipid Langmuir monolayers. Its hydrolysis product was followed by monitoring the absorbance at 410 nm in situ in the monolayer subphase of the Langmuir trough. Several surface pressures, corresponding to different molecular surface densities, were attained by lateral compression of the monolayers. The morphology of the monolayers, observed by fluorescence microscopy, showed three different types of domains owing to the heterogeneous partition of the enzyme within the mixed enzyme/lipid film. The catalytic activity was modulated by the enzyme surface density, and it increased until a pressure of 18 mN/m was reached, but it decreased significantly when the equilibrium in-plane elasticity (surface compressional modulus) increased more noticeably, resulting in alterations in the interface morphology. A model for the modulation of the enzyme orientation and catalytic activity by lipid/enzyme surface morphology and enzyme surface packing at the air/liquid interface is proposed. The results might have an important impact on the comprehension of the enzymatic activity regulation of GPI-anchored proteins in biomembranes.