Langmuir-Blodgett-Kuhn and self-assembled films of asymmetrically substituted poly(paraphenylene)

Asymmetrically substituted poly(paraphenylene) (PhPPP) with hydrophilic and hydrophobic side chains was investigated. The polymer behavior at the air-water interface was studied on the basis of surface pressure-area (pi-A) isotherms and compression/expansion hysteresis measurements. PhPPP can form stable monolayers with an area per repeat unit of A=0.20+/-0.02 nm2 and a collapse pressure in the range of pi=25 mN/m. Then, Langmuir-Blodgett-Kuhn (LBK) films of PhPPP were prepared by horizontally and vertically transferring the Langmuir monolayers onto hydrophilic solid substrates at pi=12 mN/m. Cross-section analysis of the AFM tapping-mode topography images of a single transferred monolayer reveals a thickness of d0=0.9+/-0.1 nm. Taking into account the obtained monolayer thickness, curve-fitting calculations of angular scan data of LB monolayers measured using surface plasmon resonance (SPR) spectroscopy lead to a value for the refractive index of n=1.78+/-0.02 at lambda=632.8 nm. Next, the spontaneous formation of a PhPPP monolayer by adsorption from solution was studied ex situ by atomic force microscopy and UV-vis spectroscopy and in situ by using SPR spectroscopy. Stable self-assembled monolayers of PhPPP can be formed on hydrophilic surfaces with a thickness similar to that of the monolayer obtained using the LB method. The characterization results confirmed the amphiphilic character and the self-assembly properties of PhPPP, as well as the possibility of preparing homogeneous monolayer and multilayer films.     

Amphiphilic siloxane phosphonate macromolecule monolayers at the air/water interface: effects of structure and temperature

A comprehensive study is reported of Langmuir-Blodgett (LB) films (spread at the air/water interface using the Langmuir balance technique) composed of surface active, nonionic, and OH-free amphiphilic siloxane phosphonate ester macromolecules. Analysis is made on three molecular structures in the form of linear polymer poly(diethylphosphono-benzyl-alphabeta-ethyl methylsiloxane) (PPEMS), cyclic oligomer methylphosphonobenzyl-alphabeta-ethyl cyclosiloxane (MPECS), and copolymer poly(PEMS-co-DMS). The surface pressure-surface area (pi -A) isotherms of homopolymer at 3-40 degrees C show a clear temperature-induced phase transition (plateaus at pit approximately 17-19 mN/m) below 10 degrees C. The magnitude of the transition substantially increases upon lowering the temperature (partial differential DeltaAt/ partial differential T approximately -0.1 nm2 unit(-1) deg(-1) and partial differential pi t / partial differential T approximately -0.25 mN m(-1) deg(-1)). The positive entropy and enthalpy gain infers that strong coupling with the subphase and excess hydration attributed to hydrogen bonding between the P=O bond and the subphase prevails at low temperatures. The cyclic oligomer MPECS forms a condensed monolayer at the air/water interface that does not display a similar transition in the experimental temperature range. The temperature sensitivity of MPECS film is observed only in the collapsed region. The nature of the interaction with the subphase is similar for MPECS and PPEMS, indicating that the size and thermal mobility are the controlling factors in these processes. The elasticity plot reveals two distinct states (above and below transition). This observation is supported by BAM images that show irregular spiral structures below 10 degrees C. The transition occurring in the copolymer at 20 degrees C is due to relaxation of the PDMS component. The two maxima shown in the elasticity plot indicate additive fractions of PPEMS and PDMS. The surface areas of these macromolecules in the relaxed (1.48 nm2/unit) and packed (0.45 nm2/unit) forms obtained by PM3 modeling agree well with the experimental data and seem to indicate that the siloxane chain is being lifted off the subphase by the hydrophobic phenylic part of the molecule.     

柔性间隔基对于香豆素取代二乙炔LB膜聚合及手性形成的影响

设计合成了2种香豆素取代二乙炔单体,7-(10,12-二十三双炔酰氧基)-香豆素(CODA)和7-(10,12-二十三双炔酰氧乙氧基)-香豆素(CO2DA),研究了柔性间隔基对香豆素取代二乙炔单体在气-液界面的组装、单体LB膜的聚合以及聚二乙炔主链螺旋结构形成的影响.利用Langmui-Blodgett(LB)技术,以纯水为亚相,膜压在35 mN/m时沉积制备了香豆素取代二乙炔单体LB膜.尽管CODA是非手性的,但其LB膜均表现出明显的宏观手性信号.这是由于在压缩过程中香豆素基团间强烈的π-π堆积,形成了螺旋排列,显示出超分子手性.而CO2DA LB膜无明显CD信号.经254 nm紫外光辐照,CODA LB膜聚合成蓝相,聚二乙炔主链表现出明显的宏观手性.而CO2DA LB膜聚合后无明显的CD信号.薄膜中香豆素功能基团的不规则排列不利于二乙炔单体的固态聚合以及聚二乙炔主链螺旋结构的形成.     

Surface behavior and peptide-lipid interactions of the cyclic neuropeptide melanin concentrating hormone

The kinetics and the thermodynamics of melanin concentrating hormone (MCH) adsorption, penetration, and mixing with membrane components are reported. MCH behaved as a surface active peptide, forming stable monolayers at a lipid-free air-water interface, with an equilibrium spreading pressure, a collapse pressure, and a minimal molecular area of 11 mN/m, 13 mN/m, and 140 A (2), respectively. Additional peptide interfacial stabilization was achieved in the presence of lipids, as evidenced by the expansion observed at pi > pi sp in monolayers containing premixtures of MCH with zwitterionic or charged lipids. The MCH-monolayer association and dissociation rate constants were 9.52 x 10 (-4) microM (-1) min (-1) and 8.83 x 10 (-4) min (-1), respectively. The binding of MCH to the dpPC-water interface had a K d = 930 nM at 10 mN/m. MCH penetration in lipid monolayers occurred even up to pi cutoff = 29-32 mN/m. The interaction stability, binding orientation, and miscibility of MCH in monolayers depended on the lipid type, the MCH molar fraction in the mixture, and the molecular packing of the monolayer. This predicted its heterogeneous distribution between different self-separated membrane domains. Our results demonstrated the ability of MCH to incorporate itself into biomembranes and supports the possibility that MCH affects the activity of mechanosensitive membrane proteins through mechanisms unrelated with binding to specific receptors.     

Use of high-performance liquid chromatography for assay of glutamic acid decarboxylase. Its limitation in use for post-mortem brain.

Rat brain, obtained 10 min after death, contained high levels of endogenous gamma-aminobutyric acid (GABA) and glutamic acid. Incubation of this brain homogenate at 37 degrees C indicated decrease of GABA with time due to degradation by GABA-transaminase. Reported high-performance liquid chromatographic (HPLC) methods for glutamic acid decarboxylase (GAD) assay depend on the difference between the GABA content of the reaction mixture after and before the incubation period. None of the methods considered the degradation of GABA during incubation. Furthermore, during determination of the Michaelis constant (KM) for the reaction none of them considered the endogenous substrate. Here we have focused on these factors which seriously affect the maximum velocity (Vmax) and KM values during GAD assay by the HPLC technique. By a simple and rapid HPLC technique we have measured GAD activity in post-mortem rat brain after removing endogenous glutamic acid by charcoal treatment and using gabaquline to prevent GABA degradation during incubation period. By this method a Vmax value of 46 +/- 4 nmol/h/mg protein and a KM value of 7.5 +/- 0.6 mM were observed for GAD activity of crude brain homogenate. For a comparative study, we have carried out radiometric assay of GAD activity from the same sample and observed a Vmax of 48 +/- 6 nmol/h/mg protein and KM of 6.9 +/- 0.4 mM.     

Effects of surface pressure on the structure of the monolayer formed at the air/water interface by a non-ionic surfactant

The monolayer formed at an air/water interface by the synthetic non-ionic surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(omega-methoxydodecakis (ethylene glycol)) (2C18E12) has been characterized using Langmuir trough measurements, Brewster angle microscopy (BAM), and neutron reflectometry. The BAM and reflectometry studies were performed at four different surface pressures (pi) in the range 15-40 mN/m. The BAM studies (which give information on the in-plane organisation of the surfactant layer) demonstrate that the 2C18E12 molecules are arranged on the water surface to form distinct, approximately circular, 5 microm diameter domains. As the surface pressure is increased these domains retain their size and shape but are made progressively more close-packed, such that the monolayer is made more or less complete at pi=40 mN/m. The neutron reflectometry measurements were made to determine the structure of the interfacial surfactant layer at pi=15, 28, 34 and 40 mN/m, providing information on the thickness of the 2C18E12 alkyl chains', head groups' and associated solvent distributions (measured along the surface normal), along with the separations between these distributions, and the effective interfacial area per molecule. Partial structure factor analyses of the reflectivity data show that the effective interfacial area occupied decreases from 217 A2 per 2C18E12 molecule at pi=15 mN/m down to 102 A2 at pi=40 mN/m. There are concomitant increases in the widths of the surfactant's alkyl chains' and head groups' distributions (modelled as Gaussians), with the former rising from 12 A (at pi=15 mN/m) up to 19 A (at pi=40 mN/m) and the latter rising from 13 A (at pi=15 mN/m) up to 24 A (at pi=40 mN/m). The compression of the monolayer is also shown to give rise to an increased surface roughness, some of which is due to the thermal roughness caused by capillary waves, but with a significant contribution also coming from the intrinsic/structural disorder in the monolayer. At all surface pressures studied, the alkyl chains and head groups of the 2C18E12 are found to exhibit a significant overlap, and this increases with increasing pi. Given the various trends noted on how the structure of the 2C18E12 monolayer changes as a function of pi, we extrapolate to consider the structure of the monolayer at pi>40 mN/m (making comparison with its single chain (CnEm) counterparts) and then relate these findings to the observations recorded on the structure and solute entrapment efficiency of 2C18E12 vesicles.     

Fabrication of phytic acid sensor based on mixed phytase-lipid Langmuir-Blodgett films

This paper reports the surface activity of phytase at the air-water interface, its interaction with lipid monolayers, and the construction of a new phytic acid biosensor on the basis of the Langmuir-Blodgett (LB) technique. Phytase was inserted in the subphase solution of dipalmitoylphosphatidylglycerol (DPPG) Langmuir monolayers, and its incorporation to the air-water interface was monitored with surface pressure measurements. Phytase was able to incorporate into DPPG monolayers even at high surface pressures, ca. 30 mN/m, under controlled ionic strength, pH, and temperature. Mixed Langmuir monolayers of phytase and DPPG were characterized by surface pressure-area and surface potential-area isotherms, and the presence of the enzyme provided an expansion in the monolayers (when compared to the pure lipid at the interface). The enzyme incorporation also led to significant changes in the equilibrium surface compressibility (in-plane elasticity), especially in liquid-expanded and liquid-condensed regions. The dynamic surface elasticity for phytase-containing interfaces was investigated using harmonic oscillation and axisymmetric drop shape analysis. The insertion of the enzyme at DPPG monolayers caused an increase in the dynamic surface elasticity at 30 mN m(-)(1), indicating a strong interaction between the enzyme and lipid molecules at a high-surface packing. Langmuir-Blodgett (LB) films containing 35 layers of mixed phytase-DPPG were characterized by ultraviolet-visible and fluorescence spectroscopy and crystal quartz microbalance nanogravimetry. The ability in detecting phytic acid was studied with voltammetric measurements.     

Thermodynamics of the liquid expanded to condensed phase transition of poly(L-lactic acid) in Langmuir monolayers

Surface pressure-area per monomer (pi-A) isotherms show that poly(L-lactic acid) (PLLA) Langmuir monolayers exhibit a liquid expanded-to-condensed (LE/LC) phase transition at low surface pressure. Brewster angle microscopy images show circular domains where the LC phase is surrounded by the LE phase during phase coexistence. Morphology studies via atomic force microscopy show that well-ordered patterns are only observed for Langmuir-Blodgett films prepared in the LC phase, while no ordered features are observed in the LE phase. The morphological differences confirm that during the LE/LC phase transition PLLA molecules form well-ordered structures at the air/water interface. Analysis by the two-dimensional Clausius-Clapeyron equation is used to predict the critical parameters (X(c)). Both critical parameters, the critical temperature (T(c)) and the critical pressure (pi(c)), increase with increasing number average molar mass (M(n)) as X(c) = X(c,infinity) - KM(n)(-1), where X(c,infinity) is the value of the critical parameter at infinite molar mass and K is a constant. For PLLA T(c,infinity) = 36.2 +/- 0.3 degrees C and pi(c,infinity) = 4.53 +/- 0.06 mN x m(-1). This study provides a model polymer system for examining critical behavior in two dimensions.     

Influence of Poly(ethylenimine) on the Monolayer of Oleic Acid at the Air/Water Interface

The effect of poly(ethylenimine) (PEI) dissolved in water on the surface pressure-area isotherm of oleic acid (OA) at the air/water interface was investigated. On a concentrated PEI solution, the isotherm of the OA monolayers exhibited a noticeable difference as a function of subphase pH. PEI caused the collapse pressure of the OA monolayer to increase up to 45 mN/m, due to a stronger acid-base-type interaction occurring between the amine group of the PEI and the carboxyl group of OA; on a pure water subphase, the collapse pressure was;28 mN/m. On the other hand, owing to a stronger OA-PEI interaction, the OA monolayers favored a liquid-expanded state more on the PEI-containing water subphase than on the pure water. From the QCM measurement, each OA molecule appeared to interact, on average, with 4.3-5.8 ethylenimine repeating units at basic pHs. We also found that OA multilayers could be assembled on a hydrophilic substrate by a Z-type Langmuir-Blodgett (LB) deposition in a PEI-containing subphase at basic pHs. The ATR-IR spectral data revealed that, in a Z-type LB film, the headgroup of OA was mostly present as carboxylate, interacting in an ionic state with the protonated amine groups of PEI. In acidic conditions, neither a Y-type nor a Z-type deposition was really accomplished. Nonetheless, the ATR-IR spectral data suggested that OA molecules should exist in a monomeric state in a LB film assembled at acidic pHs without PEI while they would form intermolecular hydrogen bridges and/or dimers in the presence of PEI. Copyright 2000 Academic Press.     

Infrared reflection absorption spectroscopy coupled with Brewster angle microscopy for studying interactions of amphiphilic triblock copolymers with phospholipid monolayers

Novel water-soluble amphiphilic triblock copolymers poly(glycerol monomethacrylate)-b-poly(propylene oxide)-b-poly(glycerol monomethacrylate) (PGMA-b-PPO-b-PGMA) were synthesized because of their expected enhanced ability to interact with biological membranes compared to the well-known poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) block copolymers. Their bulkier hydrophilic PGMA blocks might induce a disturbance in the packing of liquid-crystalline lipid bilayers in addition to the effect caused by the hydrophobic PPO block alone. To gain a better insight into the polymer-membrane interactions at the molecular level, the adsorption kinetics and concomitant interactions of (PGMA14)(2-)PPO(34) with model membranes of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were monitored using infrared reflection absorption spectroscopy (IRRAS) coupled with Brewster angle microscopy (BAM) and surface pressure (pi) measurements. The maximum penetration surface pressure of ca. 39 mN/m suggests that (PGMA14)(2-)PPO(34) is able to insert into lipid monolayers even above the so-called monolayer-bilayer equivalent pressure of 30-35 mN/m. Copolymer adsorption to a liquid-expanded DPPC-d62 monolayer proceeds in a two-step mechanism: (i) initially only the more hydrophobic PPO middle block penetrates the lipid monolayer; (ii) following the liquid-expanded-liquid-condensed (LE-LC) phase transition, the bulky PGMA hydrophilic blocks are dragged into the headgroup region as the PPO block inserts further into the fatty acid region. The adsorption kinetics is considerably faster for DMPC-d54 monolayers due to their higher fluidity. Copolymer adsorption to an LC-DPPC-d62 monolayer leads to a change in the monolayer packing by forcing the lipid alkyl chains into a more vertical orientation, their tilt angle with respect to the surface normal being reduced from initially 30 degrees +/- 3 degrees to 18 degrees +/- 3 degrees. BAM images rule out macroscopic phase separation and show that coalescence of DPPC-d62 LC domains takes place at relatively low surface pressures of pi > or = 23 mN/m, suggesting that (PGMA14)(2-)PPO (34) partitions into both LE as well as LC domains.     

An investigation into the mechanisms of DNA strand breakage by direct ionization of variably hydrated plasmid DNA

The mechanisms by which ionizing radiation directly causes strand breaks in DNA were investigated by comparing the chemical yield of DNA-trapped free radicals to the chemical yield of DNA single strand break (ssb) and double strand break (dsb), as a function of hydration (Gamma). Solid-state films of plasmid pUC18, hydrated to 2.5 < Gamma < 22.5 mol, were X-irradiated at 4 K, warmed to room temperature, and dissolved in water. Free radical yields were determined by EPR at 4 K. With use of the same samples, Gel electrophoresis was used to measure the chemical yield of total strand breaks, which includes prompt plus heat labile ssb; G'total(ssb) decreased from 0.092 +/- 0.016 micromol/J at Gamma= 2.5 to 0.066 +/- 0.008 micromol/J at Gamma= 22.5. Most provocative is that at Gamma= 2.5 the yield of total ssb exceeds the yield of trapped deoxyribose radicals: G'total(ssb) - G'sugar(fr) = 0.06 +/- 0.02 micromol/J. Nearly 2/3 of the strand breaks are derived from precursors other than radicals trapped on the deoxyribose moiety. To account for these nonradical precursors, we hypothesize that strand breaks are produced by two one-electron oxidations at a single deoxyribose residue within an ionization cluster.     

Protein-poly(silicic) acid interactions at the air/solution interface

The structure of the interface generated by a spread layer of beta-casein on an aqueous colloidal poly(silicic) acid subphase is described. The results are compared with data for the protein alone spread at the air/water interface and the silicate solution. Films develop at the air-solution interface and a strong pH dependence of the interaction causing this is demonstrated. Reflectometry with X-rays and neutrons was used to probe the interaction as a function of subphase pH and film compression. Film thickness, tau/A, scattering length density, rho/A(-2), water volume fraction, phi(w), and surface coverage, Gamma/mg m(-2), were used to quantify the interfacial structure. Where possible, the X-ray and neutron data sets were co-refined enabling phi(w) to be evaluated without assumption regarding the protein density. At pH 5-7, strong protein-silicate interaction occurred, the interface comprising three regions: a discrete protein upper layer on top of a 15 +/- 2 A layer of silicated material followed by a diffuse layer that extended into the subphase.     

Microstructure and Ion Exchange in Stearic Acid Langmuir-Blodgett Films Studied by Fourier Transform Infrared-Attenuated Total Reflection Spectroscopy

Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectra have been recorded of 11-layer Langmuir-Blodgett (LB) films of stearic acid deposited at various surface pressures (0.1, 15, and 35 mN/m), and the molecular orientation angles were evaluated quantitatively, which supplied insight into the molecular order with the alkyl chains tightly packed like crystal in the LB films deposited at the zero and higher surface pressures. These experimental results indicate that, in the Langmuir film as the precursor of LB films, stearic acid molecules self-aggregate to form two-dimensional crystalline domains already even at the zero surface pressure, which results in the inhomogeneity of monolayer. The analysis of dependence of nu(C=O) intensity on the surface pressure, surface density, and subphase temperature leads to the conclusion that the defects in LB films originate from the Langmuir film and be conserved upon deposition. Annealing below 50 degrees C and cooling could improve the monolayer homogeneity, and thus a defect-free or low-defect LB films can be deposited. Furthermore, ion exchange conducted in the LB films, on the other hand, confirms the existence of structure defects in LB films of stearic acid. The polar plane microstructure, lateral transport along the polar planes and the coordination types of stearic acid/cation system may be the rate-limiting process. The results have implication on the possible uses of stearic acid LB films as ion-exchange materials or sensors. Copyright 2001 Academic Press.     

Studies of Langmuir-Blodgett films of an ion pair metal complex containing Eu(III)-Ru(II) dual chromophores

A surfactant ion-pair complex, [Ru(bpy)(2)L][Eu(NTA)(4)](2) (in which L = 1-docosyl-2-(2- pyridyl)benzimidazole, bpy = 2,2'-bipyridine, and NTA = 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedionato) has been synthesized. The surface pressure-area isotherm measurements show that the complex forms a stable Langmuir film at the air-water interface without adding any electrolytes into the subphase. The monolayers formed at the surface pressures of 5 mN m(-1) and 20 mN m(-1), have been successfully transferred onto glass and quartz substrates with the transfer ratios close to unity. The Langmuir-Blodgett films were studied by UV-visible, infrared, and emission spectroscopies, atomic force microscopy, and cyclic voltammetry. The optical, redox, and morphology properties of the LB films were found to be significantly affected by the target surface pressures used for the film depositions.     

Two-dimensional self-assembly of linear poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers at the air-water interface

The interfacial properties of amphiphilic linear diblock copolymers based on poly(ethylene oxide) and poly(epsilon-caprolactone) (PEO-b-PCL) were studied at the air-water (A/W) interface by surface pressure measurements (isotherms and hysteresis experiments). The resulting Langmuir monolayers were transferred onto mica substrates and the Langmuir-Blodgett (LB) film morphologies were investigated by atomic force microscopy (AFM). All block copolymers had the same PEO segment (Mn = 2670 g/mol) and different PCL chain lengths (Mn = 1270; 2110; 3110 and 4010 g/mol). Isothermal characterization of the block copolymer samples indicated the presence of three distinct phase transitions around 6.5, 10.5, and 13.5 mN/m. The phase transitions at 6.5 and 13.5 mN/m correspond to the dissolution of the PEO segments in the water subphase and crystallization of the PCL blocks above the interface similarly as for the corresponding homopolymers, respectively. The phase transition at 10.5 mN/m was not observed for the homopolymers alone or for their blends and arises from a brush formation of the PEO segments anchored underneath the adsorbed hydrophobic PCL segments. AFM analysis confirmed the presence of PCL crystals in the LB films with unusual hairlike/needlelike architectures significantly different from those obtained for PCL homopolymers.     

Interfacial energetics of protein adsorption from aqueous buffer to surfaces with varying hydrophilicity

Adsorption isotherms constructed from time-and-concentration-dependent advancing contact angles thetaa show that the profound biochemical diversity among ten different blood proteins with molecular weight spanning 10-1000 kDa has little discernible effect on the amount adsorbed from aqueous phosphate-buffered saline (PBS) solution after 1 h contact with a particular test surface selected from the full range of observable water wettability (as quantified by PBS adhesion tension tauoa=gammaolv cos thetaoa; where gammaolv is the liquid-vapor interfacial tension and thetaoa is the advancing PBS contact angle). The maximum advancing spreading pressure, Pimaxa, determined from adsorption isotherms decreases systematically with tauoa for methyl-terminated self-assembled monolayers (CH3 SAM, tauo=-15 mN/m), polystyrene spun-coated onto electronic-grade SiOx wafers (PS, tauo=7.2 mN/m), aminopropyltriethoxysilane-treated SiOx surfaces (APTES, tauo = 42 mN/m), and fully water wettable SiOx (tauo=72 mN/m). Likewise, the apparent Gibbs' surface excess [Gammasl-Gammasv], which measures the difference in the amount of protein adsorbed Gamma (mol/cm2) at solid-vapor (SV) and solid-liquid (SL) interfaces, decreases with tauo from maximal values measured on the CH3 SAM surface through zero (no protein adsorption in excess of bulk solution concentration) near tauo=30 mN/m (thetaa=65 degrees). These latter results corroborate the conclusion drawn from independent studies that water is too strongly bound to surfaces with tauo>or=30 mN/m to be displaced by adsorbing protein and that, as a consequence, protein does not accumulate within the interfacial region of such surfaces at concentrations exceeding that of bulk solution ([Gammasl-Gammasv]=0 at tauo=30 mN/m). Results are collectively interpreted to mean that water controls protein adsorption to surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different amino acid compositions.     

N-Myristoylated Phosphatidylethanolamine: Interfacial Behavior and Interaction with Cholesterol

The interfacial packing behavior of N-myristoyldimyristoylphosphatidylethanolamine (N-14:0 DMPE) and its interaction with cholesterol were characterized and compared to the behavior of dimyristoylphosphatidylethanolamine (DMPE) using an automated Langmuir type film balance. Surface pressure and surface potential were monitored as a function of lipid cross-sectional molecular area. N-14:0 DMPE exhibited two-dimensional (2D) phase transitions of a liquid-expanded to condensed nature at many temperatures in the 15-30 °C range, but isotherms showed only condensed behavior at 15 °C. The sharp decline in the surface compressional moduli upon entering the 2D-transition region is consistent with differences in the partial molar areas of coexisting liquid-expanded (chain-disordered) and condensed (chain-ordered) phases. Including Ca(2+) in the subphase beneath the negatively charged N-14:0 DMPE caused a downward shift in the 2D-transition onset pressure even in the presence of 100 mM NaCl. The average dipole moments perpendicular to the lipid-water interface for N-14:0 DMPE's liquid-expanded and condensed phases were higher than those of DMPE. At surface pressures sufficiently low (<10 mN/m) to produce liquid-expanded phase behavior in pure N-14:0 DMPE, mixing with cholesterol resulted in a classic "condensing effect". Maximal area condensation was observed near equimolar N-14:0 DMPE/cholesterol. Insights into mixing behavior at high surface pressures that mimic the lipid cross-sectional areas of biomembranes were provided by analyzing the surface compressional moduli as a function of cholesterol mole fraction. Complex mixing patterns were observed that deviated significantly from theoretical ideal mixing behavior suggesting the presence of lipid "complexes" and/or a liquid-ordered phase at high sterol mole fractions (>0.35) and low to intermediate surface pressures (<20 mN/m) as well as the possible coexistence of relatively immiscible solid phases at higher surface pressures (e.g., 35 mN/m).     

FT-Raman and FTIR spectroscopic studies of N-octadecanoyl-L-alanine amphiphiles

FTIR spectroscopy is used to compare the difference in molecular structure between Langmuir-Blodgett (LB) films (transferred at the surface pressure 40 mN/m with the vertical method and 0 mN/m with the horizontal method) and bulk sample of N-Octadecanoyl-L-alanine amphiphiles. The bulk sample possesses a very similar microstructure (intermolecular hydrogen-bonding interaction and triclinic chain packing) to the well-ordered LB films. Much information on molecular structure of the bulk sample is obtained using FT-Raman spectroscopy, and several weak Raman scattering peaks are assigned.     

Thermodynamic characteristics and Langmuir-Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.     

Thin films of hydrophobically modified poly(N,N-dimethylacrylamide)

The behavior of a poly(N,N-dimethylacrylamide) hydrophobically modified by incorporating 0.33 mol % of a pyrenyl derivative, [4-(1-pyrenyl)butyl]amine hydrochloride (PY) and 3.56 mol % of dodecylamine (DO) has been studied at the air/water interface. Surface pressure-area isotherm measurements show that the film is initially anchored by the hydrophobic groups at the air-water interface with a pancake-like structure and, with increasing surface pressure, evolves to a quasi mushroom structure, finally reaching a brush configuration at high pressures. Monolayers of this polymer were transferred to silica substrates using the Langmuir-Blodgett (LB) technique at 5, 15, and 25 mN.m(-1). The properties of the LB films were studied by steady-state and time-resolved fluorescence as well as by atomic force microscopy. The results show that the aggregates formed at low pressures are disrupted by pressure increase, while the water-soluble poly(N,N-dimethylacrylamide) becomes dissolved in the water subphase.