Monolayers Of Dipalmitoylphosphatidic Acid Research Articles

Effect of pH and Salt on Surface pKa of Phosphatidic Acid Monolayers.

The pH-induced surface speciation of organic surfactants such as fatty acids and phospholipids in monolayers and coatings is considered to be an important factor controlling their interfacial organization and properties. Yet, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface pKa) of the protic functional group(s) present. Here, we use three independent methods-compression isotherms, surface tension pH titration, and infrared reflection-absorption spectroscopy (IRRAS)-to study the protonation state of dipalmitoylphosphatidic acid (DPPA) monolayers on water and NaCl solutions. By examining the molecular area expansion at basic pH, the pKa to remove the second proton of DPPA (surface pKa2) at the aqueous interface is estimated. In addition, utilizing IRRAS combined with density functional theory calculations, the vibrational modes of the phosphate headgroup were directly probed and assigned to understand DPPA charge speciation with increasing pH. We find that all three experimental techniques give consistent surface pKa2 values in good agreement with each other. Results show that a condensed DPPA monolayer has a surface pKa2 of 11.5, a value higher than previously reported (∼7.9-8.5). This surface pKa2 was further altered by the presence of Na+ cations in the aqueous subphase, which reduced the surface pKa2 from 11.5 to 10.5. It was also found that the surface pKa2 value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface pKa2 as low as 9.2 for DPPA monolayers in the two-dimensional gaseous phase over NaCl solutions. The experimentally determined surface pKa2 values are also found to be in agreement with those predicted by Gouy-Chapman theory, validating these methods and proving that surface charge density is the driving factor behind changes to the surface pKa2.

Cation effects on phosphatidic acid monolayers at various pH conditions

The impact of pH and cations on phase behavior, stability, and surface morphology for dipalmitoylphosphatidic acid (DPPA) monolayers was investigated. At pH<10, DPPA monolayers on water are predominantly populated by neutral species and display the highest packing density. Cations are found to expand and stabilize the monolayer in the following order of increasing magnitude at pH 5.6: Na+>K+∼Mg2+>Ca2+. Additionally, cation complexation is tied to the pH and protonation state of DPPA, which are the primary factors controlling the monolayer surface behavior. The binding affinity of cations to the headgroup and thus deprotonation capability of the cation, ranked in the order of Ca2+>Mg2+>Na+>K+, is found to be well explained by the law of matching water affinities. Nucleation of surface 3D lipid structures is observed from Ca2+, Mg2+, and Na+, but not from K+, consistent with the lowest binding affinity of K+. Unraveling cation and pH effects on DPPA monolayers is useful in further understanding the surface properties of complex systems such as organic-coated marine aerosols where organic films are directly influenced by the pH and ionic composition of the underlying aqueous phase.

Effects of surface pressure on the properties of Langmuir monolayers and interfacial water at the air-water interface.

The effects of surface pressure on the physical properties of Langmuir monolayers of palmitic acid (PA) and dipalmitoylphosphatidic acid (DPPA) at the air/water interface are investigated through molecular dynamics simulations with atomistic force fields. The structure and dynamics of both monolayers and interfacial water are compared across the range of surface pressures at which stable monolayers can form. For PA monolayers at T = 300 K, the untilted condensed phase with a hexagonal lattice structure is found at high surface pressure, while the uniformly tilted condensed phase with a centered rectangular lattice structure is observed at low surface pressure, in agreement with the available experimental data. A state with uniform chain tilt but no periodic spatial ordering is observed for DPPA monolayers on a Na(+)/water subphase at both high and low surface pressures. The hydrophobic acyl chains of both monolayers pack efficiently at all surface pressures, resulting in a very small number of gauche defects. The analysis of the hydrogen-bonding structure/dynamics at the monolayer/water interface indicates that water molecules hydrogen-bonded to the DPPA head groups reorient more slowly than those hydrogen-bonded to the PA head groups, with the orientational dynamics becoming significantly slower at high surface pressure. Possible implications for physicochemical processes taking place on marine aerosols in the atmosphere are discussed.

Evidence for heterodimers of 2,4,5-trichlorophenol on planar lipid layers. A FTIR-ATR investigation

Trichlorophenols are weak acids of high hydrophobicity and are able to transport protons across the mitochondrial membrane. Thus the proton motive force is dissipated and the ATP production decreased. In situ Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR) experiments with 2,4,5-trichlorophenol (TCP) adsorbed to model membranes resulted in good evidence for the formation of the TCP-heterodimer. Two surfaces were examined: a dipalmitoyl phosphatidic acid (DPPA) monolayer and a planar DPPA/1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC) bilayer. TCP was adsorbed from 1 to 3 mM solutions at pH 6.0 to the lipid layers leading to surface layers at the water/lipid interface. Difference spectra showed an effect on DPPA acyl chains even when it was covered with POPC. Time-resolved measurements revealed two distinct adsorption processes, which were assigned to TCP and its deprotonated anion (phenoxide), respectively. For DPPA/POPC bilayers, the adsorption of TCP was faster than that of its phenoxide, whereas adsorption of both species to DPPA monolayers proceeded with similar velocity. In both cases, phenoxide formation at the membrane was found to be delayed with respect to phenol adsorption. Phenoxide and phenol were retained after replacing the TCP solution with buffer. For the retained species, we estimated a phenol/phenoxide molar ratio of 1 at pH 6.0 (p K a=6.94 for TCP), demonstrating strong evidence for heterodimer formation.

Evidence for heterodimers of 2,4,5-trichlorophenol on planar lipid layers. A FTIR-ATR investigation

Trichlorophenols are weak acids of high hydrophobicity and are able to transport protons across the mitochondrial membrane. Thus the proton motive force is dissipated and the ATP production decreased. In situ Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR) experiments with 2,4,5-trichlorophenol (TCP) adsorbed to model membranes resulted in good evidence for the formation of the TCP-heterodimer. Two surfaces were examined: a dipalmitoyl phosphatidic acid (DPPA) monolayer and a planar DPPA/1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine (POPC) bilayer. TCP was adsorbed from 1 to 3 mM solutions at pH 6.0 to the lipid layers leading to surface layers at the water/lipid interface. Difference spectra showed an effect on DPPA acyl chains even when it was covered with POPC. Time-resolved measurements revealed two distinct adsorption processes, which were assigned to TCP and its deprotonated anion (phenoxide), respectively. For DPPA/POPC bilayers, the adsorption of TCP was faster than that of its phenoxide, whereas adsorption of both species to DPPA monolayers proceeded with similar velocity. In both cases, phenoxide formation at the membrane was found to be delayed with respect to phenol adsorption. Phenoxide and phenol were retained after replacing the TCP solution with buffer. For the retained species, we estimated a phenol/phenoxide molar ratio of 1 at pH 6.0 (p K a =6.94 for TCP), demonstrating strong evidence for heterodimer formation.

Electrode Coating With Ultrathin Films Containing Electroactive Molecules for Biosensor Applications

Redox molecules, i.e., ferrocene (Fc), ferrocene carboxylic acid (Fc-COOH), and 3-3', 5-5'-tetramethylbenzidine (TMB), are good mediators for typical coenzymes of several dehydrogenases. Such mediators were immobilized in monolayers of dipalmitoylphosphatidic acid (DPPA) by means of the cospreading technique at the water-air interface. TMB was found to interact more strongly than Fc and Fc-COOH with the DPPA matrix; moreover, we observed that DPPA/TMB monolayers could be transferred on different solid supports with the Langmuir-Blodgett (LB) technique. LB films, containing TMB immobilized by means of cospreading and incubation procedure, were characterized recording UV-Vis absorption spectra. Such investigation evidenced the presence of TMB with different oxidation states. Hybrid films containing the mediators were prepared combining two techniques: LB and self-assembly of octadecylmercaptane and octylmercaptane on gold. The resulting structures were characterized by means of ellipsometry and contact angle measurements. Information on the influence of the multilayer structure on the electrode currents and on the redox activity of the immobilized mediators was obtained by cyclic voltammetry. A significant electrochemical signal was observed for TMB included in LB films. The sets of results independently evidenced that TMB can be quantitatively embedded in the ultrathin films of a lipid matrix maintaining its electroactive behavior.

Interaction of a Bacterial Endotoxin with Different Surfaces Investigated by in Situ Fourier Transform Infrared Attenuated Total Reflection Spectroscopy

The interaction of negatively charged lipopolysaccharide (LPS) from Pseudomonas aeruginosa (serotype 10) with different kinds of surfaces was investigated by FTIR ATR spectroscopy. The surfaces comprise a hydrophilic germanium plate, a hydrophobic dipalmitoylphosphatidic acid (DPPA) monolayer, a positively charged cross-linked aminopropyltriethoxysilane (ATS) polymer, and a positively charged lipid bilayer consisting of a DPPA inner monolayer attached to the internal reflection element (IRE) and an outer monolayer consisting of equal molar amounts of palmitoyloleoylphosphatidylcholine (POPC) and hexadecylpyridinium (HDPyr) as positively charged component. Whereas the hydrophilic and negatively charged LPS exhibited only weak adsorption to a hydrophilic Ge IRE and to a hydrophobic DPPA monolayer, it bound strongly to the positively charged surface of ATS and interacted also with the POPC/HDPyr membrane. However, in the latter case, the positively charged component (HDPyr) turned out to be not sufficiently strong anchored in the lipid matrix, because it was completely extracted by the endotoxin. The remaining POPC layer exhibited a significantly reduced molecular ordering compared with a compact POPC monolayer. The results demonstrate distinct interactions of the negatively charged LPS with positively charged surfaces and the ability of LPS to remove positively charged molecules from a biomembrane.

Structural and Topographical Characteristics of Dipalmitoyl Phosphatidic Acid in Langmuir Monolayers

Dipalmitoyl phosphatidic acid (DPPA) monolayers at the air–water interface were studied from surface pressure (Π)–area (A) isotherms and at the microscopic level with Brewster angle microscopy (BAM) under different conditions of temperature, pH, and ionic strength. BAM images were recorded simultaneously with Π–A isotherms during the monolayer compression–expansion cycles. DPPA monolayers show a structural polymorphism from the liquid-expanded (LE)–liquid-condensed (LC) transition region at lower surface pressures toward liquid-condensed and solid (S) structures at higher surface pressures. An increase in temperature, pH, or ionic strength provokes an expansion in the monolayer structure. The results obtained from the Π–A measurements are confirmed by the monolayer topography and relative reflectivity. The measurements of relative reflectivity upon monolayer compression showed an increase in relative monolayer thickness of 1.25 and 3.3 times throughout the full monolayer compression from the liquid-expanded to the liquid-condensed and solid states, respectively.

Direct Influence of the Interaction between the First Layer and a Hydrophilic Substrate on the Transition from Y- to Z-Type Transfer during Deposition of Phospholipid Langmuir−Blodgett Films

In view of obtaining homogeneous LB films, monolayers of dipalmitoylphosphatidic acid (DPPA) and of mixed DPPA and dipalmitoylphosphatidylcholine (DPPC) (molar ratio, 2:1) were transferred onto hyd...

Characterization of Modified Staphylococcal Protein A Containing Phospholipid Monolayer on Both Solution and Slide Surfaces

A method is described for incorporation of water-soluble protein Staphylococcal protein A (SpA) into phospholipid monolayer using covalent protein-lipid conjugates in detergent solution. The amphiphilic conjugates have solubility properties very similar to intergral membrane proteins. When the conjugates are applied into dipalmitoyl-phosphatidic acid monolayer, a protein containing monolayer is formed on subphase surface. The monolayer is transferred to pre-coated substrate surface to form an artificial membrane. Results show that unmodified SpA is readily ejected from the monolayer when compressing the monolayer but modified SpA incorporates into the monolayer stably. The incorporation of the protein is proportional to the lipid coupling degree. When the protein is excessively modified, the IgG binding activity of the SpA in the membrane is lost significantly.

Thermal desorption of Langmuir-Blodgett monolayers

A quartz crystal microbalance was used to study thermal desorption of LB layers of stearic acid (SA) and dipalmitoyl phosphatidic acid (DPPA) from Si(111) and Si(100) surfaces. Desorption follows first-order kinetics. A single monolayer of SA desorbs with an activation energy E a of 129±8 kJ mol −1, and a DPPA monolayer has E a = 42±7 kJ mol −1. In both cases E a is independent of coverage. For both SA and DPPA, the second and third monolayers of a three-monolayer LB film desorb at room temperature leaving a more strongly bound first layer for which E a now depends on coverage. Changes in the properties of the first layer are attributed to structural alteration induced by the application of higher layers.