Tz. Ivanova

Behavior of pure and mixed DPPC liposomes spread or adsorbed at the air-water interface

Liposomes from pure dipalmitoylphosphatidylcholine (DPPC) and mixed DPPC: distearoylphosphatidylcholine (DSPC): soybean lecithin (SL) prepared by the Bangham method with sonication were dispersed into solution or spread at the interface and the kinetics of the surface film formation was studied by measuring and recording the evolution of superficial tension, surface potential, and superficial (14C labeled) DPPC density.A simple theoretical approach can describe these kinetics by two processes: irreversible diffusion of closed vesicles into or from the bulk phase, and irrevers ible transformation of closed spherical vesicles into destroyed ones which form the surface film. Diffusion controls the phenomenon for small initial amounts of liposomes.Transformation controls the phenomenon for important initial amounts of liposomes. The kinetic constant of the transformation,K, does not depend on the technique used to form the surface film (spreading or adsorption).The equilibrium and rheological properties of surface films formed after liposome spreading are compared to those of monolayers

Effect of hydrophobic protein SP-C on structure and dilatational properties of the model monolayers of pulmonary surfactant

Abstract The role of hydrophobic protein SPC on the structure and dilatational rheological properties of the model monolayers of alveolar surfactant was investigated, using a developed rheological approach and atomic force microscopy (AFM) imaging. The non-equilibrium effects due to the reorganization of the cluster lipid monolayers increase with increasing protein content. The results obtained confirm the idea that the hydrophobic pulmonary proteins help to disrupt the bilayers, thereby facilitating the formation of a monolayer at the alveolar surface and the re-spreading of the collapsed phase. The properties of model lipid-protein monolayers were compared with the behaviour of a spread film of commercial seminatural surfactant Curosurf.

Hydrolysis kinetics of poly(d,l-lactide) monolayers spread on basic or acidic aqueous subphases

Abstract The hydrolysis kinetics of insoluble poly( d,l -lactide) monolayers spread on basic or acidic aqueous substrates was studied with a barostat surface balance. A theoretical approach based on the random fragmentation of polymer molecules leading to the appearance of small soluble fragments was developed. Hydrolysis rate constant values were obtained. The role of interfacial organization of the reaction products is discussed. The process of fragmentation of the interfacial polymer structures was visualized by atomic force microscopy imaging.

Comparative kinetics of phospholipase A2 action on liposomes and monolayers of phosphatidylcholine spread at the air-water interface

Abstract A detailed kinetic model describing the surface transformation of spread liposomes along with their enzymatic hydrolysis was developed. The model was applied to the hydrolysis of the long-chain phosphatidylcholine generating reaction products which remain at the interface and to medium-chain substrates from which the products desorb rapidly into the bulk phase. The overall kinetic constants of the hydrolysis in liposomal systems were compared with those obtained with monolayers under barostatic conditions. The values of the interfacial Michaelis-Menten constant were estimated.

Spreading kinetics of dimyristoylphosphatidylcholine liposomes at the air/water interface below and above the main phase-transition temperature

The spreading kinetics of dimyristoylphosphatidylcholine liposomes at the air/water interface, below and above the main phase-transition temperature are studied by measuring the evolution of the surface pressure. The reorganization mechanism of the closed liposomal bilayers into a surface film is dependent on the state of the bilayer. The estimated values of the interfacial reorganization rate constant for liposomes in a liquid-crystal state are larger than those for liposomes in crystal-gel state. Atomic force microscopic imaging demonstrates the different morphology of the interfacial mesophases obtained.

Interfacial properties of adsorbed films made of a PEG2000 and PLA50 mixture or a copolymer at the dichloromethane–water interface

Adsorption kinetics of films of poly(ethylene glycol) (PEG2000) studied by the dynamic pendant drop method showed that PEG2000 was more tensioactive at the dichloromethane (DCM)-water interface than at the air-water interface. When initially solubilized into DCM, PEG2000 segments would form an adsorbed layer with hydrophobic segments buried into the polymer chains turned toward the organic phase. Compression of this layer, accompanied by viscoelastic effects, led to expulsion of some hydrophilic tails toward the water phase. When initially dissolved in water, adsorption of PEG2000 segments led to an elastic PEG2000 layer organized on both sides of the interface. Results showed that when the PEG2000-PLA50 (poly(D,L-lactide)) copolymer film was adsorbed at the DCM-water interface, it resulted in a mixed layer exclusively turned toward DCM and its rheological properties were governed by PLA50. When adsorption at the DCM-water interface resulted from a physical mixture of PEG2000 and PLA50, rheological properties of the film were influenced by the initial localization of PEG2000 in the bulk phases. In the case of a mixed film formed by the adsorption of PLA50 from DCM and PEG2000 from water, results showed that PEG2000 segments totally pushed those of PLA50 away from the interface and exclusively influenced the behavior of the mixed film.

Comparative study of lipolysis by PLA2 of DOPC substrates organized as monolayers, bilayer vesicles and nanocapsules.

The water-soluble lipolytic enzymes act at the interface of insoluble lipid substrates, where the catalytical step is coupled with various interfacial phenomena as enzyme penetration, solubilization of reaction products, loss of mechanical stability of organized assemblies of phospholipids molecule, etc. One biologically relevant example is the enzymatic hydrolysis of DOPC by PLA(2), which results in cleavage of phospholipids molecules into water insoluble lipolytic products, namely oleic acid and lysophospholipid. In general, the enzymatic activity depends on the substrate organization and molecular environment of the catalytic reaction. The lipolysis by phospholipase A(2) of dioleoylphosphatidylcholine substrates organized as monolayer, bilayers vesicles and lipid nanocapsules was studied by measuring the decrease of the surface area at constant surface pressure or increase of the surface pressure at constant area at air-water interface. A kinetic model describing the coupling of the catalytic act with corresponding interfacial phenomena was developed. By using the kinetic model the values for the global hydrolytic kinetic constants were obtained. The obtained value for the monolayer is five orders of magnitude higher than this obtained with small unilamellar vesicles and six orders of magnitude higher then those obtained with lipid nanocapsules. The comparison shows that the enzymatic catalytic act occurring in the lipid environment of the monolayer is more efficacious than at the vesicle and nanocapsules interfaces.

KINETICS OF SPREADING AT THE AIR-WATER INTERFACE OF DIOLEOYLPHOSPHATIDYLCHOLINE LIPOSOMES INFLUENCED BY PHOTODYNAMIC LIPID PEROXIDATION

Abstract The kinetics of surface film formation at the air-water interface after spreading of small unilamellar dioleoylphosphatidylcholine liposomes are studied by measuring the evolution of surface parameters: surface pressure π and surface potential ΔV. Various proposed kinetic models are analysed. The surface transformation of perfectly closed spherical liposomes into a surface film is accelerated by various chemical reactions. An important biologically relevant case is studied: photodynamic peroxidation of lipids by methylene blue. A theoretical approach describing the coupling of this chemical interaction with the process of surface transformation is developed.

Kinetics of lipid layer formation at interfaces

Abstract A formal kinetic model of lipid layer formation at an interface in contact with a liposomal suspension is developed and investigated. Neglecting diffusion (for sufficiently high bulk concentrations) the kinetic scheme consists of two consecutive processes: (I) irreversible transformation of ‘soluble’ intact vesicles from the ‘subsurface’ layer into ‘adsorbed’ ones (‘defected’ or ‘ruptured’ liposomes, ‘mesophases’); and (II) irreversible transformation of the ‘adsorbed’ vesicles into a lipid monolayer. The resulting set of two differential equations is analyzed making use of the ‘steady-state concentration’ approach (with ‘adsorbed’ vesicles as intermediate compound). Numerical results illustrate the predicted kinetic behavior which depends on the relative magnitude of the rates of the two consecutive processes. Approximate analytical solutions in the case of a much slower process I are obtained in some limiting cases. The model is used to estimate rate constants from previously established experimental kinetic data at the air/water interface.

Savinase proteolysis of insulin Langmuir monolayers studied by surface pressure and surface potential measurements accompanied by atomic force microscopy (AFM) imaging

The mechanism of the enzymatic action of Savinase on an insulin substrate organized in a monolayer at the air-water interface was studied. We followed two steps experimental approach classical surface pressure and surface potential measurements in combination with atomic force microscopy imaging. Utilizing the barostat surface balance, the hydrolysis kinetic was followed by measuring simultaneously the decrease in the surface area and the change of the surface potential versus time. The decrease in the surface area is a result of the random scission of the peptide bonds of polypeptide chain, progressively appearance of amino acid residues, and their solubilization in the aqueous subphase. The interpretation of the surface potential data was based on the contribution of the dipole moments of the intact and broken peptide groups which remain at the interface during the proteolysis. An appropriate kinetic model for the Savinase action was applied, and the global kinetic constant was obtained. The application of the AFM revealed the state of the insulin monolayers before and after the Savinase action. The comparison of the topography of the films and the roughness analysis showed that insulin Langmuir-Blodgett (LB) films transferred before the enzyme action were flat, while at the end of hydrolysis, roughness of films has increased and the appearance of 3D structures was observed.