I. Panaiotov

Dynamics of insoluble monolayers: I. Dilatational or elastic modulus, friction coefficient, and Marangoni effect for dipalmitoyl lecithin monolayers

The linear propagation of a continuous local surface pressure perturbation Δπ is studied using an original method. The theoretical approach allows the interpretation of the results by considering the dynamics of the liquid substrate and of the film—the Marangoni effect. It is shown that the spreading of Δπ under continuous compression can be described by a diffusion of particle velocity mechanism with a constant Du = (E/αμ), where E is the dilatational modulus of the elastic film, α the Bressler-Talmud coefficient depending on the substrate hydrodynamics, and μ the substrate viscosity. The values of Du are in the range 350–3400 cm2 sec−1. Those of α → 6.8 ± 8 cm−1. The relation of this type of continuous compression experiment with periodical longitudinal waves is discussed. It is stressed that the system studied and the interpretation may be useful for the study of stagnant aqueous films, on solids, bearing a monolayer on their free surface.

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

Interfacial and temporal organization of enzymatic lipolysis

Water-soluble lipolytic enzymes act mainly at the water/lipid interface where their catalytic reactions are coupled with various interfacial phenomena such as penetration and activation of the enzyme, as well as desorption of the soluble products, solubilization in the presence of acceptors or molecular reorganization of the insoluble reaction products, inhibition, and so on. Various models have been proposed to analyze the kinetic data.

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 of monomolecular films of long chain phosphatidylcholine by phospholipase A2 in the presence of β-cyclodextrin

Abstract The desorption rates of monomolecular films of oleic acid (OA) and lysophosphatidylcholine (lyso PC) at the air/water interface by water soluble β-cyclodextrin (β-CD) were studied. The desorption of OA and lyso PC involves the complexation of the single acyl chain into the β-CD cavity and the solubilization of the β-CD/OA or β-CD/lyso PC complex into the aqueous subphase, associated with a rapid decrease in the surface pressure. In the case of monomolecular films of egg phosphatidylcholine (egg PC), 1,2-dihexadecanoyl- sn -glycero-3-phosphatidylcholine (DPPC); 1,2-di( cis -9-octadecenoyl)- sn -glycero-3-phosphatidycholine (DOPC), no detectable changes in the surface pressure occurred after β-CD injection into the subphase. The hydrolysis of medium and long chain PC monomolecular films by phospholipase A 2 (PLA 2 ) was investigated in the absence of β-CD in the subphase. The considerable difference between the apparent kinetic constants was attributed to physical steps possibly involving molecular reorganization in the interface of long chain lipolytic products, associated with enzyme product inhibition. In the presence of β-CD in the subphase, the enzymatic hydrolysis of monomolecular films of long chain PC was found to have kinetic constants which were comparable to those measured with medium chain lipids. Furthermore, comparisons between the desorption rates of the β-CD/lipolytic product complexes and the enzymatic hydrolysis rates of long chain PC, in the presence of β-CD in the aqueous subphase, showed that the rate limiting step is neither the formation of the β-CD/lipolytic product complexes nor their desorption into the water subphase but the hydrolysis of the PC monomolecular films by PLA 2 . The presence of β-CD in the water subphase made it possible for the first time to perform kinetic measurements on the rates of hydrolysis of monomolecular films long chain PC by PLA 2 .

Dynamics of insoluble monolayers: II. Viscoelastic behavior and marangoni effect for mixed protein phospholipid films

Abstract Mixed films of phosphatidylinositol monophosphate and of the apoprotein of the Folch—Lees proteolipid from myelin were continuously compressed and the rates of their surface pressure increase with time were measured at various positions in the film. These rates decreased when the distance from the barrier increased. This behavior may be interpreted by using the theory developed in Part I [Dimitrov et al., J. Colloid Interface Sci. 65, 483 (1978)] and a Maxwell model for a viscoelastic two-dimensional fluid. The specific relaxation time θ thus deduced is very large for the lipid-rich mixed films for our experimental conditions. It is smaller when an excess of protein charges is present.

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.

Effect of surface pressure on mixed dipalmitoyl-lecithin—serum albumin monolayer composition

The lipoprotein surface film which covers the lung alveoli determines their stability and the observed lung pressure—volume hysteresis [1,2]. Serum albumin (SA) and dipalmitoyllecithin (DPL) mixed monolayers may serve as models for this alveolar surface film [3]. The change of surface pressure II of the mixed monolayers was measured during the cyclic alteration of the film area in the range 0–45 dyn cm−1 or 0–70 dyn cm−1. The initial monolayer composition changes, following the increase in surface pressure, as a result of three processes: 1. Conformational changes of SA molecules and reversible expulsion of protein residues. 2. Desorption (irreversible) of the biopolymer from the highly compressed mixed monolayers [7]. 3. Collapse of dipalmitoyllecithin. Various approaches were used for the evaluation of compositional changes of mixed monolayers during monolayer compression. In the range of surface pressures between 13 and 45 dyn cm−1 processes (1) and (2) above involve the displacement of practically all SA residues from the surface plane. At II > 70 dyn cm−1 the monolayers were found to collapse irreversibly and DPL molecules were found to leave the surface. We conclude that the effect of surface pressure on mixed monolayer composition in the range of surface pressures 0–45 dyn cm−1 is due to processes (1) and (2). In the range 0–70 dyn cm−1, processes (1), (2) and (3) occur.

MECHANISMS UNDERLYING THE DESORPTION OF LONG-CHAIN LIPOLYTIC PRODUCTS BY CYCLODEXTRINS : APPLICATION TO LIPASE KINETICS IN MONOLAYER

Abstract The desorption rates of monolayers of insoluble lipolytic products, oleic acid (OA) and mono-olein (MO), spread at the air-water interface, were measured in the presence of β-cyclodextrin (β-CD) or α-cyclodextrin (α-CD) at various subphase pH values. The desorption rates of the CD-OA complex at pH 2 can be satisfactorily described by a kinetic Langmuir equation revealing the existence of an energy barrier in the complex formation of the CD-OA. The dramatic increase observed in the desorption rates of the CD-OA complex at alkaline pH is in agreement with the theoretical prediction that a diffusion process is likely to occur from the surface to the bulk phase. Our results are consistent with the formation of hydrogen bonds between the ionized OA molecule and the O(6)H groups in the CD cavity. In the presence of β-CD in the subphase, comparable rates of hydrolysis of medium and long-chain monomolecular films of various glycerides by human pancreatic lipase (HPL) and Humicola lanuginosa lipase (HL) were observed. Comparisons between the above rates of enzymatic hydrolysis and the desorption rates of the CD-lipolytic product complexes indicated that the monomolecular film hydrolysis is rate limiting and does not depend on the type of CD used.