Interactions between hen egg-white lysozyme, PEG2,000, and PLA50 at the air–water interface

In this paper, we compared the efficiency of polymer films, made of a poly(ethylene glycol) (PEG2,000)/poly(d,l-lactide) (PLA50) mixture, or a PEG2,000-PLA50 copolymer, to prevent adsorption of a model protein, the hen egg-white lysozyme (HEWL), at the air-water interface. This was achieved by analyzing the surface pressure/surface area curves, and the X-ray reflectivity data of the polymer films spread on a Langmuir trough, obtained in absence or in presence of the protein. For both the mixture and the copolymer, the amount of protein adsorbed at the air-water interface decreases when the density of the polymer surface coverage increases. It was shown that even in a condensed state, the polymer film made by the mixture can not totally prevent HEWL molecules to adsorb and penetrate the polymer mixed film, but however, protein molecules would not be directly exposed to the more hydrophobic phase, i.e. the air phase. It was also shown that the configuration adopted by the copolymer at the interface in its condensed state would prevent adsorption of HEWL molecules for several hours; this would be due in particular to the presence of PEG segments in the interfacial film.

Spectroscopic studies on poly(ethylene glycol)-lysozyme interactions

In the present paper, different spectroscopic methods were applied to evaluate conformational changes of hen egg-white lysozyme (HEWL) in various solvents and in the presence of poly(ethylene glycol) (PEG). In citrate (0.007M, pH=6), or in Tris (0.1M, pH=7.4), no conformational change of the protein was measured across the range of concentrations tested. In addition, HEWL in ultra-pure water revealed no irreversible conformational change and no activity loss, at least at low concentrations (< or =0.2mg/ml). Whereas PEG can induce a reorganization of water molecules, no change of the secondary and tertiary protein conformations was observed in the presence of PEG. In addition, in the presence of PEG of various molecular weights, no change of enzymatic activity of the HEWL was observed across the range of concentrations tested.

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.

Properties of polyethylene glycol 660 12-hydroxy stearate at a triglyceride/water interface

This study proposed a method to understand the surfactant role in the first step of the formulation of a novel generation of lipidic nanocapsules. A dynamic rheological protocol was applied using a pendant drop tensiometer in order to determine the interfacial properties of the initial mixture implied in the first formulation step. The response, in terms of interfacial elasticities, described how this mixture led to monodisperse nanometer size range structures after a physico-chemical constraint.

Influence of some formulation parameters on lysozyme adsorption and on its stability in solution

According to our results concerning the behavior of lysozyme at interfaces, its secondary structure and its enzymatic activity, successful protein encapsulation would need to maintain a pH value far from the enzyme isoelectric point value during the formulation to reduce, in particular, the adsorption of lysozyme molecules at the created interfaces. Moreover, buffers or salt solution must be used in order to keep intact the native secondary conformation of lysozyme, and preserve its enzymatic activity.

Development and characterization of solid lipid nanoparticles loaded with magnetite

This paper describes the preparation of colloidal lipid particles containing magnetite from warm emulsions. A two step method was used to obtain the nanoparticles: (i) formulation of a transparent phase by heating a O/W emulsion (aqueous surfactant solution melted with a lipid phase, containing the ethyl oleate and soybean lecithin) in which modified lipophilic magnetite is incorporated, and (ii) preparation of the nanoparticles by dispersing the warm transparent phase in cold water (7 degrees C) under mechanical stirring. The latter method gives spherical nanoparticles of a mean size of 62 nm measured by Photon Correlation Spectroscopy and Transmission Electronic Microscopy. The magnetite entrapment efficiency was determined by use of a magnetophoretic sedimentation method.

Structural Properties of Asymmetric Mixed-Chain Phosphatidylethanolamine Films

By use of Langmuir isotherms and atomic force microscopy (AFM) experiments, we compare the structural properties of an asymmetric 1-palmitoyl-2-oleoyl-glycero-3-phosphoethanolamine (PE 181/160) film sampled at 21 degrees C on mica plates with those of a film composed of a mixture of the related saturated phospholipid (PE 160/160) and unsaturated phospholipid (PE 181/181). From PE 181/160, the Pi-A curve obtained presents a plateau (35 mN/m) where AFM images allowed us to determine a nucleation process, which governs the growth of condensed domains. This surface transition is interpreted as reflecting hydrophobic interactions mainly between the alkyl chains of the molecules. Moreover Pi-A curves of a mixture (v/v) of the two related saturated and unsaturated phospholipids (PE 160/160 and PE 181/181) show a "double plateau" (50 and 54 mN/m) and we have characterized a partial surface demixing at every surface pressure. Finally we show that only PE 181/160 is able to form bilayers above the collapse pressure. Copyright 1999 Academic Press.

Why does PEG 400 co-encapsulation improve NGF stability and release from PLGA biodegradable microspheres?

PURPOSE

The aim of this work was to understand the mechanism by which co-encapsulated PEG 400 improved the stability of NGF and allowed a continuous release from PLGA 37.5/25 microspheres.

METHODS

Microparticles were prepared according to the double emulsion method. PEG 400 was added with NGF in the internal aqueous phase (PEG/PLGA ratio 1/1 and 1.8/1). Its effect was investigated through interfacial tension studies. Protein stability was assessed by ELISA.

RESULTS

A novel application of PEG in protein stabilization during encapsulation was evidenced by adsorption kinetics studies. PEG 400 limited the penetration of NGF in the interfacial film of the primary emulsion. Consequently, it stabilized the NGF by reducing the contact with the organic phase. In addition, it avoided the NGF release profile to level off by limiting the irreversible NGF anchorage in the polymer layers. On the other hand, the amount of active NGF released in the early stages was increased. During microparticle preparation, NaCl could be added in the external aqueous phase to modify the structure of microparticles. This allowed to reduce the initial release rate without affecting the protein stability always encountered in the absence of PEG.

CONCLUSIONS

PEG 400 appeared of major interest to achieve a continuous delivery of NGF over seven weeks from biodegradable microparticles prepared by the double emulsion technique.

Tensioactivity and supramolecular organization of the palmitoyl prodrug of timolol

PURPOSE

To improve the bioavailability of the ocular drug timolol by facilitating its transport through the cornea, an amphiphilic prodrug was synthesized via the addition of a palmitic chain by esterification. The present study was undertaken to investigate the physicochemical and tensioactive properties of the prodrug.

METHODS

The amphiphilic properties of the prodrug were firstly investigated by the Wilhelmy plate method. The textures generated by the supramolecular organizations of the ester were visualized by optical microscopy.

RESULTS

The prodrug clearly decreased the surface tension. Optical microscopy provided excellent evidence for the existence of lyotropic liquid crystalline phases: two isotropic but organized phases and a birefringent lamellar phase.

CONCLUSIONS

The results from the ensemble of studies undertaken to determine the amphiphilic properties of the prodrug were all in accord with its ability to form liquid crystalline phases. The liquid crystalline state of the prodrug is believed to introduce a delay in the drug pharmacological effect.

Surface characterization of poly(alpha-hydroxy acid) microspheres prepared by a solvent evaporation/extraction process

This work constitutes the first attempt to characterize the wettability of poly(alpha-hydroxy acid) (PAHA) microspheres in situ, prepared according to a complex process involving emulsification, solvent evaporation, washing and freeze-drying. The analysis of the flotation profile of the microspheres has allowed us to determine both advancing and receding contact angles at the microsphere/air/water interface and furnished information on the organization of poly(vinyl alcohol) (PVA) and bovine serum albumin (BSA) at the surface of the PAHA coating. By the comparison of contact angles measured from model surfaces obtained by sampling pure PAHA, PVA, BSA and mixed PVA/PAHA monolayers on glass and poly(methyl methacrylate) (PMMA) substrates, it was concluded that the emulsifier (PVA or BSA) was strongly anchored to the surfaces of the microspheres. The use of BSA to formulate the microspheres from a single oil-in-water emulsion led to dry particles having a hydrophobic surface. The unfolding of the hydrophilic segments of the BSA embedded at the surface of the microspheres, following immersion in water, increased the wettability of the microspheres by water. The same qualitative results were obtained when PVA was used to stabilize single emulsions. On the other hand, microspheres formulated from a double water-in-oil-in-water emulsion displayed no modifications of their wettability when immersed in water. This can be explained by the absence of mobility of the hydrophilic segments of the emulsifier which are blocked in the surface or at the subsurface of the polymer matrix.

Kinetics of liposome disintegration from foam film studies: Effect of the lipid bilayer phase state

The kinetics of interfacial liposome breakdown is investigated in the thin liquid film microinterferometric set up of Scheludko et al. Suspensions of small unilamellar vesicles of dimyristoylphosphatidylcholine are studied at temperatures above and below the temperature of the main gel-liquid crystal first order phase transition. The experimentally established time traces of the velocity of thinning of foam films are used to estimate the kinetic (rate) constants of interfacial liposome disintegration. New and previously established data for other lipids are summarized and compared with results from kinetic measurements of lipid monolayer formation. The thin film experiments confirm the existence of interfacial liposomal aggregates. A change in the kinetic behaviour is observed, due to the 'melting' of the hydrophobic tails in the lipid aggregates. This may have various consequences of biological and pharmacological importance.

A structural study of interfacial phospholipid and lung surfactant layers by transmission electron microscopy after Blodgett sampling: influence of surface pressure and temperature

Monolayer studies of the lung surfactant extract (LSE), dipalmitoyl phosphatidilcholine (DPPC) and dioleyl phosphatidilcholine (DOPC) have been performed in the dynamic condition at various temperatures. These compounds were also studied by differential scanning calorimetry, and the Langmuir Blodgett films were examined by electron microscopy. The combination of these techniques allowed us to describe precisely the collapse process, which was found to be different above and below the transition temperature of the lipids. However, whereas a phase separation for DPPC/DOPC mixtures occurred at all temperatures studied, this separation was observed for LSE only at temperatures lower than that characteristic of the "rigid state" to "liquid-like state" transition temperature. The ability of LSE to rapidly respread upon decompression appears to be due to the formation of piled amorphous aggregates formed during compression of its monolayers.

Wettability of polymers by mucin aqueous solutions

The wettability of poly(methyl methacrylate) and polyethylene by water and aqueous mucin solutions have been studied by sessile drop and under-water captive air bubble contact angles, respectively. From the sessile drop and octane under-water contact angles the polymer-water interfaces have been characterized in terms of works of adhesion and acid-base (polar) interactions. A large water-air contact angle hysteresis observed with poly(methyl methacrylate) surfaces has been attributed to side-chain beta relaxations of polymer ester methyl groups. The wettabilities of the polymers by mucin aqueous solutions have been studied as a function of protein concentration and related to the surface tensions. A positive slope of adhesion tension vs surface tension line, characteristic of polar surfaces, was found with poly(methyl methacrylate). By contrast, a change in the slope, explained as a change in mucin relative adsorption densities at solid/liquid and solid/vapour interfaces, was observed with polyethylene. This adhesion tension behavior appeared to be in agreement with previous data we have published concerning the quantity and state of mucin which are adsorbed to polymers characterized by different surface properties.

Collagen at interfaces. II: Competitive adsorption of collagen against albumin and fibrinogen

Adsorption of chemically radiolabeled [14C] collagen from binary mixtures with albumin or fibrinogen was studied on the solution/air and solution/polyethylene interfaces and revealed the preferential adsorption of albumin. This phenomenon is confirmed by the data of surface tension measurements of single protein, collagen-albumin, and collagen-fibrinogen solutions. Desorption experiments clearly show that more irreversibly adsorbed collagen was found on polyethylene surfaces when adsorption was performed from collagen-fibrinogen than from collagen-albumin solutions. The combined adsorption-desorption and the surface tension data show that competitive adsorption of collagen at the hydrophobic surfaces is strongly influenced by the surface tension properties of the proteins in solution.

Collagen at interfaces. I. In situ collagen adsorption at solution/air and solution/polymer interfaces

Collagen was isolated from rat tail tendons and acetylated with 1-14C acetic anhydride. In situ adsorption of this collagen from a buffer solution (pH = 2.7) was measured at the interfaces to air, polyethylene and polyethylene grafted with poly(maleic acid), respectively. The kinetics of adsorption were recorded for all surfaces studied and the corresponding diffusion coefficients for collagen in solution with various protein concentrations were calculated. The desorption of collagen from polymer surfaces was also studied. These experiments reveal the existence of both a reversibly and an irreversibly adsorbed collagen layer on the polymers tested. The desorption/adsorption ratio for the polyethylene is higher than that for the grafted polyethylene indicating stronger interactions of collagen with the grafted surface than with the non-modified polyethylene.

Adsorption of bovine submaxillary mucin on silicone contact lenses grafted with poly(vinyl pyrrolidone)

Bovine submaxillary mucin is considered to be an analogue of the high molecular protein present in the conjunctival mucus. This mucin was isolated from fresh salivary glands and acetylated with [1-14C]acetic anhydride. In situ adsorption of the bovine submaxillary mucin on silicone contact lenses ungrafted and grafted with poly(vinyl pyrrolidone) was performed for the first time using an original radiotracer technique. The results show that the adsorbed amounts of mucin are higher on grafted samples and that thick layers are adsorbed when mucin concentration in the bulk solution is increased. Desorption experiments reveal that in addition to the tightly adsorbed protein layer, a loosely bound mucin layer of the same thickness exists on grafted and ungrafted silicones.