Competitive adsorption of fibrinogen and dipalmitoylphosphatidylcholine at the air/aqueous interface

Structural effects of zinc on phosphatidylserine-containing lipid membranes: kinetic analysis of membrane reorganization

Zinc induces reorganization of phosphatidylserine-containing lipid membranes.

Fibrinogen: a journey into biotechnology

Fibrinogen has been known since the mid-nineteenth century. Although initially its interest had been within the field of physiology over time its study has spread to new disciplines such as biochemistry, colloids and interfaces or biotechnology. First, we will describe the bulk properties of the molecule as well as its supramolecular assembly with different ligands by using different techniques and theoretical models. In the next step we will analyze the interfacial properties, an important topic because fibrinogen is considered to be a major inhibitor of lung surfactants' function at the lining layer of alveoli. The final step will be devoted to its main application in biotechnology. Thus, the adsorption of fibrinogen at solid/electrolyte interfaces and at carrier particles will be discussed. The reversibility of adsorption, fibrinogen molecule orientation, and maximum coverage will be thoroughly discussed. The stability of fibrinogen monolayers formed at these surfaces with respect to pH and ionic strength cyclic changes will also be presented. Based on the physicochemical data, adsorption kinetics and colloid particle deposition measurements, probable adsorption mechanisms of fibrinogen on solid/electrolyte interfaces will be defined.

Application of Three-Dimensionally Printed Probe and Reservoir to Critical Micelle Concentration Determination by Microvolume Surface Tension Measurement

It is important to determine a critical micelle concentration (CMC) of a surfactant in a protein formulation for stabilizing the protein at maximum by preventing it from interfacial denaturation. There are several techniques for CMC determination. Among them, surface tensiometry is the most common approach because this has a long history and much data at many research fields. However, large amount of sample solution is usually required for the measurement (e.g., more than 1 mL is necessary when a standard reservoir like a glass petri dish is used). This is one of the hurdles for protein formulators because only a small amount of protein could be used at the early-stage development. In this research, we tried to minimize the required amount of sample solution for surface tension measurement by developing appropriate probe and reservoir using a three-dimensional printer (3D printer). The advantages and capabilities of 3D printer are (1) to control the shape and size of the printed material precisely, (2) to change the figure freely, and (3) to prepare the prototype quickly. After the experiments and thereby the refinement of probe as well as reservoir, we found that CMCs of polysorbate 20, polysorbate 80, and poloxamer 188 in water and protein formulations could be precisely detected using a probe 0.5 mm in diameter and small reservoir with a pocket of 7.5 mm in diameter/0.25 mm in depth which were made by a 3D printer. Furthermore, the required sample solution per each measurement could be reduced to 80 μL, which means more than 90% reduction against a standard reservoir.

Subphase exchange experiments with the pendant drop technique

INTRODUCTION

The development of the coaxial double capillary 15 years ago opened up the possibility to undertake accurate desorption and penetration studies of interfacial layers in the pendant drop technique. Drop and bubble methods offer several advantages with respect to other interfacial techniques. They allow a more stringent control of the environmental conditions, use smaller amounts of material and provide a much higher interface/volume ratio than in conventional Langmuir Troughs.

EXPERIMENTAL

The coaxial capillary was developed 15 years ago at the University of Granada as an accessory for the pendant drop surface film balance. It allows exchanging the subphase of the drop without disturbing the surface film and preserving the drop volume throughout the subphase exchange. Hence, this methodology enables one to carry out a great variety of interfacial studies well beyond the usual adsorption profiles. Penetration studies, sequential adsorption measurements, desorption kinetics, reversibility of adsorption and testing of enzymatic treatments on interfacial layers are amongst the principal applications. The coaxial capillary has been recently upgraded to a multi-exchange device which has boosted its applicability. It can be now used to address multilayer formation, create soft interfacial nano-composites such as membranes, polyelectrolyte assemblies and simulate in vitro digestion in a single droplet.

APPLICATIONS

This review aims to compile the experimental work done, using the pendant drop subphase exchange in the last decade, and how its use has provided new insights into the surface/interfacial properties of many different materials. Special emphasis is placed on recent work regarding simulation of in vitro digestion in order to address issues relating to metabolism degradation profiles. The use of this methodology when dealing with interfacial studies allows setting the foundations of interfacial engineering technology. Based on subphase exchange experiments, we aim to develop models for competitive adsorption of different compounds at the interface and build up layer-by-layer interfacial structures. Future challenges comprise the design of finely adjusted nanoengineering systems, based on multilayer assemblies with tailored functionalities, to match the application demand.

Idiopathic preretinal glia in aging and age-related macular degeneration

During analysis of glia in wholemount aged human retinas, frequent projections onto the vitreal surface of the inner limiting membrane (ILM) were noted. The present study characterized these preretinal glial structures. The amount of glial cells on the vitreal side of the ILM was compared between eyes with age-related macular degeneration (AMD) and age-matched control eyes. Retinal wholemounts were stained for markers of retinal astrocytes and activated Müller cells (glial fibrillary acidic protein, GFAP), Müller cells (vimentin, glutamine synthetase) and microglia/hyalocytes (IBA-1). Retinal vessels were labeled with UEA lectin. Images were collected using a Zeiss LSM 710 confocal microscope. Retinas were then cryopreserved. Laminin labeling of cryosections determined the location of glial structures in relation to the ILM. All retinas investigated herein had varied amounts of preretinal glia. These glial structures were classified into three groups based on size: sprouts, blooms, and membranes. The simplest of the glial structures observed were focal sprouts of singular GFAP-positive cells or processes on the vitreal surface of the ILM. The intermediate structures observed, glial blooms, were created by multiple cells/processes exiting from a single point and extending along the vitreoretinal surface. The most extensive structures, glial membranes, consisted of compact networks of cells and processes. Preretinal glia were observed in all areas of the retina but they were most prominent over large vessels. While all glial blooms and membranes contained vimentin and GFAP-positive cells, these proteins did not always co-localize. Many areas had no preretinal GFAP but had numerous vimentin only glial sprouts. In double labeled glial sprouts, vimentin staining extended beyond that of GFAP. Hyalocytes and microglia were detected along with glial sprouts, blooms, and membranes. They did not, however, concentrate in the retina below these structures. Cross sectional analysis identified small breaks in the ILM above large retinal vessels through which glial cells exited the retina. Preretinal glial structures of varied sizes are a common occurrence in aged retinas and, in most cases, are subclinical. While all retinal glia are found in blooms, vimentin labeling suggests that Müller cells form the leading edge. All retinas investigated from eyes with active choroidal neovascularization (CNV) had extensive glial membranes on the vitreal surface of the ILM. Although these structures may be benign, they may exert traction on the retina as they spread along the vitreoretinal interface. In cases with CNV, glial cells in the vitreous could bind intravitreally injected anti-vascular endothelial growth factor. These preretinal glial structures indicate the remodeling of both astrocytes and Müller cells in aged retinas, in particular those with advanced AMD.

Investigating the effect of an arterial hypertension drug on the structural properties of plasma protein

Propanolol is a betablocker drug used in the treatment of arterial hypertension related diseases. In order to achieve an optimal performance of this drug it is important to consider the possible interactions of propanolol with plasma proteins. In this work, we have used several experimental techniques to characterise the effect of addition of the betablocker propanolol on the properties of bovine plasma fibrinogen (FB). Differential scanning calorimeter (DSC), circular dichroism (CD), dynamic light scattering (DLS), surface tension techniques and atomic force microscopy (AFM) measurements have been combined to carry out a detailed physicochemical and surface characterization of the mixed system. As a result, DSC measurements show that propranolol can play two opposite roles, either acting as a structure stabilizer at low molar concentrations or as a structure destabilizer at higher concentrations, in different domains of fibrinogen. CD measurements have revealed that the effect of propanolol on the secondary structure of fibrinogen depends on the temperature and the drug concentration and the DLS analysis showed evidence for protein aggregation. Interestingly, surface tension measurements provided further evidence of the conformational change induced by propanolol on the secondary structure of FB by importantly increasing the surface tension of the system. Finally, AFM imaging of the fibrinogen system provided direct visualization of the protein structure in the presence of propanolol. Combination of these techniques has produced complementary information on the behavior of the mixed system, providing new insights into the structural properties of proteins with potential medical interest.

Effect of a PEGylated lipid on the dispersion stability and dynamic surface tension of aqueous DPPC and on the interactions with albumin

Dispersions of dipalmitoylphosphatidylcholine (DPPC) vesicles at 0.1 wt % (1000 ppm) in aqueous isotonic buffer solutions produced by extensive sonication were found to be colloidally stable for hours and days. They also had very low (<10 mN/m) dynamic surface tension minima (DSTM) under pulsating area conditions at 37 degrees C at 20 rpm area pulsation rate. When a 1000 ppm DPPC dispersion was mixed with a stable solution of 1000 ppm bovine serum albumin (BSA), it became colloidally unstable, aggregating within minutes, implying that heterocoagulation between lipid vesicles and albumin takes place. The heterocoagulated dispersion produced high DSTM because the lipid transport rate to the interface became slower. Moreover, the protein may have been transported to the surface faster and adsorbed more than the lipid at the surface. DPPC lipid vesicles were modified for reducing aggregation with other vesicles or with the protein with the addition of a small weight fraction of a neutral "PEGylated" lipid, with a covalently bonded poly(ethylene glycol) (PEG) group. The mixed vesicles were found to be quite more stable than the DPPC vesicles, remaining stable for months, apparently stabilized by steric forces. The colloidal stability at the initial stages of coagulation was evaluated quantitatively from the Fuchs-Smoluchowski stability ratio W. When the modified lipid vesicle dispersion was mixed with the albumin, the vesicles showed no tendency to aggregate with the albumin molecules for days, also probably because of steric repulsion between the PEGylated lipid and the protein. Finally, the mixed lipid dispersions maintained their low DSTM as did the DPPC vesicles without the albumin, and also in the presence of albumin. The results have implications on the use of DPPC or DPPC-based lipids in treating alveolar respiratory diseases without albumin inhibition of their surface tension lowering ability.

Effect of buffer composition and preparation protocol on the dispersion stability and interfacial behavior of aqueous DPPC dispersions

The effect of the buffer composition and the preparation protocol on the dynamic surface tension (DST) and vesicle sizes of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions was studied. Four isotonic buffers were used in preparing DPPC dispersions at physiological conditions for possible biological applications: (1) a standard PBS solution; (2) the above PBS with 1mM CaCl(2); (3) PBS with one tenth the previous standard phosphate salt concentrations and 2.5 mM CaCl(2); and (4) 150 mM NaCl with 2.5 mM CaCl(2) and 10mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid). Two protocols, with a new method and an old method (Bangham method), were used in preparing the DPPC dispersions. The DPPC dispersions prepared with the new method contained mostly vesicles and were quite stable at 25 or 37 degrees C. Dynamic light scattering (DLS) and spectroturbidimetry (ST) results showed that the DPPC vesicle sizes in buffer (4) were much smaller than those in the other buffers. When the DPPC dispersions were prepared with the new method, the diameter of the DPPC particles was smaller than those with the old method. The DPPC vesicles with the new method were more stable than those with the other method. The DPPC dispersions of 1000 ppm at 37 degrees C with the new method produced, at pulsating area conditions at 20 cycles per minute, low tension minima (gamma(min)), lower than 10 mN/m, in buffers (1), (2), and (4). With buffer (4) the DSTs were lower and were achieved faster than with the other buffers. A minimum concentration of 1000 or 250 ppm DPPC was needed to produce DSTs lower than 10 mN/m within 10 min or less, with buffer (2) or (4), respectively. IRRAS results suggest that DPPC in buffer (2) or (4) forms a close-packed monolayer at the interface. These results have implications for designing efficient protocols of lipid dispersion preparation and lung surfactant replacement formulations in treating respiratory disease.

Protein-directed assembly of binary monolayers at the interface and surface patterns of protein on the monolayers

Ferritin-directed assembly of binary monolayers of zwitterionic dipalmitoylphosphatidylcholine and cationic dioctadecyldimethylammonium bromide (DOMA) at the interface and surface patterns of ferritin on the monolayers have been investigated using a combination of infrared reflection absorption spectroscopy, surface plasmon resonance, and atomic force microscopy. Ferritin binding to the binary monolayers at the air-water interface at the surface pressure 30 mN/m, primarily driven by the electrostatic interaction, gives rise to a change in tilt angle of hydrocarbon chains from 15 degrees +/- 1 degrees to 10 degrees +/- 1 degrees with respect to the normal of the monolayer at the mole fraction of DOMA (XDOMA) of 0.1. The chains at XDOMA = 0.3 are oriented vertical to the water surface before and after protein binding. A new mechanism for protein binding to the binary monolayers is proposed. The secondary structures of the adsorbed ferritin are prevented from changing to some extent due to the existence of the monolayers. The amounts of the bound protein on the monolayers at the air-water interface are increased in comparison with those on the pre-immobilized monolayers at low XDOMA. The increased amounts and different patterns of the adsorbed protein at the monolayers are mostly attributed to the formation of multiple binding sites available for ferritin, which is due to the lateral reorganization of the lipid components in the monolayers induced by the protein in the subphase. The created multiple binding sites on the monolayer surfaces through the protein-directed assembly can be preserved for subsequent protein binding.

Effect of sonication and freezing–thawing on the aggregate size and dynamic surface tension of aqueous DPPC dispersions

The effect of sonication and freezing-thawing on the aggregate size and dynamic surface tension of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions was studied by cryogenic-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), UV-vis spectroturbidimetry, and surface tensiometry. When 1000 ppm (0.1 wt%) DPPC dispersions were prepared with a certain protocol, including extensive sonication, they contained mostly frozen vesicles and were quite clear, transparent, and stable for at least 30 days. The average dispersed vesicles diameter was 80 nm in water and 90 nm in standard phosphate saline buffer. After a freeze-thaw cycle, this dispersion became turbid, and precipitates of coagulated vesicles were observed with large particles of average size of 1.5x10(3) nm. The vesicle coagulation is due to the local salt concentration increase during the freezing of water. This dispersion has much higher equilibrium and dynamic surface tension than those before freezing. When this freeze-thawed dispersion was subjected to a resonication at 55 degrees C, smaller vesicles with sizes of ca. 70 nm were produced, and a lower surface tension behavior was restored as before freezing. Similar behavior was observed at 30 ppm DPPC. These results indicate that the freeze-thaw cycle causes substantial aggregation and precipitation of the vesicles. These results have implications for designing efficient protocols of lipid dispersion preparation and lung surfactant replacement formulations in treating respiratory disease and for effective administration.

Stability and state of aggregation of aqueous fibrinogen and dipalmitoylphosphatidylcholine lipid vesicles

The stability and state of aggregation of aqueous fibrinogen (FB) and dipalmitoylphosphatidylcholine (DPPC) vesicles in water or buffer at 25 degrees C were studied with dynamic light scattering (DLS), UV-vis spectroturbidimetry (ST), and cryo-transmission electron microscopy (cryo-TEM). In water, when 1000 ppm (0.10 wt %) DPPC dispersions were prepared with a protocol including extensive sonication, they contained mostly vesicles and were quite clear, transparent, and stable for at least 30 days. FB mixtures with water (0.075 wt %) were quite unstable and biphasic. They formed large aggregates which eventually precipitated. The addition of DPPC vesicles into these unstable FB dispersions reversed FB aggregation and precipitation and produced stable translucent microdispersions. The inferred lipid/protein aggregates were limited in size, with average diameters ranging from 200 to 300 nm. In buffer, DPPC dispersions were also clear and quite stable, with average dispersed particles diameter of ca. 90 nm. FB dissolved in aqueous buffer and formed transparent and stable solutions. Adding salt to an aggregated FB dispersion in water reversed the aggregation. FB aggregated and redissolved in the presence of the citrate and after the citrate was removed. There was no effect of citrate (present in FB initially) in the FB aggregation or redissolution. FB molecules in buffer form dimers or higher aggregates. Their average aggregation number is 2, determined with Rayleigh scattering analysis of turbidity data. The average hydrodynamic diameter of FB solutions from DLS was 30 nm. Mixing a stable FB solution in buffer and a stable DPPC dispersion in buffer produced highly unstable mixtures, in which large aggregates precipitated. These results have implications in understanding the interactions of lipids and proteins in many biological applications and food processing applications.

Directed Assembly of Binary Monolayers with a High Protein Affinity: Infrared Reflection Absorption Spectroscopy (IRRAS) and Surface Plasmon Resonance (SPR)

Infrared reflection absorption spectroscopy (IRRAS) and surface plasmon resonance (SPR) techniques have been employed to investigate human serum albumin (HSA) binding to binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA). At the air-water interface, the favorable electrostatic interaction between DPPC and DOMA leads to a dense chain packing. The tilt angle of the hydrocarbon chains decreases with increasing mole fraction of DOMA (X(DOMA)) in the monolayers at the surface pressure 30 mN/m: DPPC ( approximately 30 degrees ), X(DOMA) = 0.1 ( approximately 15 degrees ), and X(DOMA) = 0.3 ( approximately 0 degrees ). Negligible protein binding to the DPPC monolayer is observed in contrast to a significant binding to the binary monolayers. After HSA binding, the hydrocarbon chains at X(DOMA) = 0.1 undergo an increase in tilt angle from 15 degrees to 25 approximately 30 degrees , and the chains at X(DOMA) = 0.3 remain almost unchanged. The two components in the monolayers deliver through lateral reorganization, induced by the protein in the subphase, to form multiple interaction sites favorable for protein binding. The surfaces with a high protein affinity are created through the directed assembly of binary monolayers for use in biosensing.

Surface Phase Transition of C12E1 at the Air/Water Interface: A Study by Dynamic Surface Tension, External RA FT-IR, and 2D IR Correlation Methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.

Infrared spectroscopy analysis of mixed DPPC/fibrinogen layer behavior at the air/liquid interface under a continuous compression-expansion condition

The mixed layer behavior of dipalmitoyl phosphatidylcholine (DPPC) with fibrinogen at continuously compressed-expanded air/liquid interfaces was analyzed in situ by infrared reflection-absorption spectroscopy (IRRAS). The reflectance-absorbance (RA) intensities and/or wavenumbers of nu(a)-CH2 and amide I bands for a mixed DPPC/fibrinogen layer at the interface were obtained directly by an infrared spectrometer with a monolayer/grazing angle accessory and a removable Langmuir trough. The nu(a)-CH2 RA intensity-area hysteresis curves of a DPPC monolayer indicate a significant loss of free DPPC molecules at the interface during the first compression stage, which is also supported by the corresponding nu(a)-CH2 wavenumber-area hysteresis curves. For a mixed DPPC/fibrinogen layer at the interface, the amide I RA intensity-area hysteresis curves suggest that the fibrinogen molecules were expelled from the interface upon compression, apparently because of the presence of insoluble DPPC molecules. The squeeze-out of fibrinogen evidently removed a pronounced amount of DPPC from the interface, as judged from the corresponding nu(a)-CH2 intensity and wavenumber data. Moreover, significant adsorption of fibrinogen was found during the subsequent interface expansion stage. With the in situ IRRAS analysis of the mixed layer behavior at the interface, the induced loss of DPPC by fibrinogen expulsion from the compressed interface and the dominant adsorption of fibrinogen to the expanded interface were clearly demonstrated.