The Displacement of Preadsorbed Protein with a Cationic Surfactant at the Hydrophilic SiO2−Water Interface

Adsorbed protein film on pump surfaces leads to particle formation during fill-finish manufacturing

During fill-finish manufacturing, therapeutic proteins may aggregate or form subvisible particles in response to the physical stresses encountered within filling pumps. Understanding and quantitating this risk is important since filling may be the last unit operation before the patient receives their dose. We studied particle formation from lab-scale to manufacturing-scale using sensitive and robust protein formulations. Filling experiments with a ceramic rotary piston pump were integrated with a rinse-stripping method to investigate the relationship between protein adsorption and particle formation. For a sensitive protein, multilayer film formation on the piston surface correlated with high levels of subvisible particles in solution. For a robust protein formulation, adsorption and subvisible particle formation were minimal. These results support an aggregation mechanism that is initiated by adsorption to pump surfaces and propagated by mechanical and/or hydrodynamic disruption of the film. The elemental analysis confirmed that ceramic wear debris remained at trace levels and did not contribute appreciably to protein aggregation.

The multi-spectroscopic approach on the interaction between rabbit serum albumin and cationic surfactant: Investigation on the formation and solubilization of the protein aggregation

The protein-surfactant interaction studies have great importance in the range of the application like cosmetics, food, pharmaceutical, detergent industries, and many more. In this study, we have studies protein (rabbit serum albumin, RSA) and a cationic surfactant (cetyltrimethylammonium bromide, CTAB) interaction at different physiological conditions (viz., pH, ionic strength, surfactants concentrations, protein concentration, and many more). They form the protein surfactant complexes. The interchange of electrostatic and hydrophobic force monitors the change in complexes. The three different pHs (below (4.0), above (7.0) and at (4.7) the isoelectric point of RSA) of the medium indicate the three different charges on the protein while surfactant is positive in charge. Critical micelle concentration (CMC) plays a significant role in protein-surfactant interaction. CTAB unfolds the protein at its specific concentration range afterward again; it starts refolded. RSA interacted, with the addition of the CTAB is characterized by many spectroscopic methods like UV-visible, fluorescence, fluorescence time-resolved, circular dichroism, and topographical changes monitored by the AFM. In fluorescence spectra, the blue shift shows the unfolding of RSA. The molecular docking indicates the binding energy of 5.8 kcal mol^-1. The changes below and above the CMC is significant.

Interfacial Adsorption of Silk Fibroin Peptides and Their Interaction with Surfactants at the Solid-Water Interface

Regenerated silk fibroin (RSF) is a Food and Drug Administration-approved material and has been widely used in many biomedical and cosmetic applications. Because of the amphiphilic nature of the primary repeat amino acid sequence (e.g., AGAGAS), RSF peptides can significantly reduce the water surface tension and therefore have the potential to be used as a surface active component for many applications, particularly in the biomedical, cosmetic, pharmaceutical, and food industries. In this paper, the adsorption of RSF peptides separated into molecular fractions of 5-30, 30-300, and >300 kDa has been studied at the solid-water interface by neutron reflection and spectroscopic ellipsometry to assess its surface active behavior. A stable layer of RSF was found to be irreversibly adsorbed at the hydrophilic SiO2-water interface. Changes in solution concentration, pH, and ionic strength all had an impact on the final adsorbed amount found at the interface. There were no significant differences between the final adsorbed amounts or layer structure among the three RSF molecular fractions studied; however, >300 kDa RSF was more stable to changes in solution ionic strength. Adsorption of conventional anionic and cationic surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C12TAB), to the preadsorbed 5-30 kDa RSF revealed penetration of the surfactant into the RSF layer, at concentrations below the critical micellar concentration (CMC). SDS was found in the preadsorbed RSF layer and gradually removed RSF from the surface with an increase in SDS concentration. At concentrations above the CMC, there is near complete removal of RSF by SDS at the interface. C12TAB adsorbed into the preadsorbed RSF layer with considerably less removal of RSF from the interface compared to SDS. At concentrations above the CMC, both C12Tab and RSF were found to coexist at the interface, forming a less thick layer but with a considerable amount of RSF still present.

Surfactant induced aggregation behavior of Merocyanine-540 adsorbed on polymer coated positively charged gold nanoparticles

Surfactant induced aggregation behavior of Merocyanine 540 adsorbed on polymer (PDD) coated gold nanoparticles (AuNP) is reported. The absorption band of the dye shifts to higher energy in the presence of free polymer and polymer coated AuNP implying aggregation. Addition of a negatively charged surfactant (SDS) induces multiple bands in the extinction spectrum of the dye adsorbed on nanoparticle surface. The highest (460nm) and lowest (564nm) energy bands of the dye become prominent at 10 and >50μM SDS concentrations respectively (dye: 10μM; AuNP: 100-200pM). Based on earlier results the high energy band is likely to originate from dye aggregates and the low energy band is likely to originate from dye monomers. This is attributed to the interplay between polymer-surfactant and polymer-dye interactions at the AuNP surface. The extinction spectra of dye adsorbed at AuNP surface remain unaffected in the presence of a positively charged (CTAB) or a neutral surfactant (Tx-100), at low surfactant concentrations. However at higher surfactant concentrations (>60μM) dye aggregation takes place which is attributed to dye-surfactant interactions. The fluorescence intensity of the dye quenched significantly but its lifetime increased in the presence of polymer coated AuNP. This is attributed to aggregation and reduction in the photoisomerization rate of the dye adsorbed on AuNP surface.

Co-adsorption of peptide amphiphile V(6)K and conventional surfactants SDS and C(12)TAB at the solid/water interface

Recent research has reported many attractive benefits from short peptide amphiphiles. A practical route for them to enter the real world of applications is through formulation with conventional surfactants. This study reports the co-adsorption of the surfactant-like peptide, V6K, with conventional anionic and cationic surfactants at the solid/water interface. The time-dependant adsorption behaviour was examined using spectroscopic ellipsometry whilst adsorbed layer composition and structural distribution of the components were investigated by neutron reflection with the use of hydrogen/deuterium labelling of the surfactant molecules. Both binary (surfactant/peptide mixtures) and sequential (peptide followed by surfactant) adsorption have been studied. It was found that at the hydrophilic SiO2/water interface, the peptide was able to form a stable, flat, defected bilayer structure however both the structure and adsorbed amount were highly dependent on the initial peptide concentration. This consequently affected surfactant adsorption. In the presence of a pre-adsorbed peptide layer anionic sodium dodecyl sulfate (SDS) could readily co-adsorb at the interface; however, cationic dodecyl trimethyl ammonium bromide (C12TAB) could not co-adsorb due to the same charge character. However on a trimethoxy octyl silane (C8) coated hydrophobic surface, V6K formed a monolayer, and subsequent exposure to cationic and anionic surfactants both led to some co-adsorption at the interface. In binary surfactant/peptide mixtures, it was found that adsorption was dependent on the molar ratio of the surfactant and peptide. For SDS mixtures below molar unity and concentrations below CMC for C12TAB, V6K was able to dominate adsorption at the interface. Above molar unity, no adsorption was detected for SDS/V6K mixtures. In contrast, C12TAB gradually replaced the peptide and became dominant at the interface. These results thus elucidate the adsorption behaviour of V6K, which was found to dominate interfacial adsorption but its exact adsorbed amount and distribution were affected by interfacial hydrophobicity and interactions with conventional surfactants.

Method for estimating protein binding capacity of polymeric systems

Composite biomaterials made from synthetic and protein-based polymers are extensively researched in tissue engineering. To successfully fabricate a protein-polymer composite, it is critical to understand how strongly the protein binds to the synthetic polymer, which occurs through protein adsorption. Currently, there is no cost-effective and simple method for characterizing this interfacial binding. To characterize this interfacial binding, we introduce a simple three-step method that involves: 1) synthetic polymer surface characterisation, 2) a quick, inexpensive and robust novel immuno-based assay that uses protein extraction compounds to characterize protein binding strength followed by 3) an in vitro 2D model of cell culture to confirm the results of the immuno-based assay. Fibrinogen, precursor of fibrin, was adsorbed (test protein) on three different polymeric surfaces: silicone, poly(acrylic acid)-coated silicone and poly(allylamine)-coated silicone. Polystyrene surface was used as a reference. Characterisation of the different surfaces revealed different chemistry and roughness. The novel immuno-based assay showed significantly stronger binding of fibrinogen to both poly(acrylic acid) and poly(allylamine) coated silicone. Finally, cell studies showed that the strength of the interaction between the protein and the polymer had an effect on cell growth. This novel immuno-based assay is a valuable tool in developing composite biomaterials of synthetic and protein-based polymers with the potential to be applied in other fields of research where protein adsorption onto surfaces plays an important role.

Selected wheat seed defense proteins exhibit competitive binding to model microbial lipid interfaces

Puroindolines (Pins) and purothionins (Pths) are basic, amphiphilic, cysteine-rich wheat proteins that play a role in plant defense against microbial pathogens. This study examined the co-adsorption and sequential addition of Pins (Pin-a, Pin-b, and a mutant form of Pin-b with Trp-44 to Arg-44 substitution) and β-purothionin (β-Pth) model anionic lipid layers using a combination of surface pressure measurements, external reflection FTIR spectroscopy, and neutron reflectometry. Results highlighted differences in the protein binding mechanisms and in the competitive binding and penetration of lipid layers between respective Pins and β-Pth. Pin-a formed a blanket-like layer of protein below the lipid surface that resulted in the reduction or inhibition of β-Pth penetration of the lipid layer. Wild-type Pin-b participated in co-operative binding with β-Pth, whereas the mutant Pin-b did not bind to the lipid layer in the presence of β-Pth. The results provide further insight into the role of hydrophobic and cationic amino acid residues in antimicrobial activity.

Silk layering as studied with neutron reflectivity

Neutron reflectivity (NR) measurements of ultrathin surface films (below 30 nm) composed of Bombyx mori silk fibroin protein in combination with atomic force microscopy and ellipsometry were used to reveal the internal structural organization in both dry and swollen states. Reconstituted aqueous silk solution deposited on a silicon substrate using the spin-assisted layer-by-layer (SA-LbL) technique resulted in a monolayer silk film composed of random nanofibrils with constant scattering length density (SLD). However, a vertically segregated ordering with two different regions has been observed in dry, thicker, seven-layer SA-LbL silk films. The vertical segregation of silk multilayer films indicates the presence of a different secondary structure of silk in direct contact with the silicon oxide surface (first 6 nm). The layered structure can be attributed to interfacial β-sheet crystallization and the formation of well-developed nanofibrillar nanoporous morphology for the initially deposited silk surface layers with the preservation of less dense, random coil secondary structure for the layers that follow. This segregated structure of solid silk films defines their complex nonuniform behavior in the D(2)O environment with thicker silk films undergoing delamination during swelling. For a silk monolayer with an initial thickness of 6 nm, we observed the increase in the effective thickness by 60% combined with surprising decrease in density. Considering the nanoporous morphology of the hydrophobic silk layer, we suggested that the apparent increase in its thickness in liquid environment is caused by the air nanobubble trapping phenomenon at the liquid-solid interface.

Thermodynamics, adsorption kinetics and rheology of mixed protein-surfactant interfacial layers

Depending on the bulk composition, adsorption layers formed from mixed protein/surfactant solutions contain different amounts of protein. Clearly, increasing amounts of surfactant should decrease the amount of adsorbed proteins successively. However, due to the much larger adsorption energy, proteins are rather strongly bound to the interface and via competitive adsorption surfactants cannot easily displace proteins. A thermodynamic theory was developed recently which describes the composition of mixed protein/surfactant adsorption layers. This theory is based on models for the single compounds and allows a prognosis of the resulting mixed layers by using the characteristic parameters of the involved components. This thermodynamic theory serves also as the respective boundary condition for the dynamics of adsorption layers formed from mixed solutions and their dilational rheological behaviour. Based on experimental studies with milk proteins (beta-casein and beta-lactoglobulin) mixed with non-ionic (decyl and dodecyl dimethyl phosphine oxide) and ionic (sodium dodecyl sulphate and dodecyl trimethyl ammonium bromide) surfactants at the water/air and water/hexane interfaces, the potential of the theoretical tools is demonstrated. The displacement of pre-adsorbed proteins by subsequently added surfactant can be successfully studied by a special experimental technique based on a drop volume exchange. In this way the drop profile analysis can provide tensiometry and dilational rheology data (via drop oscillation experiments) for two adsorption routes--sequential adsorption of the single compounds in addition to the traditional simultaneous adsorption from a mixed solution. Complementary measurements of the surface shear rheology and the adsorption layer thickness via ellipsometry are added in order to support the proposed mechanisms drawn from tensiometry and dilational rheology, i.e. to show that the formation of mixed adsorption layer is based on a modification of the protein molecules via electrostatic (ionic) and/or hydrophobic interactions by the surfactant molecules and a competitive adsorption of the resulting complexes with the free, unbound surfactant. Under certain conditions, the properties of the sequentially formed layers differ from those formed simultaneously, which can be explained by the different locations of complex formation.

Rapid screening of surfactant and biosurfactant surface cleaning performance

Surface Plasmon Resonance (SPR) and rubisco protein stain were used as tools to screen the effectiveness of detergent formulations in cleaning a protein stain from solid surfaces. Surfactant and biosurfactant-based formulations, with and without added protease, were screened for cleaning performance. Enzyme-free detergent formulations at 1500 ppm total surfactant were insufficient to cause complete surface cleaning, despite the high concentration of surfactant. The cleaning performance of a "home-made" formulation containing 2 ppm subtilisin A (SA) and 2 ppm sodium dodecyl benzyl sulphonate (SDOBS) was as efficient as the best amongst the three enzyme-free 1500 ppm formulations. The cleaning performance of 2 ppm SA in the absence of SDOBS was less effective than the combined formulation, even though 2 ppm SDOBS alone did not cause any protein removal. The observed synergistic performance was attributed to the cooperative mechanisms (chemical and physical attack) by which these two agents act on a rubisco stain. Replacing SDOBS in the enzyme-surfactant formulation with the same amount of surfactin biosurfactant (2 ppm) gave the best rubisco removal of all formulations examined in this study, irrespective of the surface chemistry underlying the protein film. It was found that 75% and 80% of immobilised rubisco stain could be removed from hydrophobic and hydrophilic surfaces, respectively, by the biosurfactant-SA formulation (compared with 60% and 65%, respectively, using the SDOBS-SA formulation). Our results suggest that it may be possible to generate fully renewable biochemical-based cleaning formulations that have superior cleaning performance to existing technologies. In developing optimised formulations, there is a pressing need for chip-based tools similar to that developed in this research.

Improved cleaning of hydrophilic protein-coated surfaces using the combination of Nanobubbles and SDS

The use of nanobubbles, the common surfactant sodium dodecyl sulfate (SDS), and nanobubbles in combination with SDS as cleaning agents to remove lysozyme from the solid-liquid interface has been investigated using a quartz crystal microbalance on both hydrophobic and hydrophilic surfaces. On the hydrophobic surface, significant amounts of protein remained on the surface after 10 cycles of nanobubble treatment for 10 s periods in phosphate buffer. The cleaning efficiency of SDS was far superior and was shown to remove approximately 90% of the protein. The use of nanobubbles in combination with SDS failed to improve the cleaning efficiency further. On the other hand, lysozyme on the hydrophilic surface cannot be removed effectively by either 10 cycles of cleaning with nanobubbles or 10 cycles of cleaning with SDS. Nevertheless, the protein can be removed completely after 6 cycles of cleaning with nanobubbles in combination with SDS.

Antimicrobial peptide-lipid binding interactions and binding selectivity

Surface pressure measurements, external reflection-Fourier transform infrared spectroscopy, and neutron reflectivity have been used to investigate the lipid-binding behavior of three antimicrobial peptides: melittin, magainin II, and cecropin P1. As expected, all three cationic peptides were shown to interact more strongly with the anionic lipid, 1,2 dihexadecanoyl-sn-glycerol-3-(phosphor-rac-(1-glycerol)) (DPPG), compared to the zwitterionic lipid, 1,2 dihexadecanoyl-sn-glycerol-3-phosphocholine (DPPC). All three peptides have been shown to penetrate DPPC lipid layers by surface pressure, and this was confirmed for the melittin-DPPC interaction by neutron reflectivity measurements. Adsorption of peptide was, however, minimal, with a maximum of 0.4 mg m(-2) seen for melittin adsorption compared to 2.1 mg m(-2) for adsorption to DPPG (from 0.7 microM solution). The mode of binding to DPPG was shown to depend on the distribution of basic residues within the peptide alpha-helix, although in all cases adsorption below the lipid layer was shown to dominate over insertion within the layer. Melittin adsorption to DPPG altered the lipid layer structure observed through changes in the external reflection-Fourier transform infrared lipid spectra and neutron reflectivity. This lipid disruption was not observed for magainin or cecropin. In addition, melittin binding to both lipids was shown to be 50% greater than for either magainin or cecropin. Adsorption to the bare air-water interface was also investigated and surface activity followed the trend melittin>magainin>cecropin. External reflection-Fourier transform infrared amide spectra revealed that melittin adopted a helical structure only in the presence of lipid, whereas magainin and cecropin adopted helical structure also at an air-water interface. This behavior has been related to the different charge distributions on the peptide amino acid sequences.

Micellar electrokinetic capillary chromatography reveals differences in intracellular metabolism between liposomal and free doxorubicin treatment of human leukemia cells

Doxil is a pegylated liposome formulation of the anthracycline doxorubicin. To better explain observed differences in the toxicity of Doxil and free doxorubicin in solution, the intracellular metabolism of the formulations after treatment in CCRF-CEM and CEM/C2 human leukemia cell lines was investigated. Using micellar electrokinetic capillary chromatography with laser-induced fluorescence detection, with a 63 zepto (10(-21)) mole doxorubicin limit of detection, five common metabolites and doxorubicin were detected upon treatment with both of these drug delivery systems. Two unique metabolites appeared with the Doxil and two unique metabolites appeared with the free doxorubicin delivery systems. For common metabolites, the relative amount of metabolite generated from Doxil was approximately 10 times higher than for free doxorubicin.

Templated surfactant readsorption on polyelectrolyte-induced depleted surfactant surfaces

Changes in the structure of a surfactant adsorbed on oxidized silicon arising from interaction with a polyelectrolyte have been studied using polarized infrared attenuated total reflection spectroscopy. Specifically, the cationic surfactant cetyltrimethylammonium bromide (CTAB) was found to form a highly ordered layer on oxidized silicon at a concentration of 5.5 x 10(-5) M and a pH of 9.6. Addition of a solution of the anionic polyelectrolyte poly(styrenesulfonate) to the ordered CTAB layer resulted in a rapid and dramatic decrease in the surface excess of CTAB. Interestingly however, the interfacial order of the residual surfactant was retained for a time period greater than 1 h, before decreasing. Reintroduction of a surfactant solution prior to destabilization of the residual interfacial CTAB resulted in the readsorption of the surfactant, the recovery of the initial equilibrium coverage, and the maintenance of an ordered CTAB conformation. This desorption/readsorption process may be subsequently repeated without destroying the order of the CTAB on the surface. If however sufficient time is allowed for the residual interfacial surfactant to destabilize prior to readdition of CTAB, the degree of surfactant order remains low, despite the rapid reobtainment of a surface excess equal to or greater than that initially measured. These results are interpreted in terms of polymer/surfactant interfacial complexation and the removal of adsorbed surfactant into solution. The ordering behavior of the residual surfactant suggests that CTAB is left on the surface in isolated patches of highly ordered species that maintain their order until two-dimensional diffusion leads to a more homogeneous surfactant surface distribution and hence the loss of conformational order. The degree of orientation order assumed by surfactant readsorbing on a depleted surface appears to be templated by the order of the residual interfacial surfactant, suggestive of a two-dimensional epitaxial growth mechanism for CTAB readsorption.

NEUTRON REFLECTION FROM LIQUID INTERFACES

Recent applications of neutron reflectometry to the study of wet interfaces are described. An outline is given of the basic principles that allow the techniques to determine composition and structure in a variety of situations. These are the adsorption of surfactant molecules at air/liquid and solid/liquid interfaces, the shape of the segment-density profiles of different types of polymer, including block copolymers and polyelectrolytes, adsorption in mixed surfactant and polymer/surfactant systems, and interfacial systems of biophysical interest.