Chitosan as a removing agent of beta-lactoglobulin from membrane models

Interaction mechanism of chitosan oligomers in pure water with cell membrane models studied by SFG vibrational spectroscopy

Chitosan is a versatile and biocompatible cationic antimicrobial polymer obtained from sustainable sources that is effective against a wide range of microorganisms. Although it is soluble only at low pH, chitosan oligomers (ChitO) are soluble in pure water and thus more appropriate for antibacterial applications. Although there is a vast literature on chitosan's antimicrobial activity, the molecular details of its interaction with biomembranes remain unclear. Here we investigate these molecular interactions by resorting to phospholipid Langmuir films (zwitterionic DPPC and anionic DPPG) as simplified membrane models (for mammalian and bacterial membranes, respectively), and using SFG vibrational spectroscopy to probe lipid tail conformation, headgroup dynamics and interfacial water orientation. For comparison, we also investigate the interactions of another simple cationic antimicrobial polyelectrolyte, poly(allylamine) hydrochloride - PAH. By forming the lipid films over the polyelectrolyte solutions, we found that both have only a very small interaction with DPPC, but PAH adsorption is able to invert the interfacial water orientation (membrane potential). This might explain why ChitO is compatible with mammalian cells, while PAH is toxic. In contrast, their interaction with DPPG films is much stronger, even more so for ChitO, with both insertion within the lipid film and interaction with the oppositely charged headgroups. Again, PAH adsorption inverts the membrane potential, while ChitO does not. Finally, ChitO interaction with DPPG is weaker if the antimicrobial is injected underneath a pre-assembled Langmuir film, and its interaction mode depends on the time interval between end of film compression and ChitO injection. These differences between ChitO and PAH effects on the model membranes highlight the importance of molecular structure and intermolecular interactions for their bioactivity, and therefore this study may provide insights for the rational design of more effective antimicrobial molecules.

Dominant hydrophobic interactions with β-glucan in nanoarchitectonics with mixed Langmuir monolayers of cholesterol/dipalmitoyl phosphatidyl choline

The polysaccharide β-glucan, found in the cell wall of cereals such as wheat, oats, and barley, is believed to lower the concentration of bad cholesterol in humans, but the molecular-level mechanisms responsible for such an action are unknown. In this study, we use Langmuir monolayers of cholesterol and dipalmitoyl phosphatidyl choline (DPPC) as cell membrane models that are made to interact with β-glucan. Neat cholesterol and mixed cholesterol/DPPC monolayers were expanded upon incorporating β-glucan from the aqueous subphase. This incorporation was found to induce ordering in mixed monolayers and dehydration of the carbonyl group at higher cholesterol concentrations. These effects are attributed to hydrophobic interactions as identified with polarization-modulated infrared reflection-absorption spectroscopy. They correlate well with the hypothesis that cholesterol levels can be lowered by the formation of soluble fibers with β-glucan through hydrophobic interactions, blocking cholesterol absorption by the organism.

Chitosan effects on monolayers of zwitterionic, anionic and a natural lipid extract from E. coli at physiological pH

Langmuir monolayers are used to simulate the biological membrane environment, acting as a mimetic system of the outer or the inner membrane leaflet. Herein, we analyze the interaction of membrane models with a partially N-acetylated chitosan (Ch35%) possessing a quasi-ideal random pattern of acetylation, full water solubility up to pH ≈ 8.5 and unusually high weight average molecular weight. Lipid monolayers containing dipalmitoyl phosphatidyl choline (DPPC), dipalmitoyl phosphatidyl ethalonamine (DPPE), dipalmitoyl phosphatidyl glycerol (DPPG) or E. coli total lipid extract were spread onto subphases buffered at pH 4.5 or 7.4. The incorporation of Ch35% chitosan caused monolayer expansion and a general trend of decreasing monolayer rigidity with Ch35% concentration. Due to its relatively high content of N-acetylglucosamine (GlcNAc) units, Ch35% interactions with negatively charged monolayers and with E. coli extract were weaker than those involving zwitterionic monolayers or lipid rafts. While the smaller interaction with negatively charged lipids was unexpected, this finding can be attributed to the degree of acetylation (35%) which imparts a small number of charged groups for Ch35% to interact. Chitosan properties are therefore determinant for interactions with model cell membranes, which explains the variability in chitosan bactericide activity in the literature. This is the first study on the effects from chitosans on realistic models of bacterial membranes under physiological pH.

Interaction of chitosan derivatives with cell membrane models in a biologically relevant medium

HYPOTHESIS

The interaction of chitosan, a natural biopolymer with various biomedical applications, with lipid Langmuir films has been widely investigated as a simple model for cell membranes. However, to ensure polymer solubility, up to now only acidic subphases with pH significantly below biological fluids have been used. To increase the biological significance of these investigations, here we evaluated the effects of two chitosan derivatives (low molecular weight - CH, and positively charged - CH-P40) on phospholipid films (either zwitterionic DPPC or anionic DPPG) using phosphate buffered saline solutions (PBS) as a subphase.

EXPERIMENTS

Surface pressure - area (π-A) isotherms were used to evaluate the expansion and changes in film elasticity, while Sum-Frequency Generation (SFG) vibrational spectroscopy provided information about the chain conformation of lipids.

FINDINGS

It was found that chitosans caused a small expansion of the DPPC film by its insertion within the monolayer. In contrast, they distinctly expanded DPPG monolayers by both chitosan insertion within the lipid monolayer and by interacting with the anionic head group. Therefore, PBS buffer can be used as a subphase for more biologically relevant studies of chitosan interactions with Langmuir films, shedding light on why chitosan is antibacterial but not toxic to mammals, as the interaction mechanism depends on lipid headgroup charge.

Influence of riboflavin on the oxidation kinetics of unsaturated fatty acids at the air/aqueous interface revealed by sum frequency generation vibrational spectroscopy

Riboflavin, a common nutrient also known as vitamin B2, is known to potentially play important roles in preventing lipid peroxidations. However, the detailed antioxidant mechanisms, especially the influence of riboflavin on lipid oxidations at biological interfaces, have not yet been fully explored. In the current study, the effect of riboflavin molecules on the oxidation kinetics of monounsaturated cis-11-eicosenoic acid (EA) at the air/water interface was systematically investigated using sum frequency generation vibrational spectroscopy (SFG-VS). It was discovered that the oxidation rates of the interfacial EA molecules can be reduced by about two to three times in the presence of riboflavin in the aqueous subphase. Further SFG-VS measurements under the protection of nitrogen purging gas showed that more tightly packed and ordered monolayer structures were formed by the surface adsorption of riboflavin molecules, making the C[double bond, length as m-dash]C bonds less accessible to the gas phase oxidative species. These results suggested that the antioxidant mechanism for riboflavin in the vicinity of biomembranes may not necessarily involve other reducing agents. They also show the great importance of interfacial molecular structures in biologically relevant chemical reactions.

Enantioselective cytotoxicity of chiral polymer vesicles with linear and hyperbranched structures

Herein, we study the enantioselective cytotoxicity of vesicles self-assembled by optically active linear polymers (LNPs) and hyperbranched polymers (HBPs). Compared to HBP vesicles, LNP vesicles exhibit properties such as a higher surface charge density and more violent interaction with simulated biomembranes which results in larger cytotoxicity against HeLa cells. Specifically, racemic-LNP vesicles exhibit the largest cytotoxicity of all. More interestingly, there is no significant enantioselective dependence of HBP vesicles on the abovementioned properties. Overall, we proved that the cytotoxicity of vesicles is deeply related to chirality and topological-structures. This research is of great fundamental value for the design of novel bio-interface materials.

Thermodynamics and kinetics of joint action of antiviral agent tilorone and DMSO on model lipid membranes

Individual and joint action of two water-soluble drugs, DMSO and tilorone, on model l-α-dipalmitoylphosphatidylcholine (DPPC) membranes were studied in equilibrium and kinetic regimes by differential scanning calorimetry (DSC). For equilibrium experiments, the drugs were introduced during preparation of the model membrane. In kinetic studies, one of the drugs was added to the DPPC membrane already containing the other drug, and the effects of drug-membrane interactions were monitored in real-time regime. It was found that tilorone and DMSO had opposite effects on the membrane melting temperature, which were non-additive under joint introduction of these drugs. Analysis of kinetics of DSC profiles under drugs introduction allowed us to discriminate two processes in drug-membrane interactions with different characteristic times, i.e., drug sorption onto the membrane (minutes) and drug diffusion through stacks of lipid bilayers (hours). It was established that 0.1 mol% DMSO effectively enhanced membrane penetration for tilorone with the rate of tilorone diffusion being dependent upon the scheme of drugs administration. A model was proposed describing how sorption of a dopant onto lipid membrane could affect the membrane permeability for other dopants. Conditions were determined for enhancement of membrane permeability, as it was observed for DPPC/DMSO/tilorone system.

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces

Investigation into nanostructured organic films has served many purposes, including the design of functionalized surfaces that may be applied in biomedical devices and tissue engineering and for studying physiological processes depending on the interaction with cell membranes. Of particular relevance are Langmuir monolayers, Langmuir-Blodgett (LB) and layer-by-layer (LbL) films used to simulate biological interfaces. In this review, we shall focus on the use of vibrational spectroscopy methods to probe molecular-level interactions at biomimetic interfaces, with special emphasis on three surface-specific techniques, namely sum frequency generation (SFG), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and surface-enhanced Raman scattering (SERS). The two types of systems selected for exemplifying the potential of the methods are the cell membrane models and the functionalized surfaces with biomolecules. Examples will be given on how SFG and PM-IRRAS can be combined to determine the effects from biomolecules on cell membrane models, which include determination of the orientation and preservation of secondary structure. Crucial information for the action of biomolecules on model membranes has also been obtained with PM-IRRAS, as is the case of chitosan removing proteins from the membrane. SERS will be shown as promising for enabling detection limits down to the single-molecule level. The strengths and limitations of these methods will also be discussed, in addition to the prospects for the near future.

Cellulase and alcohol dehydrogenase immobilized in Langmuir and Langmuir-Blodgett films and their molecular-level effects upon contact with cellulose and ethanol

The key challenges for producing devices based on nanostructured films with control over the molecular architecture are to preserve the catalytic activity of the immobilized biomolecules and to provide a reliable method for determining the intermolecular interactions and the accommodation of molecules at very small scales. In this work, the enzymes cellulase and alcohol dehydrogenase (ADH) were coimmobilized with dipalmitoylphosphatidylcholine (DPPC) as Langmuir-Blodgett (LB) films, and their biological activities were assayed by accommodating the structure formed in contact with cellulose. For this purpose, the polysaccharide was dissolved in an ionic liquid, 1-buthyl-3-methylimidazolium chloride (BMImCl), and dropped on the top of the hybrid cellulase-ADH-DPPC LB film. The interactions between cellulose and ethanol, which are the catalytic substrates of the enzymes as well as important elements in the production of second-generation fuels, were then investigated using polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Investigation of the secondary structures of the enzymes was performed using PM-IRRAS, through which the presence of ethanol and cellulose was observed to highly affect the structures of ADH and cellulase, respectively. The detection of products formed from the catalyzed reactions as well as the changes of secondary structure of the enzymes immobilization could be carried out, which opens the possibility to produce a means for producing second-generation ethanol using nanoscale arrangements.

Adsorption and enzyme activity of sucrose phosphorylase on lipid Langmuir and Langmuir-Blodgett films

The production of bioelectronic devices, including biosensors, can be conducted using enzymes immobilized in ultrathin solid films, for which preserving the enzymatic catalytic activity is crucial for optimal performance. In this sense, nanostructured films that allow for control over molecular architectures are of interest. In this paper, we investigate the adsorption of sucrose phosphorylase onto Langmuir monolayers of the phospholipid dimyristoylphosphatidic acid, which caused the surface pressure isotherms to expand. With polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), the amide bands from the enzyme could be identified, with the C-N and C=O dipole moments lying parallel to the air-water interface. Structuring of the enzyme into an α-helix was noted, and this structure was preserved when the mixed enzyme-phospholipid monolayer was transferred in the form of a Langmuir-Blodgett (LB) film. The latter was demonstrated with measurements of the catalytic activity of sucrose phosphorylase, which presented the highest enzyme activity for multilayer LB film. The approach presented in this study not only allows for optimized catalytic activity toward sucrose but also permits to explain why certain film architectures exhibit superior performance.

Interaction of O-acylated chitosans with biomembrane models: probing the effects from hydrophobic interactions and hydrogen bonding

One of the major challenges in establishing the mechanisms responsible for the chitosan action in biomedical applications lies in the determination of the molecular-level interactions with the cell membrane. In this study, we probed hydrophobic interactions and H-bonding in experiments with O,O'-diacetylchitosan (DACT) and O,O'-dipropionylchitosan (DPPCT) incorporated into monolayers of distinct phospholipids, the zwitterionic dipalmitoyl phosphatidyl choline (DPPC), and the negatively charged dipalmitoyl phosphatidyl glycerol (DPPG) and dimyristoyl phosphatidic acid (DMPA). The importance of hydrophobic interactions was confirmed with the larger effects observed for DACT and DPPCT than for parent chitosan (Chi), particularly for the more hydrophobic DPPCT. Such larger effects were noted in surface pressure isotherms and elasticity of the monolayers. Since H-bonding is hampered for the chitosan derivatives, which have part of their hydroxyl groups shielded by O-acylation, these effects indicate that H-bonding does not play an important role in the chitosan-membrane interactions. Using polarization-modulated infrared reflection absorption (PM-IRRAS) spectroscopy, we found that the chitosan derivatives were incorporated into the hydrophobic chain of the phospholipids, even at high surface pressures comparable to those in a real cell membrane. Taken together, these results indicate that the chitosan derivatives containing hydrophobic moieties would probably be more efficient than parent chitosan as antimicrobial agents, where interaction with the cell membrane is crucial.

Cationic polymers inhibit the conductance of lysenin channels

The pore-forming toxin lysenin self-assembles large and stable conductance channels in natural and artificial lipid membranes. The lysenin channels exhibit unique regulation capabilities, which open unexplored possibilities to control the transport of ions and molecules through artificial and natural lipid membranes. Our investigations demonstrate that the positively charged polymers polyethyleneimine and chitosan inhibit the conducting properties of lysenin channels inserted into planar lipid membranes. The preservation of the inhibitory effect following addition of charged polymers on either side of the supporting membrane suggests the presence of multiple binding sites within the channel's structure and a multistep inhibition mechanism that involves binding and trapping. Complete blockage of the binding sites with divalent cations prevents further inhibition in conductance induced by the addition of cationic polymers and supports the hypothesis that the binding sites are identical for both multivalent metal cations and charged polymers. The investigation at the single-channel level has shown distinct complete blockages of each of the inserted channels. These findings reveal key structural characteristics which may provide insight into lysenin's functionality while opening innovative approaches for the development of applications such as transient cell permeabilization and advanced drug delivery systems.

Enzyme activity of catalase immobilized in Langmuir-Blodgett films of phospholipids

A major challenge for producing low cost biosensors based on nanostructured films with control of molecular architectures is to preserve the catalytic activity of the immobilized biomolecules. In this study, we show that catalase (HRP) keeps its activity if immobilized in Langmuir-Blodgett (LB) films of dipalmitoyl phosphatidylglycerol (DPPG). The incorporation of catalase into a DPPG monolayer at the air-water interface was demonstrated with surface pressure and surface potential isotherms, in addition to polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). According to the PM-IRRAS data, catalase was not denatured upon adsorption on a preformed DPPG monolayer and could be transferred onto a solid substrate. The catalytic activity of catalase in a mixed LB film with DPPG was ca. 13% higher than in solution. The control of molecular architecture and choice of a suitable phospholipid matrix allows catalase-containing LB films to be used in sensing hydrogen peroxide.

Electrostatic interactions are not sufficient to account for chitosan bioactivity

Recent studies involving chitosan interacting with phospholipid monolayers that mimic cell membranes have brought molecular-level evidence for some of the physiological actions of chitosan, as in removing a protein from the membrane. This interaction has been proven to be primarily of electrostatic origin because of the positive charge of chitosan in low pH solutions, but indirect evidence has also appeared of the presence of hydrophobic interactions. In this study, we provide definitive proof that model membranes are not affected merely by the charges in the amine groups of chitosan. Such a proof was obtained by comparing surface pressure and surface potential isotherms of dipalmitoyl phosphatidyl choline (DPPC) and dipalmitoyl phosphatidyl glycerol (DPPG) monolayers incorporating either chitosan or poly(allylamine hydrochloride) (PAH). As the latter is also positively charged and with the same charged functional group as chitosan, similar effects should be observed in case the electrical charge was the only relevant parameter. Instead, we observed a large expansion in the surface pressure isotherms upon interaction with chitosan, whereas PAH had much smaller effects. Of particular relevance for biological implications, chitosan considerably reduced the monolayer elasticity, whereas PAH had almost no effect. It is clear therefore that chitosan action depends strongly either on its functional uncharged groups and/or on its specific conformation in solution.

Cholesterol mediates chitosan activity on phospholipid monolayers and Langmuir-Blodgett films

The polysaccharide chitosan has been largely used in many biological applications as a fat and cholesterol reducer, bactericide agent, and wound healing material. While the efficacy for some of such uses is proven, little is known about the molecular-level interactions involved in these applications. In this study, we employ mixed Langmuir and Langmuir-Blodgett (LB) films of negatively charged dimyristoyl phosphatidic acid (DMPA) and cholesterol as cell membrane models to investigate the role of cholesterol in the molecular-level action of chitosan. Chitosan does not remove cholesterol from the monolayer. The interaction with chitosan tends to expand the DMPA monolayer due to its interpenetration within the film. On the other hand, cholesterol induces condensation of the DMPA monolayer. The competing effects cause the surface pressure isotherms of mixed DMPA-cholesterol films on a chitosan subphase to be unaffected by the cholesterol mole fraction, due to distinct degrees of chitosan penetration into the film in the presence of cholesterol. By combining polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation spectroscopy (SFG), we showed that chitosan induces order into negatively charged phospholipid layers, whereas the opposite occurs for cholesterol. In conclusion, chitosan has its penetration in the film modulated by cholesterol, and electrostatic interactions with negatively charged phospholipids, such as DMPA, are crucial for the action of chitosan.

Interaction of a C-terminal peptide of Bos taurus diacylglycerol acyltransferase 1 with model membranes

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final and dedicated step in the synthesis of triacylglycerol, which is believed to involve the lipids oleoyl coenzyme A (OCoA) and dioleoyl-sn-glycerol (DOG) as substrates. In this work we investigated the interaction of a specific peptide, referred to as SIT2, on the C-terminal of DGAT1 (HKWCIRHFYKP) with model membranes made with OCoA and DOG in Langmuir monolayers and liposomes. According to the circular dichroism and fluorescence data, conformational changes on SIT2 were seen only on liposomes containing OCoA and DOG. In Langmuir monolayers, SIT2 causes the isotherms of neat OCoA and DOG monolayers to be expanded, but has negligible effect on mixed monolayers of OCoA and DOG. This synergistic interaction between SIT2 and DOG+OCoA may be rationalized in terms of a molecular model in which SIT2 may serve as a linkage between the two lipids. Our results therefore provide molecular-level evidence for the interaction between this domain and the substrates OCoA and DOG for the synthesis of triacylglycerol.

Immobilization of alcohol dehydrogenase in phospholipid Langmuir-Blodgett films to detect ethanol

Enzyme immobilization in nanostructured films may be useful for a number of biomimetic systems, particularly if suitable matrixes are identified. Here we show that alcohol dehydrogenase (ADH) has high affinity toward a negatively charged phospholipid, dimyristoylphosphatidic acid (DMPA), which forms a Langmuir monolayer at an air-water interface. Incorporation of ADH into the DMPA monolayer was monitored with surface pressure measurements and polarization-modulation infrared reflection absorption spectroscopy, with the alpha-helices from ADH being mainly oriented parallel to the water surface. ADH remained at the interface even at high surface pressures, thus allowing deposition of Langmuir-Blodgett (LB) films from the DMPA-ADH film. Indeed, interaction with DMPA enhances the transfer of ADH, where the mass transferred onto a solid support increased from 134 ng for ADH on a Gibbs monolayer to 178 ng for an LB film with DMPA. With fluorescence spectroscopy it was possible to confirm that the ADH structure was preserved even after one month of the LB deposition. ADH-containing films deposited onto gold-interdigitated electrodes were employed in a sensor array capable of detecting ethanol at concentrations down to 10 ppb (in volume), using impedance spectroscopy as the method of detection.