Enhanced interfacial properties of novel amino acid-derived surfactants: Effects of headgroup chemistry and of alkyl chain length and unsaturation

Influence of tail group length, amide functionality and added salt ion identity on the behaviour of betaine surfactants

Hypothesis The behaviour of surfactants in solution and at interfaces is governed by a combination of steric and electrostatic effects experienced by surfactant molecules as they interact with solvent, other species in solution, and each other. It would therefore be anticipated that highly interacting groups would significantly influence surfactant behaviour. The widely used amide functionality has polar H-bond donor/acceptor properties, and therefore its inclusion into a surfactant structure should have a profound effect on surface activity and self-assembly of that surfactant when compared to the equivalent molecule without an amide linker. Further, chaotropic or kosmotropic salt ions that affect water structuring and hydrogen bonding may provide opportunities for further tuning surfactant interactions in such cases. Experiments A library of betaine surfactant with tail lengths n=14-22 both with and without an amidopropyl linker were synthesised to study the effect of the amide functionality on surfactant properties. Characterisation of the molecules interfacial properties were performed using pendant drop tensiometry and their solution state formulation properties were probed using small-angle neutron scattering (SANS) and rheological measurements. Findings Presence of an amidopropyl linker had little effect on aggregation propensity (as evidenced by critical micelle concentration) and aggregate morphology of betaine surfactants, but did increase the Krafft temperature of these surfactants. SANS analysis indicated that aggregate morphology of alkyl betaine surfactants could be influenced by the addition of sodium salts with chaotropic counterions (I- and SCN-), but they were insensitive to more kosmotropic anions (SO42-, F- and Cl-), providing unique and novel solution control methods for this (supposedly salt-insensitive) class of surfactants.

The relationship between the chemical structure, physicochemical properties, and mucosal adjuvanticity of sugar-based surfactants

The relationship between the chemical structure, physicochemical properties, and mucosal adjuvanticity of sugar-based surfactants (SBSs) has not been sufficiently elucidated. Thus, in the present study, we systematically analyzed 11 SBSs for mucosal adjuvanticity. Ovalbumin (OVA)-specific antibody titers were measured in mice immunized intranasally with OVA plus SBS. We found that four SBSs (trehalose monododecanoate, sucrose monododecanoate, n-dodecyl-α-d-maltopyranoside, and n-dodecyl-β-d-maltopyranoside) exhibited the most potent adjuvanticity. We identified the following associations between chemical structure and adjuvanticity: 1) OVA-specific antibody titer increased with an increasing number of carbon atoms in the alkyl chain; 2) the adjuvanticity was not affected by the type of sugar or bond between the sugar and alkyl chain; and 3) SBSs with rigid structures exhibited less adjuvanticity. The relationship between physicochemical properties and adjuvanticity was as follows: 1) SBSs exhibited adjuvanticity above the critical micelle concentration and 2) in the SBSs with potent adjuvanticity, the diameter of the SBS-OVA complex was 70-75 nm. Our study indicates evidence for the direct involvement of chemical structure and physicochemical properties in determining adjuvanticity in SBSs.

Effective cytocompatible nanovectors based on serine-derived gemini surfactants and monoolein for small interfering RNA delivery

Non-viral gene therapy based on gene silencing with small interfering RNA (siRNA) has attracted great interest over recent years. Among various types of cationic complexation agents, amino acid-based surfactants have been recently explored for nucleic acid delivery due to their low toxicity and high biocompatibility. Monoolein (MO), in turn, has been used as helper lipid in liposomal systems due to its ability to form inverted nonbilayer structures that enhance fusogenicity, thus contributing to higher transfection efficiency. In this work, we focused on the development of nanovectors for siRNA delivery based on three gemini amino acid-based surfactants derived from serine - (12Ser)₂N12, amine derivative; (12Ser)₂COO12, ester derivative; and (12Ser)₂CON12, amide derivative - individually combined with MO as helper lipid. The inclusion of MO in the cationic surfactant system influences the morphology and size of the mixed aggregates. Furthermore, the gemini surfactant:MO systems showed the ability to efficiently complex siRNA, forming stable lipoplexes, in some cases clearly depending on the MO content, without inducing significant levels of cytotoxicity. High levels of gene silencing were achieved in comparison with a commercially available standard indicating that these gemini:MO systems are promising candidates as lipofection vectors for RNA interference (RNAi)-based therapies.

Nuclear Magnetic Resonance Investigation of the Effect of pH on Micelle Formation by the Amino Acid-Based Surfactant Undecyl l-Phenylalaninate

Micelle formation by the anionic amino acid-based surfactant undecyl l-phenylalaninate (und-Phe) was investigated as a function of pH in solutions containing either Na⁺, l-arginine, l-lysine, or l-ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing l-arginine and l-lysine had lower CMC than the corresponding solutions with Na⁺ counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und-Phe micelles with Na⁺ counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, l-arginine, l-lysine, and l-ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und-Phe/l-arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with l-lysine and l-ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that l-arginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, l-lysine and l-ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two-dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two-dimensional NMR also showed that in und-Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle.

Chitosan-based nanosystems and their exploited antimicrobial activity

Chitosan is a biodegradable and biocompatible natural polysaccharide that has a wide range of applications in the field of pharmaceutics, biomedical, chemical, cosmetics, textile and food industry. One of the most interesting characteristics of chitosan is its antibacterial and antifungal activity, and together with its excellent safety profile in human, it has attracted considerable attention in various research disciplines. The antimicrobial activity of chitosan is dependent on a number of factors, including its molecular weight, degree of deacetylation, degree of substitution, physical form, as well as structural properties of the cell wall of the target microorganisms. While the sole use of chitosan may not be sufficient to produce an adequate antimicrobial effect to fulfil different purposes, the incorporation of this biopolymer with other active substances such as drugs, metals and natural compounds in nanosystems is a commonly employed strategy to enhance its antimicrobial potential. In this review, we aim to provide an overview on the different approaches that exploit the antimicrobial activity of chitosan-based nanosystems and their applications, and highlight the latest advances in this field.

Sustainable Production of Fine Chemicals and Materials Using Nontoxic Renewable Sources

Due to declining hydrocarbon resources and strengthening environmental regulations, significant attention is directed toward sustainable and nontoxic supplies for the development of green technologies in a variety of industries. This account provides an overview on the sources and recent applications of such materials surveying the most common nontoxic and renewable resources that can be obtained from biological sources. Developing a broad array of technologies based on these materials would establish a truly sustainable green chemical industry. The study thematically discusses various compound groups, eg, carbohydrates, proteins, and triglycerides (oils). Since often the monomers or building blocks of these biopolymers are of significant importance and produced in large amounts, the applications of these compounds are also reviewed.

Physicochemical Properties of Amino Acid Surfactants and Their Use in Dyeing with Natural Plant Dyes

Environment-friendly green amino acid surfactants were prepared by the reaction of glutamic acid with various fatty acids (C2–C14). These green amino acid surfactants have been found to exhibit good surface activities i.e. surface tension, foaming, wetting power and emulsifying ability. The critical micelle concentration (CMC) increases with an increase of the alkyl chain of amino acid surfactants. After 5 days the biodegradation of these amino acid surfactants was larger than 60% and consequently, the surfactants should be regarded as readily biodegradable. Finally, the green amino acid surfactants were added to natural plant dyes to test their applicability for dyeing wool fibers. The surfactant developed in this study will contribute to green and sustainability chemistry.

Synthesis and Surface Activity of Cationic Amino Acid-Based Surfactants in Aqueous Solution

I studied the possibility of using amino acid-based surfactants as emulsifiers at the same time as preservatives. Fourteen lipopeptides were synthesized employing a solid phase peptide synthesis procedure. All compounds were designed to be positively charged from +1 to +4 and acylated with fatty acid chain—palmitic and miristic. The surface activity of the obtained lipopeptides was tested using a semi-automatic tensiometer to calculate parameters describing the behavior of lipopeptides in the air/water interface. Such parameters as CMC, surface tension at the CMC point (σ_CMC), effectiveness (π_CMC), and efficiency (pC20) were measured. Emulsifying properties of all lipopeptides were also examined. The studies reveal that the surface active properties of synthesized compounds strongly depend on the length of alkyl chains as well as on the composition of amino acid polar heads. The critical micelle concentration decreases with increasing alkyl chain length of lipopeptides with the same polar head. The effectiveness and efficiency decrease when the number of amino acids in the polar head increases. All lipopeptides established a very weak emulsification power and created unstable water/Miglyol 812 and water/paraffin oil emulsions. Results suggest that lipopeptides cannot be used as emulsifiers; nonetheless, it is possible to use them as auxiliary surfactants with disinfectant properties in combination with more potent emulsifiers.

Atypical cleavage of protonated N-fatty acyl amino acids derived from aspartic acid evidenced by sequential MS3 experiments

Lipidomics calls for information on detected lipids and conjugates whose structural elucidation by mass spectrometry requires to rationalization of their gas phase dissociations toward collision-induced dissociation (CID) processes. This study focused on activated dissociations of two lipoamino acid (LAA) systems composed of N-palmitoyl acyl coupled with aspartic and glutamic acid mono ethyl esters (as LAA_(*D) and LAA_(*E)). Although in MS/MS, their CID spectra show similar trends, e.g., release of water and ethanol, the [(LAA_(*D/*E)+H)-C₂H₅OH]⁺ product ions dissociate via distinct pathways in sequential MS³ experiments. The formation of all the product ions is rationalized by charge-promoted cleavages often involving stepwise processes with ion isomerization into ion-dipole prior to dissociation. The latter explains the maleic anhydride or ketene neutral losses from N-palmitoyl acyl aspartate and glutamate anhydride fragment ions, respectively. Consequently, protonated palmitoyl acid amide is generated from LAA_(*D), whereas LAA_(*E) leads to the [*E+H-H₂O]⁺ anhydride. The former releases ammonia to provide acylium, which gives the C _n H_(2n-1) and C _n H_(2n-3) carbenium series. This should offer structural information, e.g., to locate either unsaturation(s) or alkyl group branching present on the various fatty acyl moieties of lipo-aspartic acid in further studies based on MS ⁿ experiments.

Synthesis, Surface Active Properties and Cytotoxicity of Sodium N-Acyl Prolines

Sodium N-acyl prolines (NaNAPro) were synthesized using mixture of fatty acids obtained from coconut, palm, karanja, Sterculia foetida and high oleic sunflower oils via Schotten-Baumann reaction in 58-75% yields to study the synergetic effect of mixture of hydrophobic fatty acyl functionalities like saturation, unsaturation and cyclopropene fatty acids with different chain lengths and aliphatic hetero cyclic proline head group on their surface and cytotoxicity activities. The products were characterized by chromatographic and spectral techniques. The synthesized products were evaluated for their surface active properties such as surface tension, wetting power, foaming characteristics, emulsion stability, calcium tolerance, critical micelle concentration (CMC) and thermodynamic properties. The results revealed that all the products exhibited superior surface active properties like CMC, calcium tolerance and emulsion stability as compared to the standard surfactant, sodium lauryl sulphate (SLS). In addition, palm, Sterculia foetida and high oleic sunflower fatty N-acyl prolines exhibited promising cytotoxicity against different tumor cell lines.

Biological and surface-active properties of double-chain cationic amino acid-based surfactants

Cationic amino acid-based surfactants were synthesized via solid phase peptide synthesis and terminal acylation of their α and ε positions with saturated fatty acids. Five new lipopeptides, N-α-acyl-N-ε-acyl lysine analogues, were obtained. Minimum inhibitory concentration and minimum bactericidal (fungicidal) concentration were determined on reference strains of bacteria and fungi to evaluate the antimicrobial activity of the lipopeptides. Toxicity to eukaryotic cells was examined via determination of the haemolytic activities. The surface-active properties of these compounds were evaluated by measuring the surface tension and formation of micelles as a function of concentration in aqueous solution. The cationic surfactants demonstrated diverse antibacterial activities dependent on the length of the fatty acid chain. Gram-negative bacteria and fungi showed a higher resistance than Gram-positive bacterial strains. It was found that the haemolytic activities were also chain length-dependent values. The surface-active properties showed a linear correlation between the alkyl chain length and the critical micelle concentration.

Morphological and nanomechanical behavior of supported lipid bilayers on addition of cationic surfactants

The addition of surfactants to lipid bilayers is important for the modulation of lipid bilayer properties (e.g., in protein reconstitution and development of nonviral gene delivery vehicles) and to provide insight on the properties of natural biomembranes. In this work, the thermal behavior, organization, and nanomechanical stability of model cationic lipid-surfactant bilayers have been investigated. Two different cationic surfactants, hexadecyltrimethylammonium bromide (CTAB) and a novel derivative of the amino acid serine (Ser16TFAc), have been added (up to 50 mol %) to both liposomes and supported lipid bilayers (SLBs) composed by the zwitterionic phospholipid DPPC. The thermal phase behavior of mixed liposomes has been probed by differential scanning calorimetry (DSC), and the morphology and nanomechanical properties of mixed SLBs by atomic force microscopy-based force spectroscopy (AFM-FS). Although DSC thermograms show different results for the two mixed liposomes, when both are deposited on mica substrates similar trends on the morphology and the mechanical response of the lipid-surfactant bilayers are observed. DSC thermograms indicate microdomain formation in both systems, but while CTAB decreases the degree of organization on the liposome bilayer, Ser16TFAc ultimately induces the opposite effect. Regarding the AFM-FS studies, they show that microphase segregation occurs for these systems and that the effect is dependent on the surfactant content. In both SLB systems, different microdomains characterized by their height and breakthrough force Fb are formed. The molecular organization and composition is critically discussed in the light of our experimental results and literature data on similar lipid-surfactant systems.

Simultaneous determination of interfacial molarities of amide bonds, carboxylate groups, and water by chemical trapping in micelles of amphiphiles containing peptide bond models

Chemical trapping is a powerful approach for obtaining experimental estimates of interfacial molarities of weakly basic nucleophiles in the interfacial regions of amphiphile aggregates. Here, we demonstrate that the chemical probe 4-hexadecyl-2,6-dimethylbenzenediazonium ion (16-ArN(2)(+)) reacts competitively with interfacial water, with the amide carbonyl followed by cleavage of the headgroups from the tail at the amide oxygen, and with the terminal carboxylate groups in micelles of two N-acyl amino-acid amphiphiles, sodium N-lauroylsarcosinate (SLS) and sodium N-lauroylglycinate (SLG), simple peptide bond model amphiphiles. Interfacial molarities (in moles per liter of interfacial volume) of these three groups were obtained from product yields, assuming that selectivity toward a particular nucleophile compared to water is the same in an aqueous reference solution and in the interfacial region. Interfacial carboxylate group molarities are ~1.5 M in both SLS and SLG micelles, but the concentration of the amide carbonyl for SLS micelles is ~4.6-5 times less (ca. 0.7 M) than that of SLG micelles (~3 M). The proton on the secondary N of SLG helps solubilize the amide bond in the aqueous region, but the methyl on the tertiary N of SLS helps solubilize the amide bond in the micellar core, reducing its reaction with 16-ArN(2)(+). Application of chemical trapping to proteins in membrane mimetic interfaces should provide insight into the topology of the protein within the interface because trapping of the amide carbonyl and cleavage at the C-N bond occurs only within the interface, and fragment characterization marks those peptide bonds located within the interface.

Novel surfactants with diglutamic acid polar head group: drug solubilization and toxicity studies

PURPOSE

Novel surfactants made of diglutamic acid (DG) polar head linked to lithocholic, arachidonic, linoleic or stearic acids were designed for drug solubilization.

METHODS

Surfactants 3-D conformer and packing parameter were determined by molecular modelling and self-assembling properties by pyrene fluorescence measurements. Cytotoxicity was assessed on Human Umbilical Vein Endothelial Cells (HUVEC) and haemolyitic activity on rat red blood cells. Drug solubilization was quantified and its interaction with hydrophobic moieties was characterized using differential scanning calorimetry and X-ray diffraction. Self organisation of stearoyl-DG was observed by cryogenic transmission electron microscopy. Toxicity after repeated injections of stearoyl-DG was investigated in Wistar rats.

RESULTS

DG-based surfactants self-assemble into water and their critical micellar concentrations are comprised between 200 and 920 μg/mL. Cytotoxicity and haemolysis were lower than for polysorbate 80. At best, stearoyl-DG solubilized the drug up to 22% (w/w). Solid-state characterization evidenced drug/lipid interactions leading to the formation of a new complex. Stearoyl-DG formed spherical micelles of 20 nm, as predicted by packing parameter calculation. However, it induced a possible liver toxicity after intravenous administration in rats.

CONCLUSIONS

Among the surfactants tested, stearoyl-DG is the more efficient for drug solubilization but its use is limited by its possible liver toxicity.