Surfactin Structures at Interfaces and in Solution: The Effect of pH and Cations

Biosurfactant-amphiphilized hyaluronic acid: A dual self-assembly anticancer nanoconjugate and drug vector for synergistic chemotherapy

Novel amphiphilic nanoconjugates of hyaluronic acid (HA), 50 kDa (HA50) and 100 kDa (HA100), and the lipopeptide biosurfactant surfactin (SF) were developed for potential anticancer applications. Physicochemical characterization indicated the formation of an ester conjugate (HA: SF molar ratio 1: 40) with the HA50-SF derivative exhibiting higher degree of substitution, hydrolytic stability, and surface activity. Self-assembly resulted in nanomicelles with smaller size and greater negative charge relative to SF micelles. Biological data demonstrated distinct anticancer activity of HA50-SF which displayed greater synergistic cytotoxicity and selectivity for MDA-MB 231 and MCF-7 breast cancer cells alongside greater modulation of apoptosis-related biomarkers leading to apoptosis. As bioactive vector for chemotherapeutic agents, the selected HA50-SF nanoconjugate efficiently (70 %) entrapped berberine (BER) producing a sustained release BER-HA50-SF synergistic anticancer nanoformulation. Lactoferrin (Lf) coating for dual HA/Lf targeting endowed Lf/BER-HA50-SF with significantly greater selectivity for both cell lines. A murine Ehrlich breast cancer model provided evidence for the efficacy and safety of Lf/BER-HA50-SF via tumoral, histological, immunohistochemical, molecular and systemic toxicity assessments. Thus, HA-SF nanoconjugates integrating the HA and SF properties and biofunctionalties present a novel biopolymer-biosurfactant platform of benefit to oncology nanomedicine and possibly other applications.

Neutron reflection and scattering in characterising peptide assemblies

Self-assemblies of de novo designed short peptides at interface and in bulk solution provide potential platforms for developing applications in many medical and technological areas. However, characterising how bioinspired supramolecular nanostructures evolve with dynamic self-assembling processes and respond to different stimuli remains challenging. Neutron scattering technologies including small angle neutron scattering (SANS) and neutron reflection (NR) can be advantageous and complementary to other state-of-the-art techniques in tracing structural changes under different conditions. With more neutron sources now available, SANS and NR are becoming increasingly popular in studying self-assembling processes of diverse peptide and protein systems, but the difficulty in experimental manipulation and data analysis can deter beginners. This review will introduce the basic theory, general experimental setup and data analysis of SANS and NR, followed by provision of their applications in characterising interfacial and solution self-assemblies of representative peptides and proteins. SANS and NR are remarkably effective in determining the morphological features self-assembled short peptides, especially size and shape transitions as a result of either sequence changes or in response to environmental stimuli, demonstrating the unique capability of NR and SANS in unravelling the interactive processes. These examples highlight the potential of NR and SANS in supporting the development of novel short peptides and proteins as biopharmaceutical candidates in the fight against many diseases and infections that share common features of membrane interactive processes.

Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual β-sheet-like domains

Low-molecular weight gelators (LMWGs) are small molecules (M_w < ∼1 kDa), which form self-assembled fibrillar network (SAFiN) hydrogels in water. A great majority of SAFiN gels are described by an entangled network of self-assembled fibers, in analogy to a polymer in a good solvent. Here, fibrillation of a biobased glycolipid bolaamphiphile is triggered by Ca²⁺ or Ag⁺ ions which are added to its diluted micellar phase. The resulting SAFiN, which forms a hydrogel above 0.5 wt%, has a "nano-fishnet" structure, characterized by a fibrous network of both entangled fibers and β-sheet-like rafts, generally observed for silk fibroin, actin hydrogels or mineral imogolite nanotubes, but generally not known for SAFiN. This work focuses on the strength of the SAFIN gels, their fast recovery after applying a mechanical stimulus (strain) and their unusual resistance to temperature, studied by coupling rheology to small angle X-ray scattering (rheo-SAXS) using synchrotron radiation. The Ca²⁺-based hydrogel maintains its properties up to 55 °C, while the Ag⁺-based gel shows a constant elastic modulus up to 70 °C, without the appearance of any gel-to-sol transition temperature. Furthermore, the glycolipid is obtained by fermentation from natural resources (glucose and rapeseed oil), thus showing that naturally engineered compounds can have unprecedented properties, when compared to the wide range of chemically derived amphiphiles.

Cation-Induced Fibrillation of Microbial Glycolipid Biosurfactant Probed by Ion-Resolved In Situ SAXS

Biological amphiphiles are molecules with a rich phase behavior. Micellar, vesicular, and even fibrillar phases can be found for the same molecule by applying a change in pH or by selecting the appropriate metal ion. The rich phase behavior paves the way toward a broad class of soft materials, from carriers to hydrogels. The present work contributes to understanding the fibrillation of a microbial glycolipid, glucolipid G-C18:1, produced by Starmerella bombicola ΔugtB1 and characterized by a micellar phase at alkaline pH and a vesicular phase at acidic pH. Fibrillation and prompt hydrogelation is triggered by adding either alkaline earth, Ca²⁺, or transition metal, Ag⁺, Fe²⁺, Al³⁺, ions to a G-C18:1 micellar solution. A specifically designed apparatus coupled to a synchrotron SAXS beamline allows the performing of simultaneous cation- and pH-resolved in situ monitoring of the morphological evolution from spheroidal micelles to crystalline fibers, when Ca²⁺ is employed, or to wormlike aggregates, when Fe²⁺ or Al³⁺ solutions are employed. The fast reactivity of Ag⁺ and the crystallinity of Ca²⁺-induced fibers suggest that fibrillation is driven by direct metal-ligand interactions, while the shape transition from spheroidal to elongated micelles with Fe²⁺ or Al³⁺ rather suggest charge screening between the lipid and the hydroxylated cation species.

A Coarse-Grained Model for Microbial Lipopeptide Surfactin and Its Application in Self-Assembly

Biosurfactants exhibit outstanding interfacial properties and unique biological activities that fairly related to their self-assembly in solutions and at interfaces. Computational simulations provide structural details of biosurfactant aggregates at the molecular level relevant to thermodynamic properties, but the understanding of kinetics of self-assembly remains limited due to lower simulation efficiency. In this work, a coarse-grained model has been developed for microbial lipopeptide surfactin, and surfactin monolayer at the octane/water interface and micelle in aqueous solution were studied using molecular dynamics simulations. Interaction parameters were optimized and validated by comparing with results obtained from experiments and atomistic molecular dynamics simulations. In particular, self-assembly of surfactin in aqueous solution was studied using the optimized parameters. Results showed that coarse-grained simulations well reproduced structural properties of surfactin monolayer and micelle and the molecular behavior such as surfactin orientation and conformation. Self-assembly features of surfactin in different stages have been captured, and the aggregation numbers of dominant clusters were in accordance with experimental data. This report suggested that the present coarse-grained model and interaction parameters allowed surfactin simulations over longer timescales and larger systems, which provide insights into characterizing both the kinetics of surfactin self-assembly and the adsorption of surfactin onto varying interfaces.

Disinfectant-like activity of lipopeptide biosurfactant produced by Bacillus tequilensis strain SDS21

Antiseptics and disinfectants are widely applied for eliminating microorganisms. However, microorganisms dwelling in the biofilm are less susceptible and in some cases resistant to biocide treatment. The present study describes isolation and characterization of lipopeptide biosurfactant exhibiting disinfectant-like activity. Biosurfactant was produced by an endo-rhizospheric bacterium Bacillus tequilensis strain SDS21. Biosurfactant reduced the surface tension of water from 72 to 30 mN/m with CMC of 40 mg/l. The Liquid Chromatography-Mass Spectrometry analysis of biosurfactant suggested it to be a mixture of C₁₄, C₁₅, C₁₆ and C₁₇ surfactin homologues. The lipopeptide biosurfactant exhibited bactericidal activity against planktonic cells and biofilm residing sessile cells. The biosurfactant treatment eradicated more than 99% of bacterial biofilm present on polystyrene, glass and stainless steel surface. The biosurfactant retained its bactericidal and biofilm eradicating activities even after exposure to extreme conditions like high temperate and extreme pH. Unlike some of the commonly used disinfectant, biosurfactant retained its bactericidal and biofilm removing activity even in the hard water containing Mg²⁺ and Ca²⁺ ions. Thus, suggesting that biosurfactant produced by strain SDS21 can be used as a disinfectant or in disinfectant-like formulations effective against both planktonic and biofilm residing bacteria.

Anticancer Activities of Surfactin and Potential Application of Nanotechnology Assisted Surfactin Delivery

Surfactin, a cyclic lipopeptide biosurfactant produced by various strains of Bacillus genus, has been shown to induce cytotoxicity against many cancer types, such as Ehrlich ascites, breast and colon cancers, leukemia and hepatoma. Surfactin treatment can inhibit cancer progression by growth inhibition, cell cycle arrest, apoptosis, and metastasis arrest. Owing to the potent effect of surfactin on cancer cells, numerous studies have recently investigated the mechanisms that underlie its anticancer activity. The amphiphilic nature of surfactin allows its easy incorporation nano-formulations, such as polymeric nanoparticles, micelles, microemulsions, liposomes, to name a few. The use of nano-formulations offers the advantage of optimizing surfactin delivery for an improved anticancer therapy. This review focuses on the current knowledge of surfactin properties and biosynthesis; anticancer activity against different cancer models and the underlying mechanisms involved; as well as the potential application of nano-formulations for optimal surfactin delivery.

Biosurfactant surfactin with pH-regulated emulsification activity for efficient oil separation when used as emulsifier

In the present study, the pH-regulated emulsification activity of surfactin was studied and its potential application in oil separation towards enhanced oil recovery (EOR) was investigated. As demonstrated, surfactin can stabilize emulsions quite well beyond pH 7.4. An oil emulsification ratio of about 98% was obtained at pH 11.0; while this emulsification activity was rapidly and completely lost when pH decreased to below 3.0, having an oil separation ratio of over 98%. This pH-sensitive property is probably due to surfactin dissolution-precipitation induced by the ionization-protonation of a carboxyl group in its structure under alkaline or acidic conditions. This property allows oil emulsification or oil separation to be readily achieved via simple pH adjustments when surfactin is used as an emulsifier. Furthermore, surfactin sustained its activity after demulsification and can be readily reused many times. The above obtained results indicated surfactin-based EOR processes have great application feasibility.

Benchmarking the Self-Assembly of Surfactin Biosurfactant at the Liquid-Air Interface to those of Synthetic Surfactants

The adsorption of surfactin, a lipopeptide biosurfactant, at the liquid–air interface has been investigated in this work. The maximum adsorption density and the nature and the extent of lateral interaction between the adsorbed surfactin molecules at the interface were estimated from surface tension data using the Frumkin model. The quantitative information obtained using the Frumkin model was also compared to those obtained using the Gibbs equation and the Langmuir–Szyszkowski model. Error analysis showed a better agreement between the experimental and the calculated values using the Frumkin model relative to the other two models. The adsorption of surfactin at the liquid–air interface was also compared to those of synthetic anionic, sodium dodecylbenzenesulphonate (SDBS), and nonionic, octaethylene glycol monotetradecyl ether (C₁₄E₈), surfactants. It has been estimated that the area occupied by a surfactin molecule at the interface is about 3- and 2.5-fold higher than those occupied by SDBS and C₁₄E₈ molecules, respectively. The interaction between the adsorbed molecules of the anionic biosurfactant (surfactin) was estimated to be attractive, unlike the mild repulsive interaction between the adsorbed SDBS molecules.

Surfactin at the Water/Air Interface and in Solution

The lipopeptide surfactin produced by certain strains of Bacillus subtillis is a potent biosurfactant with high amphiphilicity and a strong tendency for self-aggregation. Surfactin possesses a number of valuable biological properties such as antiviral, antibacterial, antifungal, and hemolytic activities. Owing to these properties, in addition to the general advantages of biosurfactants over synthetic surfactants, surfactin has potential biotechnological and biomedical applications. Here, the aggregation properties of surfactin in solution together with its behavior at the water/air interface were studied using classical molecular dynamics simulations (MD) at three different pH values. Validation of the MD structural data was performed by comparing neutron reflectivity and volume fraction profiles computed from the simulations with their experimental counterparts. Analysis of the MD trajectories supported conclusions about the distribution, conformations, and interactions of surfactin in solution and at the water-air interface. Considering altogether, the work presented provides atomistic models for the rationalization of some of the structural and dynamic characteristics as well as the modes of action of surfactin at different pH values.

Spray drying as a strategy for biosurfactant recovery, concentration and storage

The objective of this study was to analyze the use of Spray Drying for concentration and preservation of biosurfactants produced by Bacillus subtilis LBBMA RI4914 isolated from a heavy oil reservoir. Kaolinite and maltodextrin 10DE or 20DE were tested as drying adjuvants. Surface activity of the biosurfactant was analyzed by preparing dilution x surface activity curves of crude biosurfactant, crude biosurfactant plus adjuvants and of the dried products, after their reconstitution in water. The shelf life of the dried products was also evaluated. Spray drying was effective in the recovery and concentration of biosurfactant, while keeping its surface activity. Drying adjuvants were required to obtain a solid product with the desired characteristics. These compounds did not interfere with tensoactive properties of the biosurfactant molecules. The dehydrated product maintained its surfactant properties during storage at room temperature during the evaluation period (120 days), with no detectable loss of activity.

Potential therapeutic applications of biosurfactants

Biosurfactants have recently emerged as promising molecules for their structural novelty, versatility, and diverse properties that are potentially useful for many therapeutic applications. Mainly due to their surface activity, these molecules interact with cell membranes of several organisms and/or with the surrounding environments, and thus can be viewed as potential cancer therapeutics or as constituents of drug delivery systems. Some types of microbial surfactants, such as lipopeptides and glycolipids, have been shown to selectively inhibit the proliferation of cancer cells and to disrupt cell membranes causing their lysis through apoptosis pathways. Moreover, biosurfactants as drug delivery vehicles offer commercially attractive and scientifically novel applications. This review covers the current state-of-the-art in biosurfactant research for therapeutic purposes, providing new directions towards the discovery and development of molecules with novel structures and diverse functions for advanced applications.

Interfacial assembly of lipopeptide surfactants on octyltrimethoxysilane-modified silica surface

The adsorption of a series of cationic lipopeptide surfactants, C14Kn (where C14 denotes the myristic acyl chain and Kn represents n number of lysine residues) at the hydrophobic solid/water interface has been studied using spectroscopic ellipsometry (SE) and neutron reflection (NR). The hydrophobic C8 surface was prepared by grafting a monolayer of octyltrimethoxysilane on the silicon surface. SE was used to follow the dynamic adsorption from these lipopeptide surfactants and the amount was found to undergo a fast increase within the first 2-3 min, followed by a much slower process tending to equilibration in the subsequent 15-20 min. Lipopetide surfactants with n = 1-4 showed similar dynamic features, indicating that the interaction between the acyl chain and the C8 surface is the main driving force for adsorption. The saturation adsorption amount of C14Kn at the C8/water interface was found to be inversely related to the increasing number of Lys residues in the head group due to the increase of steric hindrance and electrostatic repulsion between the head groups. Solution concentration had a significant effect on the initial adsorption rate, similar to the feature observed from nonionic surfactants CmEn. The structure of the adsorbed layers was studied by NR in conjunction with isotopic contrasts. The layer formed by the head groups of C14K1 was 10 Å thick, and those formed by C14K2, C14K3 and C14K4 head groups were all about 13 Å thick. In contrast, the thicknesses of the layers formed by hydrophobic tails of C14K1, C14K2 C14K3, and C14K4 were found to be 17, 13, 10, and 10 Å, respectively, resulting in the steady increase of area per molecule at the interface from 29 ± 2 Å(2) for C14K1 to 65 ± 2 Å(2) for C14K4. Thus, with an increase in the head group length, the molecules in the adsorbed layer tended to lie down upon adsorption.

Interaction of the biosurfactant, Surfactin with betaines in aqueous solution

The interactions between the lipopeptide Surfactin and four betaines, N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SDDAB), N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (STDAB), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SHDAB), and N-dodecyl-N,N-dimethyl-2-ammonio-acetate (C12BE) are studied by surface tension and small-angle neutron scattering (SANS). SDDAB, STDAB, and SHDAB have the same headgroup but different hydrophobic chains. C12BE has different headgroup but the same hydrophobic chain with SDDAB. According to the interfacial parameters calculated from surface tension, the synergism between Surfactin and betaine is relevant with the molecule structure of betaine and the mole ratio of them. For betaines, the optimum alkyl chain length (STDAB) and long enough separation between positive charge and negative charge in headgroup are responsible for highest synergetic interaction with Surfactin. The aggregates of individual Surfactin and the mixtures of Surfactin and sulfopropyl betaines are predicted to be spherical based on the packing parameter (pp) and the average packing parameter (P(av)), which is in close qualitative agreement with SANS data analysis, while Surfactin/C12BE forms ellipsoidal micelles due to the smaller headgroup of C12BE.

Temperature influence on the structure and interfacial properties of surfactin micelle: a molecular dynamics simulation study

Surfactin is an efficient biosurfactant excreted by different strains of Bacillus subtilis. Our study provides a molecular view of the temperature dependence of the structure and the interfacial properties of un-ionized surfactin micelles. The overall size and shape, the surface area, the radial density distribution of the micelles, the conformation of the hydrocarbon chain, and the intramolecular/intermolecular hydrogen bonds formed in surfactin molecules were investigated. The micelles were mostly in sphere shapes, and the radii of surfactin micelle were estimated to be around 2.2 nm. The peptide rings occupied most of the surface of the micelles. Small amounts of β-turn and γ-turn structures were found in the conformations of the peptide rings. When the temperature increased, the shape of the peptide rings became planar; the solvent accessible surface area decreased as temperature dehydration occurred. At 343 K some hydrocarbon chains reversed their orientation (flip-flopped). In addition, the stability of the hydrogen bond interactions in the micelles decreases with the increasing temperature.

Self-assembly of hydrophobin and hydrophobin/surfactant mixtures in aqueous solution

The self-assembly of the protein hydrophobin, HFBII, and its self-assembly with cationic, anionic, and nonionic surfactants hexadecylterimethyl ammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), in aqueous solution have been studied by small-angle neutron scattering, SANS. HFBII self-assembles in solution as small globular aggregates, consistent with the formation of trimers or tetramers. Its self-assembly is not substantially affected by the pH or electrolytes. In the presence of CTAB, SDS, or C(12)E(6), HFBII/surfactant complexes are formed. The structure of the HFBII/surfactant complexes has been identified using contrast variation and is in the form of HFBII molecules bound to the outer surface of globular surfactant micelles. The binding of HFBII decreases the surfactant micelle aggregation number for increasing HFBII concentration in solution, and the number of hydrophobin molecules bound/micelle increases.