Adsorption and thermodynamics of biosurfactant, surfactin, monolayers at the air-buffered liquid interface

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.

Study on the interaction between surfactin and alkaline protease in aqueous solution

The interaction between surfactin and alkaline protease in aqueous solution has been studied. Ultraviolet visible absorption spectra (UV-vis) show that surfactin causes the extension of peptide chain of the alkaline protease resulting in the weakening of hydrophobic interaction between the hydrophobic groups. Fluorescence spectra indicate that the interaction of surfactin with the tryptophan and tyrosine residues led to a change of conformation of the alkaline protease. Fourier transform infrared spectroscopy (FTIR) proves complex weak interactions between surfactin and alkaline protease, especially hydrogen bonds. Enzyme activity measurements demonstrate that low concentration of surfactin can increase the enzyme activity of alkaline protease, while high concentrations inhibit it. The particle size and Zeta potential measurements confirm that the system particle size and Zeta potential are dependent on the concentration of surfactin, in addition, there is the electrostatic interaction between surfactin and alkaline protease. Surface tension measurements indicate that the binds of surfactin and alkaline protease molecules are spontaneous. Based on experimental results, the composite model of surfactin and alkaline protease in aqueous solution is proposed.

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.