Palmitic acid sophorolipid biosurfactant: from self-assembled fibrillar network (SAFiN) to hydrogels with fast recovery

Nanofibres are an interesting phase into which amphiphilic molecules can self-assemble. Described for a large number of synthetic lipids, they were seldom reported for natural lipids like microbial amphiphiles, known as biosurfactants. In this work, we show that the palmitic acid congener of sophorolipids (SLC16:0), one of the most studied families of biosurfactants, spontaneously forms a self-assembled fibre network (SAFiN) at pH below 6 through a pH jump process. pH-resolved in situ small-angle X-ray scattering (SAXS) shows a continuous micelle-to-fibre transition, characterized by an enhanced core–shell contrast between pH 9 and pH 7 and micellar fusion into a flat membrane between pH 7 and pH 6, approximately. Below pH 6, homogeneous, infinitely long nanofibres form by peeling off the membranes. Eventually, the nanofibre network spontaneously forms a thixotropic hydrogel with fast recovery rates after applying an oscillatory strain amplitude out of the linear viscoelastic regime: after being submitted to strain amplitudes during 5 min, the hydrogel recovers about 80% and 100% of its initial elastic modulus after, respectively, 20 s and 10 min. Finally, the strength of the hydrogel depends on the medium's final pH, with an elastic modulus fivefold higher at pH 3 than at pH 6. This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)’.

ANALYSIS OF AMINE HARDENERS FOR ADHESIVE USING pH-METRIC TITRATION IN MICELLAR MEDIA OF SODIUM DODECYLSULFATE

One of the primary tasks in the development of amine hardeners for adhesive and epoxy resins is the control of amino group quantities in their composition. The main parameter that indicates the rate of the polymerization reaction and characterizes the quality of the hardener is the amine number. It is determined by the number of primary and secondary amino groups contained in the hardener molecule, because these functional groups are involved in reactions with epoxy resins. The most common methods of analysis of amine hardeners are mainly based on titration in organic solvents and require a procedure of derivatization of primary and secondary amino groups using formaldehyde and acetic anhydride. The search for a simple, cheap and environmentally friendly alternative to such titrimetric methods is still ongoing. In this paper on the example of industrial samples of polyamide PO-300, polyethylene polyamine (PEPA) and diethylenetriamine (DETA) shows the prospects of using the method of pH-metric titration in water-micellar medium of sodium dodecylsulfate (SDS) to determine the content of primary and secondary amino groups in the adhesive hardeners. According to the developed techniques, working solutions of PO-300, PEPA and DETA were prepared by dissolving their exact mass in 20 ml of 2.0 M SDS solution. The values of PO-300, PEPA and DETA samples were 0.1040 g, 0.0225 g and 0,0200 g, respectively. Titration of the obtained solutions was performed with 0.05 M HCl solution. The percentage of primary amino groups, calculated on the basis of the obtained differential titration curves, is equal to 5,56% for PO-300, 23,6% for PEPA and 31,6% for DETA. The content of secondary amino groups in PO-300, PEPA and DETA samples is 3,03%, 15,0% and 19,6%. Founded amine number for PO-300, PEPA and DETA is well correlated with data declared by the manufacturer and equals to 302, 1381 and 1890, respectively. Unfortunately, it was not possible to establish the presence and quantity of tertiary amino groups in the samples of adhesive hardeners by this technique. The effect of cationic surfactant cetylpyridinium chloride, nonionic Triton X-100 and anionic surfactant SDS on the value of the pH jump of diethylenetriamine (DETA) was also studied. It was found that anionic SDS has the greatest differentiating effect on the acid-base properties of amino groups DETA in comparison with other studied surfactants. At that, the primary amino groups are titrated in the first place.

Application of millisecond time-resolved solid state NMR to the kinetics and mechanism of melittin self-assembly

Common experimental approaches for characterizing structural conversion processes such as protein folding and self-assembly do not report on all aspects of the evolution from an initial state to the final state. Here, we demonstrate an approach that is based on rapid mixing, freeze-trapping, and low-temperature solid-state NMR (ssNMR) with signal enhancements from dynamic nuclear polarization (DNP). Experiments on the folding and tetramerization of the 26-residue peptide melittin following a rapid pH jump show that multiple aspects of molecular structure can be followed with millisecond time resolution, including secondary structure at specific isotopically labeled sites, intramolecular and intermolecular contacts between specific pairs of labeled residues, and overall structural order. DNP-enhanced ssNMR data reveal that conversion of conformationally disordered melittin monomers at low pH to α-helical conformations at neutral pH occurs on nearly the same timescale as formation of antiparallel melittin dimers, about 6 to 9 ms for 0.3 mM melittin at 24 °C in aqueous solution containing 20% (vol/vol) glycerol and 75 mM sodium phosphate. Although stopped-flow fluorescence data suggest that melittin tetramers form quickly after dimerization, ssNMR spectra show that full structural order within melittin tetramers develops more slowly, in ∼60 ms. Time-resolved ssNMR is likely to find many applications to biomolecular structural conversion processes, including early stages of amyloid formation, viral capsid formation, and protein–protein recognition.

Remote activation of nanoparticulate biomimetic activity by light triggered pH-jump

By incorporating flash photolysis reagents, a facile and versatile method for the photo-regulation of pH-dependent activities of artificial enzymes is presented.

XFELs for structure and dynamics in biology

The development and application of the free-electron X-ray laser (XFEL) to structure and dynamics in biology since its inception in 2009 are reviewed. The research opportunities which result from the ability to outrun most radiation-damage effects are outlined, and some grand challenges are suggested. By avoiding the need to cool samples to minimize damage, the XFEL has permitted atomic resolution imaging of molecular processes on the 100 fs timescale under near-physiological conditions and in the correct thermal bath in which molecular machines operate. Radiation damage, comparisons of XFEL and synchrotron work, single-particle diffraction, fast solution scattering, pump–probe studies on photosensitive proteins, mix-and-inject experiments, caged molecules, pH jump and other reaction-initiation methods, and the study of molecular machines are all discussed. Sample-delivery methods and data-analysis algorithms for the various modes, from serial femtosecond crystallography to fast solution scattering, fluctuation X-ray scattering, mixing jet experiments and single-particle diffraction, are also reviewed.