Towards a molecular understanding of the water purification properties of Moringa seed proteins.

Seed extracts from Moringa oleifera are of wide interest for use in water purification where they can play an important role in flocculation; they also have potential as anti-microbial agents. Previous work has focused on the crude protein extract. Here we describe the detailed biophysical characterization of individual proteins from these seeds. The results provide new insights relating to the active compounds involved. One fraction, designated Mo-CBP3, has been characterized at a molecular level using a range of biochemical and biophysical techniques including liquid chromatography, X-ray diffraction, mass spectrometry, and neutron reflection. The interfacial behavior is of particular interest in considering water purification applications and interactions with both charged (e.g. silica) and uncharged (alumina) surfaces were studied. The reflection studies show that, in marked contrast to the crude extract, only a single layer of the purified Mo-CBP3 binds to a silica interface and that there is no binding to an alumina interface. These observations are consistent with the crystallographic structure of Mo-CBP3-4, which is one of the main isoforms of the Mo-CBP3 fraction. The results are put in context of previous studies of the properties of the crude extract. This work shows possible routes to development of separation processes that would be based on the specific properties of individual proteins.

Dynamic self-assembly of DNA minor groove-binding ligand DB921 into nanotubes triggered by an alkali halide.

We describe a novel self-assembling supramolecular nanotube system formed by a heterocyclic cationic molecule which was originally designed for its potential as an antiparasitic and DNA sequence recognition agent. Our structural characterisation work indicates that the nanotubes form via a hierarchical assembly mechanism that can be triggered and tuned by well-defined concentrations of simple alkali halide salts in water. The nanotubes assembled in NaCl have inner and outer diameters of ca. 22 nm and 26 nm respectively, with lengths that reach into several microns. Our results suggest the tubes consist of DB921 molecules stacked along the direction of the nanotube long axis. The tubes are stabilised by face-to-face π-π stacking and ionic interactions between the charged amidinium groups of the ligand and the negative halide ions. The assembly process of the nanotubes was followed using small-angle X-ray and neutron scattering, transmission electron microscopy and ultraviolet/visible spectroscopy. Our data demonstrate that assembly occurs through the formation of intermediate ribbon-like structures that in turn form helices that tighten and compact to form the final stable filament. This assembly process was tested using different alkali-metal salts, showing a strong preference for chloride or bromide anions and with little dependency on the type of cation. Our data further demonstrates the existence of a critical anion concentration above which the rate of self-assembly is greatly enhanced.