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.

Infrared and circular dichroism spectroscopic characterisation of secondary structure components of a water treatment coagulant protein extracted from Moringa oleifera seeds.

The secondary structure of a water treatment coagulant protein extracted from Moringa oleifera (MO) seeds has been investigated by Fourier transform infrared spectroscopy (FTIR) in the dried state, and by circular dichroism (CD) spectroscopy. The FTIR and CD spectra indicate that the secondary structure of the protein is dominated by alpha-helix. The FTIR spectrum recorded two distinct and strong absorption bands at 1656 cm(-1) and 1542 cm(-1), in the usual range of absorption of helices of proteins. The CD spectrum showed the shape of mainly alpha-helical secondary structure (estimated to be 58+/-4%) characteristic of negative ellipticity bands near 222 nm and 208 nm and a positive band at 192 nm. The beta-sheet structure composition was estimated to be 10+/-3% whereas unordered structures were around 33%. Changes in solution pH affected the protein secondary structure significantly only at pH values above 10, as indicated by CD spectra, whereas ionic strength had minimal effect. CD data also showed that sodium dodecyl sulphate (SDS) interacts with the coagulant protein and modifies the protein conformation. The surfactant-induced conformational change of the coagulant protein was confirmed by quenching of tryptophan fluorescence of the protein.

A fluorescence spectroscopic study of a coagulating protein extracted from Moringa oleifera seeds.

The fluorescence studies of coagulating protein extracted from Moringa oleifera seeds have been studied using steady-state intrinsic fluorescence. The fluorescence spectra are dominated by tryptophan emission and the emission peak maximum (lambda(max)=343+ or -2nm) indicated that the tryptophan residue is not located in the hydrophobic core of the protein. Changes in solution pH affected the protein conformation as indicated by changes in the tryptophan fluorescence above pH 9 whereas the ionic strength had minimal effect. The exposure and environments of the tryptophan residue were determined using collisional quenchers.

Interfacial properties and fluorescence of a coagulating protein extracted from Moringa oleifera seeds and its interaction with sodium dodecyl sulphate.

The surfactant behaviour of aqueous coagulating protein extracted from Moringa oleifera seeds has been investigated by surface tension measurements. The interaction of the coagulant protein with an anionic surfactant sodium dodecyl sulphate (SDS) has been monitored by surface tension and intrinsic protein fluorescence measurements. The extracted protein shows some weak surface activity at low concentrations. To achieve maximum surface activity (i.e. maximum reduction in surface tension of water), substantially higher concentrations of protein are required. The coagulant protein-SDS interaction scheme did not exhibit the behaviour of weakly interacting polymer-surfactant systems and the SDS interacts in a monomeric form with the protein. The association process of SDS with the coagulant protein is supported by protein fluorescence measurements. SDS has an effect on the fluorescence of the coagulant protein indicating that the local environment of tryptophan in the protein changes as SDS concentration below its critical micelle concentration is increased. These results have led us to the conclusions that: (1) the protein extracted from M. oleifera seeds has significant surfactant behaviour; (2) the coagulant protein interacts strongly with SDS and the protein might have specific binding sites for SDS; (3) there is formation of protein-SDS complex.