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

Abstract

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 α-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 α-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 β-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.