Study of octenyl succinic anhydride-modified waxy maize starch by nuclear magnetic resonance spectroscopy

Granular waxy maize starch was reacted with two levels (3 and 15%, based on the weight of starch) of octenyl succinic anhydride (OSA). Structure of the OSA and modified starches was studied by one-dimensional (1D) 1H and 13C and two-dimensional (2D) homonuclear correlation and heteronuclear correlation nuclear magnetic resonance (NMR) spectroscopy. The modified starches were converted to α-limit dextrins prior to NMR analysis. By applying the 1D and 2D NMR techniques, complete assignments of 1H and 13C NMR spectra of the OSA reagent were achieved, and the position of the double bond and ratio of trans to cis isomers were determined. As level of OSA substitution increased, the peak (≈5.38 ppm) for the anomeric proton of internal α-1,4-d-glucosyl units became broader in 1H NMR spectra, suggesting that substitution occurred at the O-2 position. Compared with the 13C NMR spectrum of the native starch, the modified starches gave additional signals at the C-4 peak region and broadening of the C-1, C-2, C-3, and C-4 resonances, but not of the C-6 signal. Those results further suggest that the OS groups were substituted at the O-2 and O-3 positions, but not at the O-6 position.