Determination of the Level and Position of Substitution in Hydroxypropylated Starch by High-Resolution1H-NMR Spectroscopy ofAlpha-limit Dextrins

Abstract Eleven hydroxypropylated starch samples were thinned by bacterial alpha -amylase at 80 °C prior to 1 H-NMR analysis on a 400 MHz instrument. Hydroxypropyl (HP) levels were calculated from the intensity of the HP methyl signal (1·18 ppm) compared with that of the methylene and methine (HCO) multiplet (3·2–4·1 ppm). HP values determined by 1 H-NMR correlated well with those determined by the colorimetric Johnson procedure. Anomeric proton signals in the spectra of alpha -limit dextrins were used to identify the positions of HP substitution. Methyl 2-O-, 3-O- and 6-O-hydroxypropyl-α- d -glucopyranosides were synthesized, and only the 2-ether shifted the H-1 signal. Eight HP starches with a molar substitution of 0·05–0·23 had 67–78% HP on O-2, 15–29% on O-3 and 2–17% on O–6.