Comparative Reactivity of the Three Glucosyl -OH Positions -2, -3 and -6 during esterification and saponification of starch by nmr spectroscopy of partially Deutero-acetylated material

International audienceThe individual reactivity of the three hydroxyle groups, (OH-2, OH-3, OH-6) of the glucosyl monomer residue in starch, under esterification and saponification conditions, were analyzed by 1H nmr spectroscopy. A first sample, obtained by direct acetylation, led to an intermediate DS of 1.5, leaving half of hydroxyle positions untouched. These free -OH groups were then esterified with deuterated acetic anhydride. In order to analyse the reactivity under saponification conditions, a second sample was fully acetylated, then, half of the acetate groups were removed by saponification with ethanolic KOH. These regenerated OH groups were then re-esterified with deuterated acetic anhydride as per the previous sample. The proton nmr spectra of each sample was recorded in deuterated chloroform. Integration of the (CH3) peaks in the 1 to 2 ppm region allowed the measurement of the relative (1H) acetate level on each of the three hydroxyle positions