Position of acetyl groups on anhydroglucose unit in acetylated starches with intermediate degrees of substitution.

Waxy, normal, and high-amylose maize starches were acetylated to various degrees of substitution (DS) in aqueous medium. A stepwise addition of acetic anhydride enhanced reaction efficiency and DS was in the order of high-amylose maize starch (DS 1.72) > waxy maize starch (DS 1.60) > normal maize starch (DS 1.03) when 240% (w/w) acetic anhydride was added. Starch acetates were further fully esterified to DS 3.0 using acetic anhydride-d6 or propionic anhydride to analyze position of acetyl groups on anhydroglucose unit (AGU) by 1H NMR. Regioselective 2-O-acetyl starches were produced when the DS was lower than 0.14. At DS 1, the distribution of acetyl groups was ca. 44%, 32%, and 24% at C-2, C-3, and C-6 positions, respectively, regardless of amylose content. Deuterated acetyl group was a preferred secondary substituent over propionyl group in subsequent 1H NMR analysis because it resulted a clean spectrum without interference from propionyl methyl protons.