TEMPO oxidation of gelatinized potato starch results in acid resistant blocks of glucuronic acid moieties

Chemical derivatization is often applied to improve polysaccharide functionality. Primary hydroxyl groups in starch can, for example, be oxidized to aldehydes by using a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated reaction. The major part of the aldehydo groups is subsequently converted to carboxyl groups by NaOCl. The exact structure of TEMPO-oxidized starch was studied to promote a better understanding of the TEMPO oxidation mechanism and the functionality of oxidized starches. By using weak and strong acidic hydrolysis, and methanolysis, at elevated temperatures, oxidized starches with different degrees of oxidation (DO) were broken down into oligomers and monomers. Analysis of the oligomers by chromatographic and mass spectrometric techniques revealed that blocks of glucuronic acid moieties are present in the oxidized starch polymers. The α(1 → 4) glucuronic acid–glucuronic acid bond was found to be very resistant to breakdown by acids. The α(1 → 4) glucuronic acid–glucose bond also showed increased resistance to acids compared to α(1 → 4) glucose–glucose bonds. The size of the blocks of glucuronic acid moieties increased when DO increased. Furthermore, the presence of clusters of aldehydes close to carboxyl groups directly after oxidation was proven. This implies that TEMPO, which is positively charged in its active state, is apparently attracted by the negatively charged carboxyl groups. Because of this, TEMPO tends to be active in areas where carboxyl groups have already been formed, which leads to a block wise distribution of the glucuronic acid moieties.