Starch can be oxidised to obtain a high-solid, low-viscous dispersion with minimum retrogradation, properties that are of technical importance especially in the paper industry. Alkaline hypochlorite, the most common commercial oxidant, introduces carbonyl and carboxylic functions, and also causes depolymerisation'. Oxidation with bromine at neutral pH selectively introduces keto groups into carbohydrates" and eventually causesM ring cleavage and the formation of carboxylic groups at C-2 and c-3.
Because of the instability of the products of oxidation, determination of the quantity and of the type of functional groups is difficult. We now report the application of n.m.r. spectroscopy to monitor the oxidation of potato starch with bromine.
Potato starch was oxidised with bromine at several different concentrations, and the products were reduced with borohydride, then subjected to sugar analysis' in order to determine the position of the keto groups. The detection of mannose and allose indicated that oxidation had occurred at positions 2 or 3 of the glucose residues.
The positions of the keto or hydrated keto groups and other functional groups were obtained by 1D and 2D 'H-and "C-n.m.r. spectroscopy. In a 2D-COSY experiment, the H-1 signals were assigned by correlation with H-2. The lack of correlation with H-2 indicated oxidation at position 2 or ring cleavage between C-2 and C-3. The relay COSY experiment revealed oxidation at position 3. The 13C signals were assigned by 2D-heteronuclear correlations and the multiplicities by the APT technique. The assignments were compared with data in the literature4*g'4 and are presented in Tables I andII.
There were several signals in the region for H-1. The signals from (1+4)-and (1+6)-linked r-D-glucopyranose residues appeared at 5.38 and 5.02 p.p.m., respectively. The signals at 4.78 and 5.15 p.p.m. had no correlations in the COSY analyses and were assigned to H-l of hydrated forms of 2-keto residues in different environments.