Structural investigation of degradation products of irradiated Kappa-carrageenan

The production of low-molecular-weight x-carrageenan by irradiation technique has been shown to enhance plant growth and stimulate defense action in various crops. The mechanism of the observed activity, however, is unknown. This study embarked on investigating the structural changes in the x-carrageenan upon irradiation, hopefully, to provide insight into the structure-activity relationship. Structural changes in x-carrageenan at absorbed doses of 10 and 20 kGy were monitored by spectroscopic techniques (FTIR, UV, 1D, and 2D NMR). Ebeam irradiation of x-carrageenan resulted in lowering of the pH providing the necessary condition for glycosidic bond cleavage through hydrolysis. UV analysis of the degraded x-carrageenan showed a peak at 265 nm and FTIR indicated the formation of C--O and reduction of sulfate ester groups. 1D and 2D NMR analyses of the degraded product revealed new peaks formed associated with the change in the chemical structure at the G4S component of x-carrageenan. Three degradation products were proposed that include oxidation of the OH group in the G4S-2 position into carbonyl moiety, replacement of sulfate ester with OH group, and a combination of these transformations. The results may help provide insights into the future investigations on the structure-activity relationship and mechanistic understanding of the plant-growth enhancing activity of degraded x-carrageenan.