Acid hydrolysis of κ- and l-carrageenan in the disordered and ordered conformations:: Characterization of partially hydrolyzed samples and single-stranded oligomers released from the ordered structures

kappa- and iota-carrageenan that were partially hydrolyzed in dilute acid while being in the ordered conformation (induced by LiI or LiCl, respectively) gave rise to bimodal molecular weight distributions (MWDs) as shown by size-exclusion chromatography (SEC) and gel filtration. Optical rotation measurements showed that the high molecular weight (MW) fraction remained conformationally ordered, whereas the low-MW fraction was in the disordered state. The relative amount of the low-MW fraction increased with degradation time. By inducing the disordered conformation (in 0.01 M LiCl) the weight average molecular weight (M-w) of the partially hydrolyzed samples decreased 1.5-3 times. These results are interpreted in favor of a double-stranded ordered conformation. Partial hydrolysis of this structure yields a metastable, partially double-stranded structure containing chain breaks that are "unexposed" due to partial overlap of the individual chain fragments. When the degree of polymerization (DP) of such fragments decrease below the critical value (DPc) for duplex stabilization the fragments are released and appear as a separate low-MW fraction in the SEC chromatograms. DPc was estimated to be approximately 100 residues in both cases. After induction of the disordered conformation in partially hydrolyzed samples the molecular weight distribution remained bimodal. For kappa-carrageenan this is attributed to the higher rate of hydrolysis in the disordered fragments than in the parent ordered chains, which also leads to a rapid accumulation of dialyzable oligomers (DP 1-4). For iota-carrageenan, where the rate of hydrolysis ideally is independent of the conformational state, the bimodality is tentatively attributed to the presence of kappa-units (partially formed by desulfation), which are hydrolyzed more rapidly in the disordered state than in the ordered state.