ACID-HYDROLYSIS OF KAPPA-CARRAGEENAN 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 a cid while being in the ordered conformation (induced by LiI or LiCl, r espectively) 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 (M W) fraction remained conformationally ordered, whereas the low-MW frac tion was in the disordered state. The relative amount of the low-MW fr action increased with degradation time. By inducing the disordered con formation (in 0.01 M LiCl) the weight average molecular weight (M-w) o f the partially hydrolyzed samples decreased 1.5-3 times. These result s are interpreted in favor of a double-stranded ordered conformation. Partial hydrolysis of this structure yields a metastable, partially do uble-stranded structure containing chain breaks that are ''unexposed'' due to partial overlap of the individual chain fragments. When the de gree of polymerization (DP) of such fragments decrease below the criti cal value (DPc) for duplex stabilization the fragments are released an d appear as a separate low-MW fraction in the SEC chromatograms. DPc w as estimated to be approximately 100 residues in both cases. After ind uction of the disordered conformation in partially hydrolyzed samples the molecular weight distribution remained bimodal. For kappa-carragee nan this is attributed to the higher rate of hydrolysis in the disorde red fragments than in the parent ordered chains, which also leads to a rapid accumulation of dialyzable oligomers (DP 1-4). For iota-carrage enan, where the rate of hydrolysis ideally is independent of the confo rmational state, the bimodality is tentatively attributed to the prese nce of kappa-units (partially formed by desulfation), which are hydrol yzed more rapidly in the disordered state than in the ordered state.