Abstract
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 ünexposed" 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.
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