Acid Hydrolysis of κ- and ι-Carrageenan in the Disordered and Ordered Conformations:  Characterization of Partially Hydrolyzed Samples and Single-Stranded Oligomers Released from the Ordered Structures

Abstract

κ- and ι-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 κ-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 ι-carrageenan, where the rate of hydrolysis ideally is independent of the conformational state, the bimodality is tentatively attributed to the presence of κ-units (partially formed by desulfation), which are hydrolyzed more rapidly in the disordered state than in the ordered state.

Analysis of the conformational properties of ?- and ?-carrageenan by size-exclusion chromatography combined with low-angle laser light scattering

“…18 -20 The hydrolysis was monitored both by the decrease in specific viscosity and increase in the number of reducing end groups, 18,19 as well as by monitoring the changes in molecular weight and molecular weight distribution. 20 These results were interpreted in terms of double-stranded structures for the ordered conformations of both -and -carrageenan, partly supported by the observation of a twofold decrease in M w when going from the ordered to the disordered state.…”

“…The critical lower M w required for duplex formation for -carrageenan at the same experimental conditions is about 20,000 (for single-stranded chains). 20 This may explain why M w increases by a factor less than 2 in the -35 sample.…”

“…No sign of hysteresis in the cooling curve was observed, in accordance with previous investigations for -carrageenan in I Ϫ salt. 2,11,20 The onset of conformational ordering upon cooling occurs at about 55°C, above which the sample appears to be 100% disordered.…”

Mathematical Modelling and Computer Simulation of Linear Polymer Degradation: Simple Scissions

“…aggregates and their location (e.g., amorphous or crystalline part of the polymer) by experimental verification. [51,63,79,80] Mechanistic modelling to connect structure/reactivity aspects have adequately dealt certain cases of faster decay processes (e.g., pyrolysis) for less complex polymer structure (e.g., synthetic or amorphous polymers) or reactor system. [17,21,22,81] The current algorithm can be extended to focus on this aspect as future investigation.…”

Polysaccharides

“…To obtain a gel the carrageenan molecules must undergo a transition from a random coil structure into helices that aggregate upon cooling. In general the ion induced network formation [345] is an intermolecular association that requires a minimum degree of polymerization of about 100 [346]. It is still under debate whether the fundamental ordered state is a single or a double-stranded helix.…”