The structure of κ/ι-hybrid carrageenans II. Coil–helix transition as a function of chain composition

Abstract

This paper describes the effect of the κ/ι-ratio on the physical properties of κ/ι-hybrid carrageenans (synonyms: kappa-2, κ-2, weak kappa, weak gelling kappa). To this end, a series of κ/ι-hybrid carrageenans ranging from almost homopolymeric κ-carrageenan (98 mol-% κ-units) to almost homopolymeric-carrageenan (99 mol-% ι-units) have been extracted from selected species of marine red algae (Rhodophyta). The κ/ι-ratio of these κ/ι-hybrids was determined by NMR spectroscopy. Their rheological properties were determined by small deformation oscillatory rheology. The gel strength (storage modulus, G′) of the κ/ι-hybrids decreases with decreasing κ-content. On the other hand, the gelation temperature of the κ-rich κ/ι-hybrids is independent of their composition. This allows one to control the gel strength independent of the gelation or melting temperature. The conformational order–disorder transition of the κ/ι-hybrids was studied using optical rotation and high-sensitivity differential scanning calorimetry. High-sensitivity DSC showed that the total transition enthalpy of the κ/ι-hybrids goes through a minimum at 60 mol-% κ-units, whereas for the mixture of κ- and ι-carrageenan, the total transition enthalpy is a linear function of the composition. With respect to the ordering capability, the κ/ι-hybrid carrageenans seem to behave as random block copolymers with length sequence distributions truncated from the side of the small lengths. Intrinsic thermodynamic properties (e.g., transition temperature and enthalpy) of κ- and ι-sequences in these copolymers are close to those of their parent homopolymers. The critical sequence length for κ-sequences is 2-fold of that for ι-sequences.