Conformational Changes in ι- and κ-Carrageenans Induced by Complex Formation with Bovine β-Casein

Abstract

The formation of electrostatic complexes between beta-casein and iota- and kappa-carrageenans is well-known. However, the molecular mechanism of the complexation has yet to be determined, particularly with respect to the conformational changes of the interacting macromolecules. High-sensitivity differential scanning calorimetry was used to study beta-casein/carrageenan mixtures at different pH values (3.0 to 7.5), ionic strengths (0.03 and 0.15 M), and various molar protein/polysaccharide ratios (3-400). The effects of these variables on the temperature, enthalpy, and width of the helix-coil transition of iota- and kappa-carrageenans were investigated. Neither pH nor the protein/polysaccharide ratio influenced the transition temperature of either carrageenan in the complexes. However, the transition enthalpy of both carrageenans in complexes with beta-casein decreased to zero with both decreasing pH and increasing protein/polysaccharide ratio. This may reflect an unwinding of the polysaccharide double helix induced by beta-casein, a conformational change which is fully reversible in conditions of sufficiently high ionic strength. The interaction of beta-casein with iota- and kappa-carrageenans was approximated in terms of the model of binding of large ligands to macromolecules, that provides the binding constants for these biopolymers.

Hydrogen bonding enhances the electrostatic complex coacervation between κ-carrageenan and gelatin

“…KC differs from Ã-and -carrageenan in the content and position of sulfate groups and the content of the 3,6-anhydrogalactose residue. For the protein/carrageenan complexation, sulfate groups on carrageenan play a decisive role, and thus Ã-carrageenan was found to have higher binding constant than KC [18].…”

The study of pH-dependent complexation between gelatin and gum arabic by morphology evolution and conformational transition

“…Thus, most researchers were only centered on protein's conformation, with little or even no reference to the polysaccharide. Burova et al investigated the conformational changes in i-and k-carrageenan upon complexation with b-casein (Burova et al, 2007). The application of high-sensitivity differential scanning calorimetry revealed that b-casein reduced the helical structure of carrageenan, as the interaction with unordered parts of the polysaccharide chains was more favored.…”

Interactions between whey protein isolate and gum Arabic

“…The charge complex may "dissolve" in water to form a "molecular solution", or it may coacervate and precipitate into a two-phase system. Electrostatic interactions are dominated by the pH and the ionic strength and play a key role in the formation of complexes [4,12,16,17]. The soluble complex is formed at a pH above the isoelectric point (IEP) of the protein.…”

“…We have suggested that at pH > IEP most of the side groups of the protein are negatively charged, but some moieties or fragments are still positively charged. The "peptide segments" are positively charged and may interact with the negatively charged hydrocolloid to form a weakly associated complex that does not precipitate [1,2,6,7,14,15,[17][18][19][20]. If a complex does not form, a one-phase mixture of protein and hydrocolloid is macroscopically observed.…”