Quantitative analysis of binding affinities and characterization of β‐lactoglobulin and λ‐carrageenan as a function of pH

Abstract

λ‐Carrageenan (λcar) interacted with β‐lactoglobulin (βlg) immediately to form βlg–λcar complexes when used as an additive in milk. The formation of complexes is the key process through which to explore the bioapplication of λcar, which is a complicated process and influenced by many factors. In this study, the formation process and effect of pH were ascertained by the binding affinity, hydrodynamic diameter, and secondary structure. Results showed that the interaction was spontaneously exothermic and the complexes were soluble. The binding affinities (Ka) decreased from 9.0 ± 1.3 × 105 to 1.3 ± 0.8 × 105 M−1, and the stoichiometry also decreased as the pH was increased from 4 to 7. Furthermore, DLS showed a larger hydrodynamic diameter of the complexes at lower pH. Moreover, the complexes induced a change in the secondary structural components of βlg at lower pH. Practical applications The secondary structure of βlg was changed by the interaction of λcar, which resulted in βlg–λcar complexes under acidic conditions. The soluble βlg–λcar complexes showed a good stability against aggregation. Thus, they can enhance the textural properties and stability of acidic dairy drinks, and can be used to accurately formulate ingredients in the food ingredient industry.

New insights into protein–polysaccharide complex coacervation: Dynamics, molecular parameters, and applications

“…[ 45,47,[58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] Entropy gain dominated complex coacervation (ΔH > 0, ΔS > 0)…”

“…[ [77][78][79][80] coacervation. [45,47,[58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] The negative entropy change originates mainly from the reduced mobility and flexibility of the biopolymers after association, that is, vibrational and conformational entropy decreases. (2) In contrast, the combinations of gelatin A-pectin, bovine serum albumin (BSA)-hyaluronic acid (HA), and β-lactoglobulin (BLG)-HA exhibit an endothermic interaction and increased entropy behavior; that is, complex coacervation is driven only by the entropy gain caused by the release of counterions and water molecules.…”

“…There are four types of thermodynamic parameters in the existing PPCC studies (Table 1): Most protein–polysaccharide combinations exhibit an exothermic interaction (Δ H < 0) and reduced entropy (Δ S < 0), that is, only exothermic dominated complex coacervation. [ 45,47,58–72 ] The negative entropy change originates mainly from the reduced mobility and flexibility of the biopolymers after association, that is, vibrational and conformational entropy decreases. In contrast, the combinations of gelatin A–pectin, bovine serum albumin (BSA)–hyaluronic acid (HA), and β‐lactoglobulin (BLG)–HA exhibit an endothermic interaction and increased entropy behavior; that is, complex coacervation is driven only by the entropy gain caused by the release of counterions and water molecules. [ 73,74 ] The unfavorable enthalpy changes are associated with a low affinity.…”

“…Most protein–polysaccharide combinations exhibit an exothermic interaction (Δ H < 0) and reduced entropy (Δ S < 0), that is, only exothermic dominated complex coacervation. [ 45,47,58–72 ] The negative entropy change originates mainly from the reduced mobility and flexibility of the biopolymers after association, that is, vibrational and conformational entropy decreases.…”

“…Although it is generally believed that strong PE–protein complex coacervation may cause more release of counterions and water molecules, the thermodynamic parameters of the protein–polysaccharide combination based on carrageenan and dextran sulfate still exhibit an exothermic/decreased entropy behavior, that is, only the exothermic contribution drives their complex coacervation. [ 47,69–72 ] This showed that the charge strength of the PE is not the only decisive factor. It should be emphasized that it is difficult to understand whether these diverse thermodynamic parameters arise from differences in the structural (molecular weight [Mw], chain length, chain flexibility) or charging characteristics (charge density, charge pattern, charge anisotropy) of proteins and polysaccharides or from different solvent conditions for complex coacervation by comparing fragmented studies.…”