Rheological behavior and molecular dynamics simulation of κ-carrageenan/casein under simulated gastrointestinal electrolyte conditions

In this study, we investigated the effects of gastrointestinal electrolytes (including simulated gastric fluid (SGF) and simulated intestinal fluid (SIF)) on the conformational transition and exposed sulfate groups of κ-carrageenan (κ-CGN) and κ-CC (κ-carrageenan/casein) during digestion, which might contribute to the colitis. The conformational transition of κ-CGN and κ-CC were assessed through zeta-potential measurements, rheological analysis, differential scanning calorimetry, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The sulfate group content measurement and molecular dynamics simulation were applied to investigate the exposed quantity of free sulfate groups. The results showed that SGF and SIF promoted the formation of a helical structure of κ-CGN and enhanced the intermolecular interactions of κ-CC. The κ-CGN self-helix formation in the presence of monovalent cations (Na+ and K+), and the helix-helix association of the neighboring κ-CGN chain in the presence of divalent cations (Mg2+ and Ca2+). In κ-CC system, the stability of helix conformation was enhanced by SGF and SIF, a greater number of hydrogen bonds were detected in Na+ and K+ solvent boxes, and Mg2+ and Ca2+ not only improved the number of hydrogen bonds through salt bridges but enhanced the electrostatic interaction of κ-CC, which were in accordance with the lower zeta-potential values, greater thermal hysteresis, and intensified binding energy results. There was an increase in hydrogen bonding sites on –SO3- groups and OH- groups of κ-CGN with the effects of electrolyte cations so that decreased the content of free sulfate groups. In conclusion, SGF and SIF enhanced the binding stability and reduced the contents of free sulfate groups of κ-CC. Our study findings provided value information for the safe processing of carrageenan.