Deciphering the interactions of fish gelatine and hyaluronic acid in aqueous solutions

The interactions of fish gelatine (FG) with hyaluronic acid (HA) are studied in an aqueous environment at 25°C by turbidimetric titration, confocal scanning laser microscopy, dynamic light scattering, zeta potentiometry, spectrophotometry with methylene blue, and construction of state diagrams. FG forms soluble complexes with HA above a boundary pH (pH_φ1), where both biopolymers are net-negatively charged, but develop insoluble complexes as liquid-state complex coacervates below pH_φ1, where the two biopolymers are oppositely charged. The insoluble complexes are continuously aggregated with further acid titration, followed by immediate visible phase-separation when another boundary pH (pH_p) is reached. The complex formation is mainly driven by electrostatic attractions rather than hydrogen bonding or hydrophobic interactions. The complex formation is promoted by increasing FG-to-HA weight ratio or total biopolymer concentration, or at a low ionic strength, but significantly suppressed in the presence of high ionic strength.