Protein-Dependent, pH-Selective Complexation in Tragacanth–Protein Systems: An Integrated FTIR–DLS–Rheology–Docking Study

Tragacanth gum (GT) was mixed with whey protein concentrate (WPC80), whey protein isolate (WPI) or rice protein (RP) across pH 3.0–7.2 in order to clarify the effect of protein type and pH on controlling association and bulk behavior. Turbidimetry at 600 nm by photographic validation, oscillatory and steady-shear rheology, dynamic light scattering (DLS), FTIR spectroscopy, and AutoDock Vina docking were employed and compared. Whey systems reflected a clear, mildly acidic window: low-strain elasticity (G′) reached near pH ~5, with increased A600 and dominant sub-100 nm DLS modes, reflecting associative complexation near the isoelectric region. WPI also reflected a secondary turbidity/viscosity rise at pH 7.2, consistent with segregative aggregationafter the associative window. RP was variable, featuring broadly increased turbidity with viscosity/DLS maxima at pH 6.4, reflecting glutelin-facilitated solubility/aggregation rather than an acid optimum. FTIR changes in the amide band and GT bands (COO− ~1400–1406 cm−1; 1015–1040 cm−1) supplemented enhanced coupling at pH 3–5. Superimposition through docking of multivalent hot-spots (Lys/Arg and H-bonding neighborhoods) corresponded to the phase-level readouts. Together, the data establish protein-dependent, pH-selective windows for GT–protein systems and uncover a mechanistic dichotomy: associative complexation in whey vs. neutral-side, solubility-regulated aggregation in RP.