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.