Charge heterogeneity induced binding and phase stability in β-lacto-globulin–gelatin B gels and coacervates at their common pI

Abstract

An understanding of the interactions between gelatin B (GB) and β-lacto-globulin (β-Lg) mainly arising from surface selective patch binding occurring at their common pI (≈5.0 ± 0.5) in the absence of added salt. Heterogeneous surface charge distribution on β-Lg facilitated such interaction at different mixing ratio ([GB]: [β-Lg] = r) and the GB–β-Lg complexes carried distinctive surface charge (seen through their zeta potential, ζ). For r < 1 : 1 (partial charge neutralization, ζ ≈ 0) a turbid solution was formed which gives the indication of formation of intermolecular soluble complexes. For r > 1 : 1 (overcharged regime, ζ > 0) the dispersion remained transparent and homogeneous which gives no phase separation, but the dispersion formed a gel on waiting. The overcharged gels were homogeneous, more rigid and higher melting temperature in comparison to coacervate. In the coacervate phase, the intensity of the scattered light I_s, and its time-correlation function [g₂(t) − 1] did not evolve with time. In contrast, the gel phase displayed considerable change with aging time t_w. For gels, as t_w → ∞ the system moved from an ergodic to non-ergodic state. At t_w = 0, the correlation function exhibited one relaxation mode due to the system residing deeply inside the ergodic phase and purely mirroring Brownian dynamics. After a characteristic waiting time, t_w an additional mode (slow relaxation) appeared which was attributed to inter-chain interaction induced reorganization of entanglements. This characteristic time was the time required for the system to get dynamically arrested, similar observation was made from rheology measurements too. A comprehensive phase diagram depicting the stability of the dispersion in various charged soft matter states of the complex under various temperature conditions was established.