AGGREGATION OF HYDROPHOBIC COLLOIDS AND PROTEINS: SIZE DEPENDENT BEHAVIOR AND CROSSOVER TO INSTABILITY

Abstract

In this study, the aggregation properties of mainly hydrophobic proteins and colloids are explored by using an extended DLVO model that included an extra short-range interaction. Two different interaction regimes are studied to isolate the effect of hydrophobic interactions on protein aggregation. In the first interaction regime, van der Waals attraction, electrostatic double-layer repulsion, and short-range surface interactions are considered, and the interaction curves are described by finite energy barriers that kinetically stabilize protein dispersions over a broad size (R = 5-100 nm) and hydrophobicity (hf = 0-20 %) range. In the second interaction regime, hydrophobic attraction is described by an exponential potential, causing the electrostatic barrier to collapse beyond a critical size, and resulting in a diffusion limited, irreversible protein aggregation. The maximum interaction force (Fmax) - size scaling analysis shows, Fmax ~ R±α, with α = 0.5 for the first interaction regime, whereas in the second (hydrophobicity-dominated) interaction regime, this scaling linearly decreases with size (α = -0.5), indicating the lack of finite energy barriers and the dominance of short-range attraction. These results demonstrate that hydrophobic interactions by themselves are capable of inducing aggregation in electrostatically balanced systems. As a test, aggregation pattern in two predominantly hydrophobic proteins, elastin and zein, are discussed within the framework of Model-2 with satisfactory outcomes. Overall, this study clearly shows a size-dependent crossover from attraction-dominated behavior at small sizes to an unstable interaction regime at larger sizes, a conclusion that can be extended to colloidal, intrinsically disordered proteins and nanoparticle systems where surface hydrophobicity is profound.