Condensation, Complex Coacervation, and Overcharging during DNA–Gelatin Interactions in Aqueous Solutions

Interaction between DNA (effective hydrodynamic radius, R_DNA ≈ 140 nm) and Gelatin A (GA) (effective hydrodynamic radius, R_GA ≈ 55 nm) with charge ratio (DNA:GA = 16:1) and persistence length ratio (5:1) was studied by using fixed DNA concentration (5 × 10^–3 % (w/v)) and varying GA concentration (C_GA = 0–0.25% (w/v)). Experimentally, three interesting regions of interaction were observed from dynamic light scattering, turbidity, zeta potential, and viscosity data: (i) C_GA < 0.05% (w/v), GA binds to DNA forming soluble complexes of size R_complex ≈ 60 nm < R_DNA (primary binding causing condensation); (ii) 0.05% < C_GA < 0.1% (w/v), R_complex ≈ 60 to 180 nm was observed up to charge-neutralization point (secondary binding); and (iii) C_GA > 0.1% (w/v) showed interesting overcharging behavior of DNA–GA complexes, followed by liquid–liquid phase separation (complex coacervation). Aforesaid regions of interaction were further examined theoretically by modeling the problem using electrostatic and van der Waals interaction potentials treating GA molecules as counterions to DNA macroion. Region (i) was explained on the basis of electrostatic screening, followed by reduction in persistence length, which resulted in condensation of DNA–GA complex. In region (ii), the dominance of van der Waals forces led to the formation of large soluble complexes through selective binding. This was possible due to closer proximity between GA and DNA–GA complexes and the absence of strong electrostatic forces. In region (iii), these oversized and overcharged complexes coarsened, leading to complex coacervation. Here the interaction energy profile showed that a greater number of counterions was required over and above the usual charge neutralization requirement for low-energy configurations to be achieved.