Structure and thermodynamics of polyelectrolyte complexes: Simulation and experiment
Ionic complexes of polyelectrolytes with molecules of opposite polarity are ubiquitous and important in both nature and synthetic arenas. DNA condensation by multivalent counterions enables long stiff DNA chains to be condensed for storage in small volumes such as nuclei and virus capsids. Applications of synthetic polyelectrolytes as complexation (or encapsulation) agents for proteins and nucleic acids have proliferated in recent decades in quest for more effective drug and gene delivery.
This dissertation investigated several aspects of the structure and thermodynamics of polyelectrolyte complexes, using both computer simulation and experimental characterization. We applied a Langevin dynamic simulation to the complexation of a semiflexible polyelectrolyte with multivalent counterions. The central issue is the interplay of polyelectrolyte intrinsic stiffness and counterion valency in shaping ordered structures such as toroid and folded-chain bundles as seen in DNA condensation studies. Also in accordance with experiments, our simulation has uncovered multiple kinetic pathways leading from disordered to ordered states. The simulation is extended to the complexation by polyelectrolytes of opposite polarities. The major issue is to differentiate enthalpic and entropic contributions to complexation in both weak and strong electrostatic coupling systems. Two regimes of complexation are delineated: (1) enthalpy-driven in weak polyelectrolytes where mutual Coulombic attraction between polycations and polyanions drives complexation; (2) entropy-driven in strong polyelectrolytes where although polycations and polyanions still attract each other strongly, a large entropy gain from releasing condensed counterions during complexation becomes dominant. We have also studied conformational properties of comb polyelectrolytes and their complexes. Static and dynamic light scattering studies reveal that polycyclooctene-g-pentalysine adopts an extended rodlike conformation due to strong electrostatic repulsion of the oligolysine side chains. It is demonstrated that rigid polycyclooctene-g-pentalysine could self-assemble with dsDNA to generate stable nanosized particles whose dimension can be finely adjusted by pH and polyelectrolyte/DNA mixing ratio. In conjunction with experiments, we also set forth to simulate electrostatic-mediated rigidity of comb polyelectrolytes. Interestingly, comb polyelectrolytes of greatest rigidity are those grafted with modestly charged side chains (like oligolysines) which could maximize inter-side chain repulsion without significant disruption from the counterions.