Molecular simulations of micellar aggregation of polysorbate 20 ester fractions and their interaction with N-phenyl-1-naphthylamine dye.

Micellar aggregation behavior of polysorbate 20 (PS20) has generated significant interest because of the wide use of PS20 as a surfactant to minimize protein surface adsorption and mitigate protein aggregation. Thus, there is a need for better molecular understanding of what drives the biophysical behavior of PS20 in solution. We observe that a complex amphipathic PS20 molecule, which contains both hydrophobic tail and relatively large hydrophilic head, self-associates strongly within the course of a molecular dynamics simulation performed with a fully atomistic representation of the molecule and an explicit water solvent model. The in silico behavior is consistent with micellar models of PS20 in solution. The dynamics of this self-association is rather complex involving both internal reorganization of the molecule and diffusion to form stable micelle-like aggregates. The micellar aggregates of PS20 are long-lived and are formed by the balance between the large hydrophobic interactions associated with the aliphatic tail of PS20, and the steric repulsion of the hydrophilic sorbitan head structure. In the present work, molecular models of PS20 that represent naturally occurring PS20 fractions were produced and characterized in silico. The study investigated the monoester and diester fractions: PS20M, and PS20D. These fractions present differences in the strength of their hydrophobic effect, which influences the aggregation behavior. Adaptive biasing force (ABF) simulations were carried out with the PS20M and PS20D molecular constructs to calculate the free energy of their pairwise interaction. The free energy barrier for the dissociation is higher for PS20D compared with PS20M. The results show that hydrogen bonds can form when head groups are in close proximity, such as in the PS20 aggregate assembly, and the free energy of interaction can be used to predict the morphology of the micellar aggregate for the different PS20 fractions. We were also able to simulate PS20 in the presence of N-phenyl-1-naphthylamine (NPN) to study the solution behavior of the hydrophobic molecule and of the mechanism in which it is sequestered in the hydrophobic core of the PS20 micellar aggregate.