Mass Exchange and Equilibration Processes in AOT Reverse Micelles.

ABSTRACT

Reverse micelles (RMs) made with sodium bis(2-ethylhexyl)sulfosuccinate suspended in isooctane are commonly used experimental models of aqueous microenvironments. However, there are important unanswered questions about the very characteristic that makes them of interest, namely their size. To explore the factors that determine the size of RMs, all-atom molecular dynamics simulations of RMs with different sizes but the same water-loading ratio were performed. An Anton 2 machine was used so that systems of the necessary size could be extended into the microsecond timescale, and mass exchange processes could be observed. Contrary to hypothesis, there were no net gains or losses of water by diffusion between RMs of different size. However, gains and losses did occur following fusion events. RM fusion followed RM contact only when waters were present among the hydrophobic surfactant chains at the point of contact. The presence of an encapsulated 40-residue amyloid beta peptide did not directly promote RM fusion, but it quickly and efficiently terminated each fusion event. Before fusion terminated, however, the size of the peptide-containing RM increased without a corresponding change in its water-loading ratio. We conclude that the mass transfer between RMs is most likely accomplished through transient fusion events, rather than through the diffusion of component molecules through the organic phase. The behavior of the amyloid beta peptide in this system underscores its propensity to embed in, and fold in response to, multiple interactions with the surfactant layer.