Dicarboxylic amino acid-based surfactants (AABSs) are unique molecules characterized by two carboxylate groups in their amino acid-based headgroup. In the past, we have investigated the aggregation behaviors of dicarboxylic AABSs featuring L-aspartate or L-glutamate headgroups, with varying tail structures, in the presence of sodium counterions. Experimentally, dicarboxylic AABSs with an 11-carbon hydrophobic tail featuring a terminal alkene have a critical micelle concentration (CMC) of ~80 mM. Similar surfactants with a 13-carbon saturated hydrophobic tail have CMCs of ~20 mM. Due to their mixture of the aforementioned structural features, it was anticipated that dicarboxylic AABSs with an 11-carbon saturated tail would have a CMC between 20 and 80 mM. However, we have not been able to dissolve these surfactants (undecanoyl-L-aspartate or undecanoyl-L-glutamate) at any concentration within this range. To understand this behavior, molecular dynamics (MD) simulations were employed. Specifically, AMBER MD was used to study the aggregation processes of 8 dicarboxylic AABSs, formed from the structural combination of L-aspartate or L-glutamate headgroups, 11- or 13- carbon tails, and saturated or terminal alkene tails. These simulations were performed on 100 surfactants at higher concentrations than their predicted CMCs to induce observable micellization. For each surfactant structure, the ff14SB force field was applied to the amino acid headgroup, while the gaff2 force field was assigned to the tail. Solvation effects were modeled by the Generalized Born (GB)-Neck2 implicit water model. Sodium counterions were modeled by the Joung-Cheatham parameters for monovalent ions. This approach enabled a closer examination of the energy components associated with specific regions (carboxylates, terminal tail bonds, and others) of each dicarboxylic AABS.

Xitlali Gallegos-Cruz

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