  Peptide surfactants (PEPS) are studied to capture and retain rare earth
elements (REEs) at air-water interfaces to enable REE separations. Peptide
sequences, designed to selectively bind REEs, depend crucially on the position
of ligands within their binding loop domain. These ligands form a coordination
sphere that wraps and retains the cation. We study variants of lanthanide
binding tags (LBTs) designed to complex strongly with Tb$^{3+}$. The peptide
LBT$^{5-}$ (with net charge -5) is known to bind Tb$^{3+}$ and adsorb with more
REE cations than peptide molecules, suggesting that undesired non-specific
Coulombic interactions occur. Rheological characterization of interfaces of
LBT$^{5-}$ and Tb$^{3+}$ solutions reveal the formation of an interfacial gel.
To probe whether this gelation reflects chelation among intact adsorbed
LBT$^{5-}$:Tb$^{3+}$ complexes or destruction of the binding loop, we study a
variant, LBT$^{3-}$, designed to form net neutral LBT$^{3-}$:Tb$^{3+}$
complexes. Solutions of LBT$^{3-}$ and Tb$^{3+}$ form purely viscous layers in
the presence of excess Tb$^{3+}$, indicating that each peptide binds a single
REE in an intact coordination sphere. We introduce the variant RR-LBT$^{3-}$
with net charge -3 and anionic ligands outside of the coordination sphere. We
find that such exposed ligands promote interfacial gelation. Thus, a nuanced
requirement for interfacial selectivity of PEPS is proposed: that anionic
ligands outside of the coordination sphere must be avoided to prevent the
non-selective recruitment of REE cations. This view is supported by simulation,
including interfacial molecular dynamics simulations, and interfacial
metadynamics simulations of the free energy landscape of the binding loop
conformational space.
