Nanomechanics of Anion-π Interaction in Aqueous Solution

Noncovalent interactions play a constitutive role in numerous biological processes, including biomolecular adhesion, recognition, and transport. Unlike π-π and cation-π interactions, anion-π interaction was only recently and computationally recognized to participate in biomolecular science. Here, we present the first experimental identification and direct quantification of the nanomechanics of anion-π interaction in aqueous solution by using a surface forces apparatus with complementary computational simulations. Anionic phosphate ester and π-conjugated catecholic moieties that abound in marine bioadhesives were used as the model system. Robust adhesion was observed and attributed to anion-π interaction, which was found to be enabled by the cooperative cation-π interaction due to the coexistence of a cation. The anion-π interaction strength follows the trend phosphate ester > HPO₄^2- > SO₄^2- > NO₃^-, affected by charge density, polarity, and hydration effect.