Noncovalent interactions in catechol/ammonium-rich adhesive motifs: Reassessing the role of cation-pi complexes?

Cation-pi interactions between ammonium and catechol groups provide a commonly held paradigm in the complex interplay of noncovalent forces that govern both biological phenomena, such as underwater adhesion in mussel byssus and other marine organisms, and aggregation processes in self-assembly of catecholamine-derived polymers. However, closer consideration of the peculiar properties of the catechol functionality, that features two adjacent phenoxyl groups endowed with chelating properties, would suggest operation of alternate interaction geometries besides typical cation-pi. Following previous studies on various alkali metal cations, we report herein compelling computational evidence in favor of a so far overlooked ?-type interaction between the ammonium cation and the catechol system, as the most important contributor to binding, far exceeding the cation-pi component. The present findings would hence prompt an experimental and theoretical reassessment of the actual importance of cation-pi against sigma-type interaction between ammonium and catechol groups in underwater adhesive patterns. In this framework, the catechol-ammonium interaction energy potential surfaces herein reported may provide an improved reference to approach more realistic natural and synthetic adhesion models encompassing other participating molecules/ions and the role of the solvent.