Principles of Cation-πInteractions for Engineering Mussel-InspiredFunctional Materials

CONSPECTUS:Supramolecular assembly is commonly driven bynoncovalent interactions (e.g., hydrogen bonding, electrostatic,hydrophobic, and aromatic interactions) and plays a predominantrole in multidisciplinary research areas ranging from materialsdesign to molecular biology. Understanding these noncovalentinteractions at the molecular level is important for studying anddesigning supramolecular assemblies in chemical and biologicalsystems. Cation-pi interactions, initially found through theirinfluence on protein structure, are generally formed betweenelectron-rich pi systems and cations (mainly alkali, alkaline-earthmetals, and ammonium). Cation-pi interactions play an essentialrole in many biological systems and processes, such as potassiumchannels, nicotinic acetylcholine receptors, biomolecular recog-nition and assembly, and the stabilization and function of biomacromolecular structures. Early fundamental studies on cation-pi interactions primarily focused on computational calculations, protein crystal structures, and gas- and solid-phase experiments. Withthe more recent development of spectroscopic and nanomechanical techniques, cation-pi interactions can be characterized directlyin aqueous media, offering opportunities for the rational manipulation and incorporation of cation-pi interactions into the design ofsupramolecular assemblies. In 2012, we reported the essential role of cation-pi interactions in the strong underwater adhesion ofAsian green mussel foot proteins deficient inL-3,4-dihydroxyphenylalanine (DOPA) via direct molecular force measurements. Inanother study in 2013, we reported the experimental quantification and nanomechanics of cation-pi interactions of various cationsand pi electron systems in aqueous solutions using a surface forces apparatus (SFA). Over the past decade, much progress has been achieved in probing cation-pi interactions in aqueous solutions, their impact on theunderwater adhesion and cohesion of different soft materials, and the fabrication of functional materials driven by cation-pi interactions, including surface coatings, complex coacervates, and hydrogels. These studies have demonstrated cation-pi interactionsas an important driving force for engineering functional materials. Nevertheless, compared to other noncovalent interactions,cation-pi interactions are relatively less investigated and underappreciated in governing the structure and function of supramolecularassemblies. Therefore, it is imperative to provide a detailed overview of recent advances in understanding of cation-pi interactionsfor supramolecular assembly, and how these interactions can be used to direct supramolecular assembly for various applications (e.g.,underwater adhesion). In this Account, we present very recent advances in probing and applying cation-pi interactions for mussel-inspired supramolecular assemblies as well as their structural and functional characteristics. Particular attention is paid toexperimental characterization techniques for quantifying cation-pi interactions in aqueous solutions. Moreover, the parametersresponsible for modulating the strengths of cation-pi interactions are discussed. This Account provides useful insights into thedesign and engineering of smart materials based on cation-pi interactions.