Multiple length-scale control of Boc-protected diphenylalanine aggregates through solvent composition

Abstract

Diphenylalanine and its variants are among the most studied building blocks in materials science due to their ability to form extended architectures with a wide range of morphologies. Given this propensity to self-assemble into very different shapes, controlling the aggregation process of the basic units is very challenging. Using a selection of techniques, this work investigates the critical role of solvation in influencing the association of Boc-protected diphenylalanine. The results indicate that the balance of solvents, specifically acetonitrile-water, significantly impacts the self-assembly process. Indeed, a change in acetonitrile content from 2% to 10% in water solutions drives the formation of different morphologies of the aggregates, spanning from spheres to plates. In a pure organic solvent a single orthorhombic crystalline phase is observed, whereas the presence of water reveals the coexistence of two phases: orthorhombic and hexagonal. The fraction of hexagonal phase relies on the solvent composition, while the peptides always adopt turn conformations, promoting the plane-to-plane stacking of aromatic rings. The architectural growth is driven by aromatic stacking and hydrophobic interactions; the development of different morphologies is dependent on the extent of the aggregate–solvent interface. Interestingly, we find a morphology re-entrant behaviour induced by finely tuning the acetonitrile content, thereby linking aggregate morphology with molecular characteristics. The selection of suitable building blocks and solvation conditions is crucial in steering the nature of aggregates toward creating smart biomaterials with enhanced properties.