Naturally occurring and designed materials which actuate in response to changes in relative humidity are receiving increased attention. Recently, it was shown that aromatic tripeptides could be designed into materials for tunable water-response governed by a unique architecture involving a dual network involving integrated ordered (H-bonding) and reconfigurable (aromatic) domains. However, a complete understanding of the rules governing the water response of peptide-based materials is still lacking. Phenylalanine (F) and diphenylalanine (FF) are widely recognized as self-assembling molecules with similar crystal structures characterized by an aromatic interlocking network supported by electrostatic backbone interactions. Given that the structure of F is less ordered than that of FF, we surmised that F would show greater water-responsive behavior than FF, as a degree of disorder is required for water molecules to travel in and out of the crystal structure in response to environmental humidity changes. Bilayer experiments, involving incorporation of a layer of peptide crystals on top of plastic films, and subsequent exposure to variable humidity conditions, followed by microscopy analysis demonstrates that F is strongly water responsive, while FF shows almost no response. We used atomic force microscopy to study the nanoscale volume changes associated with water-responsive behavior of F and FF crystals at relative humidities between 5% and 95%. The structural changes associated with the humidity change were further analyzed using powder X-ray diffraction and Fourier-transform infrared spectroscopy. Molecular dynamics simulations were used to observe the effects of humidity change on a single water channel. The work provides further insight towards the structural elements and the specific roles of supramolecular disorder that are integral to water-response in peptide-based materials.

Investigating the role of structural disorder in water-responsive, peptide-based supramolcular crystals

ACS Spring 2022

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