Fahmeed Sheehan1,2,3,Haozhen Wang4,Darjan Podbevsek2,Rein Ulijn2,1,3,Xi Chen2,1,5
The Graduate Center at the City University of New York1,The Advanced Science Research Center at the Graduate Center2,Hunter College3,Stanford University4,The City College of New York5
Fahmeed Sheehan1,2,3,Haozhen Wang4,Darjan Podbevsek2,Rein Ulijn2,1,3,Xi Chen2,1,5
The Graduate Center at the City University of New York1,The Advanced Science Research Center at the Graduate Center2,Hunter College3,Stanford University4,The City College of New York5
Water-responsive (WR) materials that reversibly change structure in response to changes in humidity are increasingly recognized for their potential in energy-harvesting and actuation applications. These materials translate the chemical potential of water into a defined mechanical actuation. Examples in nature include pinecones which expand and release seeds when the local environment is dry. Peptides are versatile designed supramolecular materials<sup>1</sup> and we recently started to explore them as WR responsive materials. Specifically, we previously developed WR peptide crystals with aqueous pores and deformable aromatic regions to understand the fundamental actuation mechanism of WR materials and found that a strong H-bonding network to bind with water was crucial to water-responsiveness<sup>2</sup>.<br/><br/>In here we systematically study the architecture and deformability of the aromatic regions and their influence on WR behavior. We selected three aromatic peptide crystals that each contain aromatic regions and aqueous pores, but distinct aromatic topologies: Phenylalanine (F), diphenylalanine (FF), and histidyl-tyrosyl-phenylalanine (HYF). Based on architecture alone, we expected FF, which is composed of a network of water pores connected through aromatic zippers, to be a strong WR material owing to its large aqueous pore and H-bonding network. Surprisingly, we found that the structural rigidity offered by its aromatic zipper structure prevented any shape change in response to changing humidity. On the other hand, F emerged as an extremely strong WR material with an energy density of 19.8 MJ/m<sup>3</sup>, exceeding that of HYF at 6.5 MJ/m<sup>3</sup>. Results from powder X-ray diffraction (PXRD) and Fourier-transform infrared spectroscopy (FTIR) confirm that F undergoes a crystal phase change between 60-70% relative humidity that results in its WR actuation. This contrasts with HYF which reversibly loses its lattice structure upon dehydration. Our observations suggest that FF, which is too rigid, cannot deform in response to humidity, whereas HYF, which is more flexible, does not respond efficiently. These findings show that water-responsiveness requires a fine balance between water-bonding and aromatic stacking domains, and they can inform the design of responsive aromatic peptide crystals and provide further insight into the WR mechanism of supramolecular materials.<br/><br/><sup>1 </sup> Sheehan, F.; Sementa, D.; Jain, A.; Kumar, M.; Tayarani-Najjaran, M.; Kroiss, D.; Ulijn, R. V. Peptide-Based Supramolecular Systems Chemistry. <i>Chem Rev</i> <b>2021</b>, <i>121</i> (22), 13869–13914. https://doi.org/ttps://doi.org/10.1021/acs.chemrev.1c00089.<br/><sup>2 </sup> Piotrowska, R.; Hesketh, T.; Wang, H.; G Martin, A. R.; Bowering, D.; Zhang, C.; Hu, C. T.; McPhee, S. A.; Wang, T.; Park, Y.; Singla, P.; McGlone, T.; Florence, A.; Tuttle, T.; Ulijn, R. V; Chen, X. Mechanistic Insights of Evaporation-Induced Actuation in Supramolecular Crystals. <i>Nat Mater</i> <b>2021</b>, <i>20</i>, 403–409. https://doi.org/10.1038/s41563-020-0799-0.