Water-responsive materials undergo reversible shape changes upon varying humidity levels. These mechanically robust, yet flexible structures can exert significant forces and hold promise as efficient actuators for energy harvesting, adaptive materials, and soft robotics. Here we demonstrate that energy transfer during evaporation-induced actuation of nanoporous tripeptide crystals results from strengthening of water H-bonding that drives the contraction of the pores. The seamless integration of mobile and structurally bound water inside these pores with a supramolecular network which contains readily deformable aromatic domains, translates dehydration-induced mechanical stresses through the crystal lattice, suggesting a general mechanism of efficient water-responsive actuation.The observed strengthening of water bonding complements accepted understanding of capillary force induced reversible contraction for this class of materials. These minimalistic peptide crystals are much simpler in composition compared to natural water-responsive materials, and the insights provided here can be applied more generally for the design of high-energy molecular actuators.

Melanin pigments have various properties that are of technological interest including photo‐ and radiation protection, rich coloration, and electronic functions. Nevertheless, laboratory‐based synthesis of melanin and melanin‐like materials with morphologies and chemical structures that are specifically optimized for these applications, is currently not possible. Here, melanin‐like materials that are produced by enzymatic oxidation of a supramolecular tripeptide structures that are rich in tyrosine and have a 1D morphology are demonstrated, that are retained during the oxidation process while conducting tracks form through oxidative tyrosine crosslinking. Specifically, a minimalistic self‐assembling peptide, Lys–Tyr–Tyr (KYY) with strong propensity to form supramolecular fibers, is utilized. Analysis by Raman spectroscopy shows that the tyrosines are pre‐organized inside these fibers and, upon enzymatic oxidation, result in connected catechols. These form 1D conducting tracks along the length of the fiber, which gives rise to a level of internal disorder, but retention of the fiber morphology. This results in highly conductive structures demonstrated to be dominated by proton conduction. This work demonstrates the ability to control oxidation but retain a well‐defined fibrous morphology that does not have a known equivalent in biology, and demonstrate exceptional conductivity that is enhanced by enzymatic oxidation.

In the version of this Article originally published, Figs. 1 and 2 and Extended Data Fig. 3 contained some errors. In Fig. 1c,d,e the units of curvature in the y-axes were incorrect as cm-1 ; they should have been mm-1. In addition, in Fig. 1d the right y-axis tick marks should have been labelled 1.

Hesketh, T., (2020). pygen-structures: A Python package to generate 3D molecular structures for simulations using the CHARMM forcefield.

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