Peptide stereochemistry effects from pKa-shift to gold nanoparticle templating in a supramolecular hydrogel.

The divergent supramolecular behavior of a series of tripeptide stereoisomers was elucidated through spectroscopic, microscopic, crystallographic and computational techniques. Only two epimers were able to effectively self-organize into amphipathic structures, leading to supramolecular hydrogels or crystals, respectively. Despite the similarity between the two peptides’ turn conformations, stereoconfiguration led to different abilities to engage in intramolecular hydrogen bonding. Self-assembly further shifted the pKa of the C-terminal side chain. As a result, across the pH range 4-6, only one epimer predominated sufficiently as a zwitterion to reach the critical molar fraction allowing gelation. By contrast, the differing pKa values and higher dipole moment of the other epimer favored crystallization. The four stereoisomers were further tested for gold nanoparticle (Au NP) formation, with the supramolecular hydrogel being key to control and stabilize Au NPs, yielding a nanocomposite that catalyzed the photodegradation of a dye. Importantly, the Au NP formation occurred without the use of reductants other than the peptide, and the redox chemistry was investigated by LC-MS and NMR. This study provides important insights for the rational design of simple peptides as minimalistic and green building blocks for functional nanocomposites.