Peptide Stereochemistry Effects from pK_a-Shift to Gold Nanoparticle Templating in a Supramolecular Hydrogel

Abstract

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 pK a value 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 pK a values and higher dipole moment of the other epimer favored crystallization. The four stereoisomers were further tested for gold nanoparticle (AuNP) formation, with the supramolecular hydrogel being the key to control and stabilize AuNPs, yielding a nanocomposite that catalyzed the photodegradation of a dye. Importantly, the AuNP formation occurred without the use of reductants other than the peptide, and the redox chemistry was investigated by LC−MS, NMR, and infrared scattering-type near field optical microscopy (IR s-SNOM). This study provides important insights for the rational design of simple peptides as minimalistic and green building blocks for functional nanocomposites.

Metal Ions Trigger the Gelation of Cysteine‐Containing Peptide‐Appended Coordination Cages

“…[29] However, there is a recent example of a gelling tripeptide featuring the Phe-Phe motif and a similar CD spectrum upon assembly. [30] It is worth noting that the CD spectrum of peptide C was characterised by a distinctive Cotton effect, with a positive maximum at 254 nm and a negative minimum at 228 nm, indicating a positive exciton coupling, which manifests when two interacting units display the long axes so as to constitute a clockwise screw sense. [31] The effects of metal binding on the peptide conformation were studied by UV/Vis and CD on cage 2, yet no changes were observed due to the predominant strong signals of the aromatic panels of the cage.…”