Hyper Thermostability and Liquid-Crystal-Like Properties of Designed α-Helical Peptide Nanofibers

Effects of perturbation of the hydrophobic coiled-coil core on the thermal transition process of α-helical self-assembling peptides with α-β conformational transition capability

We designed a 29-residue peptide (CCP1) with helical nanofiber-forming ability, in which the interface of the coiled-coil motif consists only of hydrophobic residues, and peptides with histidine residues substituted in the hydrophobic core (CCP2 and CCP3), and analyzed the effects of perturbations caused by the substitutions on the intermolecular association and conformational transitions. Based on the results of atomic force microscopy and circular dichroism measurements, it was found that CCP1 and CCP2 form α-helical fibers under pH 4, while CCP3 adopts the α-helix structure but lacks the association ability. Furthermore, the heating processes of CCP1 and CCP2 were followed by using spectroscopic, thermal, and morphological techniques, and it was observed that CCP1 undergoes an irreversible structural transition from α-helical to β-sheet fibers with a high degree of cooperativity, while a more gradual or non-cooperative structural transition was observed in CCP2. These results indicate that the introduction of histidine residues in the hydrophobic core significantly affects the intermolecular interactions and the rate of structural transition, providing a new design principle for the development of functional nanomaterials with biocompatibility.

Structural Analyses of Designed α-Helix and β-Sheet Peptide Nanofibers Using Solid-State Nuclear Magnetic Resonance and Cryo-Electron Microscopy and Introduction of Structure-Based Metal-Responsive Properties

The 21-residue peptide α3, which is artificially designed and consists of three repeats of 7 residues, is known to rapidly assemble into the α-helix nanofiber. However, its molecular structure within the fiber has not yet been fully elucidated. Thus, we conducted a thorough investigation of the fiber's molecular structure using solid-state NMR and other techniques. The molecules were found to be primarily composed of the α-helix structure, with some regions near the C- and N-terminal adopting a 310-helix structure. Furthermore, it was discovered that β-sheet hydrogen bonds were formed between the molecules at both ends. These intermolecular interactions caused the molecules to assemble parallelly in the same direction, forming helical fibers. In contrast, we designed two molecules, CaRP2 and βKE, that can form β-sheet intermolecular hydrogen bonds using the entire molecule instead of just the ends. Cryo-EM and other measurements confirmed that the nanofibers formed in a cross β structure, albeit at a slow rate, with the formation times ranging from 1 to 42 days. To create peptide nanofibers that instantaneously respond to changes in the external environment, we designed several molecules (HDM1-3) based on α3 by introducing metal-binding sites. One of these molecules was found to be highly responsive to the addition of metal ions, inducing α-helix formation and simultaneously assembling into nanofibers. The nanofibers lost their structure upon removal of the metal ion. The change occurred promptly and was reversible, demonstrating that the intended level of responsiveness was attained.