Jonathan P. Wojciechowski

A Capped Dipeptide Which Simultaneously Exhibits Gelation and Crystallization Behavior

Short peptides capped at their N-terminus are often highly efficient gelators, yet notoriously difficult to crystallize. This is due to strong unidirectional interactions within fibers, resulting in structure propagation only along one direction. Here, we synthesize the N-capped dipeptide, benzimidazole-diphenylalanine, which forms both hydrogels and single crystals. Even more remarkably, we show using atomic force microscopy the coexistence of these two distinct phases. We then use powder X-ray diffraction to investigate whether the single crystal structure can be extrapolated to the molecular arrangement within the hydrogel. The results suggest parallel β-sheet arrangement as the dominant structural motif, challenging existing models for gelation of short peptides, and providing new directions for the future rational design of short peptide gelators.

Effect of heterocyclic capping groups on the self-assembly of a dipeptide hydrogel

The mechanism and design rules associated with the self-assembly of short peptides into hydrogels is currently not well understood. In this work, four diphenylalanine-based peptides have been synthesised, bearing heterocyclic capping groups which have different degrees of hydrogen bonding potential and nitrogen substitution. For these four peptides, zeta potential and electrical impedance spectroscopy measurements were undertaken to monitor gelation, with the impedance data showing different gelation times for each peptide hydrogel. Through a combination of atomic force microscopy and rheological measurmeents, including dynamic strain and frequency sweeps, and thixotropic tests, the relationship between the mechanism of self-assembly in these hydrogels and their macroscopic behaviour can be established. It is observed that the degree of nitrogen substitution affects the self-assembly mechanisms of the hydrogels and as such, that there is an interplay between branching and bundling self-assembly pathways that are responsible for the final properties of each hydrogel.

Controlling self-assembly of diphenylalanine peptides at high pH using heterocyclic capping groups

Using small angle neutron scattering (SANS), it is shown that the existence of pre-assembled structures at high pH for a capped diphenylalanine hydrogel is controlled by the selection of N-terminal heterocyclic capping group, namely indole or carbazole. At high pH, changing from a somewhat hydrophilic indole capping group to a more hydrophobic carbazole capping group results in a shift from a high proportion of monomers to self-assembled fibers or wormlike micelles. The presence of these different self-assembled structures at high pH is confirmed through NMR and circular dichroism spectroscopy, scanning probe microscopy and cryogenic transmission electron microscopy.

Halogen bonding influences perylene-core twists in non-core substituted perylene tetraesters

Halogen bonding has emerged as a popular synthon in supramolecular architectures. Its effects on supramolecular perylene systems however have been under investigated. We report X⋯O halogen bonding of perylene-3,4,9,10-tetracarboxylic tetra esters. This templates a twisted perylene core which affects its spectral properties. We show this interaction exists in both the solution and solid state through spectroscopy studies and single crystal analysis.

Chiral effects in peptide-substituted perylene imide nanofibres

A series of peptide-functionalised perylene imide molecules was studied to examine the effect of peptide chirality on the self-assembly of the perylene imides core into nanofibres. Peptide stereogenic positions, stereochemical configurations, amphiphilic substitution and perylene core modification were spectroscopically evaluated with respect to chiral assembly. For dipeptide molecules, stereocenters in peptide residue positions proximal to the perylene core (1–5 units) were found to impart helical chirality to the perylene core, while stereocenters in more distal residue positions did not exert a chiral influence. Diastereomers involving stereocenter inversions within the proximal residues consequently manifested spectroscopically as pseudo-enantiomers. Increased side-chain steric demand in the proximal positions gave similar chiral influence but exhibited diminished Cotton effect intensity with additional longer wavelength features attributed to interchain excimers. Replacing one of the two peptide substituents with an alkyl chain to create strongly amphiphilic perylene bisimides disrupted chiral self-assembly. On the other hand, amphiphilic structures achieved through the modification of the perylene imide core with a bisester moiety prompted strongly exciton-coupled, solvent-responsive self-assembly into long, chiral nanofilaments.

Design, Synthesis, and Evaluation of N- and C-Terminal Protein Bioconjugates as G Protein-Coupled Receptor Agonists

A G protein-coupled receptor (GPCR) agonist protein, thaumatin, was site-specifically conjugated at the N- or C-terminus with a fluorophore for visualization of GPCR:agonist interactions. The N-terminus was specifically conjugated using a synthetic 2-pyridinecarboxyaldehyde reagent. The interaction profiles observed for N- and C-terminal conjugates were varied; N-terminal conjugates interacted very weakly with the GPCR of interest, whereas C-terminal conjugates bound to the receptor. These chemical biology tools allow interactions of therapeutic proteins:GPCR to be monitored and visualized. The methodology used for site-specific bioconjugation represents an advance in application of 2-pyridinecarboxyaldehydes for N-terminal specific bioconjugations.