Impact of NDI‐Core Substitution on the pH‐Responsive Nature of Peptide‐Tethered Luminescent Supramolecular Polymers

Abstract

The pH‐responsive nature of two self‐assembled NDI‐peptide amphiphile conjugates is reported. The diethoxy substituted NDI showed a pH‐dependent assembly behaviour, as expected. In contrast, the isopropylamino‐ and ethoxy‐substituted NDI based supramolecular polymer was stable at acidic and basic aqueous conditions. This finding highlights how subtle changes in the molecular design of π‐stacked chromophore‐peptide conjugates have a drastic impact on their equilibrium structure and ultimately functional properties.

Hierarchical self-assembly of aromatic peptide conjugates into supramolecular polymers: it takes two to tango

“…The nature of the aromatic core may also modulate the pH-responsiveness as recently reported by the groups of Besenius and George using different core-substituted NDI derivatives. 168 Thirdly, the ability to self-assemble small molecules into chiral supramolecular polymers opens up applications in different areas. For instance, exploring asymmetric catalysis, the group of Bouteiller and Raynal used chiral BTA ligands for rhodiumcatalyzed asymmetric hydrogenation, 178 and hydrosilylation reactions.…”

“… 164,165 Other examples of NDI–peptide conjugates have been reported for supramolecular polymerization of 1D helical nanostructures in aqueous media 166 and nanotubes, 167 some assemblies even displaying pH-responsiveness due to the presence of ionizable amino acids. 168 …”

Core-Substituted Naphthalene-Diimides (cNDI) and Related Derivatives: Versatile Scaffold for Supramolecular Assembly and Functional Materials

“…This modulated the overall pH-responsive behaviour of the supramolecular assemblies of 12 and 13. cNDI derivative 12 showed pH-responsive supramolecular polymerization, whereas the larger isopropyl moiety and slightly more electron-rich nature of the cNDI scaffold in 13 resulted in stable supramolecular polymers even in acidic medium. 14 Our group has reported a diamine-substituted D-A-D type cNDI gelator (14, Figure 6) that showed supramolecular polymerization by H-bonding among the imides and J-type π-stacking among the cNDI units, yielding micrometre-long nanotubes. 15 Analysis of the cooling curves by UV-Vis spectroscopy revealed a highly co-operative nature of the supramolecular polymerization with the degree of polymerization in the range of 20-25.…”

“…13 George and co-workers demonstrated supramolecular polymerizations of two β-sheet forming peptide-tethered amphiphilic cNDI derivatives (12 and 13) and their pHresponsive behaviour (Figure 5). 14 One of the cNDI building blocks (12) was symmetrically functionalized with the diethoxy groups in the aromatic core, while the other one ( 13) was unsymmetrically substituted with isopropylamine and ethoxy groups. The difference in the nature of these hydrophobic substitutions in the cNDI moiety influenced the overall balance between the π-stacking, H-bonding, attractive hydrophobic interactions and repulsive sidechain interactions, operating in the dendritic peptide segments.…”

pH-Responsive Self-Assembly of Designer Aromatic Peptide Amphiphiles and Enzymatic Post-Modification of Assembled Structures

“…Since supramolecular organizations are dependent on relatively weak noncovalent interactions, they can be susceptible to external stimuli, such as light, temperature, and pH. PAs often contain pH-responsive side chains or an N- or C-terminus; therefore, pH is one of the most common stimuli used to control the assembly [ 11 , 12 ]. Indeed, many studies have shown pH-induced changes in morphology [ 13 ] and mechanical properties [ 14 ], which resulted in functional materials, such as drug-releasing capsules [ 15 , 16 ] and injectable cell delivery vehicles [ 14 ].…”