Polycyclic amino acids yield fluorescent hydrogel

Hydrogels are attractive biomaterials for the controlled release of drugs, owing to their unique properties. Under pressure, such a gel becomes fluid enough to be injected subcutaneously, but almost immediately afterwards it reforms into a gel. This gel then erodes slowly, allowing the stored biologically active components to be released gradually. Researchers at the Vrije Universiteit Brussel (VUB), including the group of chemists Steven Ballet, Charlotte Martin and Ulrich Hennecke, are working on hydrogels composed of natural peptides that are consequently easy to break down in the body.

In developing these peptide hydrogels, the VUB researchers are working on the synthesis of modified amino acids that bring particular properties with them. They then incorporate these amino acids into a peptide sequence known to assemble into a hydrogel. Now, aided by a degree of serendipity, they have developed a new amino acid based on 1,8-naphthalimide that forms a fluorescent peptide hydrogel, with enhanced emission upon aggregation. Steven Ballet, professor of organic chemistry, explains: ‘Our peptide sequence previously consisted, amongst other things, of phenylalanines that self-assemble into beta-sheet structures. But when we changed the side chains of the amino acids, we discovered that the gels became fluorescent.’

By chance

The peptide sequence the researchers work with consists of amino acids with hydrophobic benzene rings on the same side of the chain. These aromatic side chains associate with one another, which drives the formation of peptide fibres and, consequently, a stable hydrogel. Originally, Ballet and colleagues were looking for ways to increase stability further and produce even stronger fibres. ‘We wanted to replace the aromatics with larger hydrophobic polycyclic systems to create even greater stability,’ says Ballet. ‘Somewhat by chance, we discovered that some of our substitutions caused the chains to become fluorescent.’

That chance discovery proved most opportune for the researchers. Their primary goal is to develop controlled release systems with tuneable properties. With the added property of fluorescence, the gel is visible after subcutaneous injection. ‘That is useful for imaging purposes, should we wish to investigate the localisation of the gels after injection,’ says Ballet. ‘We can now monitor in vivo for ten to twenty days how long the gel remains at the injection site and how quickly it erodes. That gives us the insights we need to develop better systems.’

Enhanced emission

The researchers synthesised their amino acids using, amongst other methods, palladium-catalysed coupling reactions. By subsequently embedding these amino acids within the peptide sequence, the fluorescence became truly pronounced. The formation of beta-sheet structures determined the orientation of the luminescent side chains and caused the emission to increase upon aggregation.

‘Within this study, the preparation of the amino acids constituted the bulk of the work,’ says Ballet. ‘There was uncertainty as to which material properties would emerge. But that is simultaneously the beauty of organic chemistry: that you can keep adapting molecules towards what you want. Once you have a lead, you can progressively rebuild the structures to refine the properties of the peptide chain.’

Antibodies

On the path towards controlled release systems, the researchers hope to gain even greater control over their biomaterials. Their current peptide gels release the active component within the first three to seven days after injection, making them less long-lasting than gels based on synthetic polymers. Ballet: ‘But thanks to the tunability of our peptides, we can develop new systems relatively quickly by modifying the structures. In this way, we aim to arrive at stronger materials with slower degradation in the body.’

In addition, the researchers wish to study further the compatibility between the active components and the material properties of the hydrogel. In the past, they have already demonstrated how their hydrogel is suitable for releasing small molecules, peptides and nanobodies (smaller variants of antibodies). ‘But that does not yet cover the entire spectrum of possible medicines,’ says Ballet. ‘We are now investigating, for instance, whether our system can also release large antibodies. That is what makes our system so appealing: rather than synthetic polymers with limited adaptability, our gel is versatile and can be rebuilt into a more “friendly” environment for therapeutic proteins.’

Jolien Bertouille, Adelaide R. Mashweu, et al., Fluorescent β-sheet-based peptide hydrogels with aggregation-enhanced emission properties, EurJOC (2025), doi:10.1002/ejoc.202500730