Effect of molecular structure on the properties of naphthalene-dipeptide hydrogelators

Dipeptide-conjugates can be efficient low molecular weight hydrogelators. However, the effective design of a gelator for a specific application is compromised by the lack of a clear understanding of the design rules that govern assembly and hence gelation. Here, we report a library of naphthalene-dipeptides, their physical chemical properties, and gelation ability. We have varied both the amino acids in the dipeptides and the substitution on the naphthalene ring to allow variation of the structure throughout the molecule. We have examined the effects of these permutations on the critical micelle concentration and air-water partition coefficient at high pH and the apparent pK(a). We show that there is a clear link between these properties and the predicted hydrophobicity of the overall conjugates, rather than the properties varying with, for example, the dipeptide sequence. The majority of these dipeptide-conjugates are effective hydrogelators, although there is no apparent link between the solution properties and whether or not a conjugate is a hydrogelator. Nevertheless, where gelation occurs, the link between hydrophobicity and apparent pK(a) allows the prediction of the pH at which a gel will be formed and hence informed choice of gelator for specific applications.