Spontaneous self-assembly of aromatic cyclic dipeptide into fibre bundles with high thermal stability and propensity for gelation

Cyclic dipeptides (CDPs) are the simplest members of cyclic peptides exhibiting exceptional rigidity and stability. A detailed study on the spontaneous self-assembly of aromatic CDP (Phe-Phe) into fibre bundles with 1-2 mu m thick and several millimetres long is described. These fibre bundles exhibit the structural hierarchy that is found in natural fibres such as spider silk and collagen. The solubility studies in various solvents provided more insights into the existence and nature of fibre bundles. Fibre bundles transform to gel in chloroform at critical concentration of added trifluoroacetic acid. Thermogravimetric analysis data indicated high thermal stability with multiple transitions attributed to the existence of structural hierarchy in fibre bundles. NMR studies revealed that aromatic pi-pi interactions along with intermolecular (N-H center dot center dot center dot O) hydrogen-bonded molecular chains are mainly responsible for the formation as well as observed high stability of fibre bundles. This detailed study of structural hierarchy, solubility, gelation and thermal stability demonstrates the robustness of aromatic CDP (Phe-Phe) to form fibre bundles.