Structural and photophysical properties of peptide micro/nanotubes functionalized with hypericin

Hypericin is a photosensitizer with promising applications in photodynamic therapy (PDT) for cancer and infectious diseases treatments. Herein, we present a basic research study of L-diphenylalanine micro/nanotubes (FF-NTs) functionalized with hypericin. The system has special properties according to the hypericin concentration, with direct consequences on both morphological and photophysical behaviors. A clear dependence between the size of the tubes and the concentration of hypericin is revealed. The generation of reactive oxygen species (ROS) is found to be improved by ∼57% in the presence of FF-NTs, as indirectly measured from the absorbance profile of 1,3-diphenylisobenzofuran (DPBF). In addition, when hypericin appears conjugated with FF-NTs, the characteristic fluorescence lifetime is significantly boosted, demonstrating the role of FF-NTs to enhance the photophysical properties and stabilizing the fluorophore in excited states. Electron paramagnetic resonance allows the proposition of a mechanism for the generation of ROS. Molecular dynamics simulations bring new insights into the interaction between hypericin and peptide assemblies, suggesting the spatial organization of the fluorophore onto the surface of the supramolecular structures as a key element to improve the photophysical properties reported here.