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.

Hybrid associates formed between peptide assemblies and fluorophores are attractive mainly because of their unique properties for biomedical applications. Recently, we demonstrated that the production of reactive oxygen species (ROS) by hypericin and their stability in excited states are enhanced upon conjugation with l,l-diphenylalanine microtubes (FF-MNTs). Although the detailed mechanisms responsible for improving the photophysical properties of ROS remain unclear, tentative hypotheses have suggested that the driving force is the growth of overall dipolar moments ascribed either to coupling between aligned H2O dipoles within the ordered structures or to the organization of hypericin molecules on peptide interfaces. To provide new insights on ROS activity in hypericin/FF-MNTs hybrids and further explore the role of water in this respect, we present results obtained from investigations on the behavior of these complexes organized into different crystalline arrangements. Specifically, we monitored and compared the photophysical performance of hypericin bound to FF-MNTs with peptides organized in both hexagonal (water-rich) and orthorhombic (water-free) symmetries. From a theoretical perspective, we present the results of new molecular dynamics simulations that highlight the distinct hypericin/peptide interaction at the interface of FF-MNTs for the different symmetries. As a conclusion, we propose that although water enhances photophysical properties, the organization induced by peptide structures and the availability of a hydrophobic environment surrounding the hypericin/peptide interface are paramount to optimizing ROS generation. The findings presented here provide useful basic research insights for designing peptide/fluorophore complexes with outstanding technological potential.

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We present the synthesis and characterization of new peptide conjugates obtained by hierarchical co-assembly of L,L-diphenylalanine (FF) and zinc phthalocyanine complexes (ZnPc) in water. Self-assembly capabilities under defined conditions were investigated by scanning electron microscopy, and photophysical properties were evaluated using UV-Vis and fluorescence spectroscopy. AFM observations demonstrated that these ZnPcs form different highly ordered arrays on the crystalline faces of the FF microplates and that surface roughness significantly changes with the presence of differently substituted phthalocyanine units. XRD assays showed that the overall molecular packing of the conjugates is organized according to a hexagonal symmetry, with ZnPcs hosted in the interstices of the peptide phase. In vitro photodynamic studies were conducted on human breast cancer MCF-7 cells to investigate both cellular uptake and cytotoxicity. It was shown that FF self-assemblies are not toxicity and enhance accumulation of ZnPc in MCF-7 cells, improving apoptotic cell death upon irradiation. Our findings demonstrate enhancement of ZnPc antitumor efficiency by FF conjugates and a proof-of-concept for new photosensitizer carriers based on peptide conjugates.