A self-assembled nanostructured material with photosensitising properties

Abstract

Using a combination of molecular modelling and spectroscopic experiments, the naturally-occurring pharmacologically active hypericin compound is shown to form a stable 1 : 1 complex with apomyoglobin that preserves its fluorescence and singlet oxygen photosensitising properties. The binding equilibrium constant has been determined as K_a = (2.4 ± 0.5) × 10⁵ M⁻¹, equivalent to a dissociation constant K_d = 4.2 ± 0.8 μM. The kinetic details of singlet oxygen production have been characterised and indicate that the protein scaffold protects hypericin from oxygen. The complex is active against Staphylococcus aureus bacteria and shows lower dark toxicity than free hypericin. In view of its superb biocompatibility, apomyoglobin should be considered as a nanovehicle for hypericin in theranostic applications.