Heptamethine cyanine dyes working as light harvesting antennas in biohybrid photosynthetic assemblies

Over billions of years, extremely efficient molecular machines have evolved to perform very specific tasks. In recent years, the idea of extending the physiological activity of biological macromolecules, through modifications designed ad hoc to carry out specific purposes, began to emerge within the scientific community. We wish to functionalize proteins and enzymes to extend their chemical and physical properties for applicative purposes. It intends to synthesize organic-biological hybrid system through bioconjugation techniques, able to form selective and covalent bonds between the biological part and its organic counterpart. These hybrid systems are potentially interesting as active materials in next-generation devices in different technological areas using as biosensors, photocatalysts and photovoltaic materials. The natural candidates capable of performing these tasks are the photosynthetic systems, in particular the photosynthetic reaction center (RC) from the purple bacterium Rhodobacter sphaeroides strain R26, whose unique photoconversion capability generates a stable charge separated state with an efficiency close to unity. On the other hand, to employ efficiently RC in hybrid devices, the light harvesting ability of the isolated protein must be refurbished to increase its absorption cross-section. About that, an organic fluorophore HCy-1 (Figure 2) belonging to the class of heptamethine cyanines[3], suitably covalently bioconjugated to the residues of the RC lysine, has recently been employed as a molecular antenna in order to improve the properties of absorption and photoconversion of the RC.