Spectroscopic Investigation on Orange Carotenoid Protein (OCP): From the Activation Mechanism to the Development of OCP-based Fluorescent Nanoparticles

Orange Carotenoid Protein (OCP) is a photoactive water-soluble protein involved in photoprotective functions. Among different strains of cyanobacteria, OPC is responsible of photoprotection processes through non-photochemical quenching (NPQ) of excess light energy absorbed by antenna systems and quenching of reactive oxygen species (ROS). The photoactivation mechanism is triggered by blue/green light absorption which induces the protein to undergo a conformational variation from a dark-adapted state (OCPO) to an active state (OCPR), the latter mastering photoprotective tasks. We have used optical FTIR spectroscopy in order to elucidate the process. As previous studies suggest that an important function is played by internal water molecules, FTIR light-minus-dark difference spectroscopy on OCP in solution was used to investigate this aspect. Furthermore, EPR spectroscopy was applied to reveal the triplet state of OCP's internal carotenoid molecule in the presence of a photosensitizer. With the perspective of applying OCP properties to the development of photochromic nanodevices, a protocol to adsorb OCP on aminopropyl functionalized mesoporous silica SBA15 nanoparticles was followed, finding that OCP remains photoactive even onto silica matrix. Fluorophores such as cyanines and flavonoids were adsorbed on SBA15 in presence of OCP in order to create switch on/off fluorescent systems with a possible application in fluorescence imaging and other domains.