Light is essential for photosynthesis, and hence in supporting life on earth, but all the steps of the light reactions may lead to the formation of dangerous oxidative species. Chlorophyll a, the green heart of oxygenic photosynthesis, is also its main Achilles' heel, due to a high intersystem crossing (ISC) probability that leads to the production of excited triplet state, an efficient singlet oxygen sensitizer. Photosynthetic organisms have consequently developed several photoprotective mechanisms aimed to avoid photo-oxidative stress originated from the excess of absorbed light, that otherwise would ultimately lead to cell death.
Photoprotection is pivotal for life on Earth, however a full comprehension of the molecular mechanisms is still lacking for several of the processes that photosynthetic organisms employ. The knowledge of the diverse adaptations of the photoprotective response to the various natural conditions in which photosynthetic organisms have evolved promises to highlight the essential characteristics that an efficient mechanism has to display. This understanding of the key molecular requirements for an efficient photoprotection may be exploited for the design of bio-mimetic molecular systems in the fields of artificial photosynthesis, photodynamic therapies and photocatalysis, to make them more durable in virtue of specifically tailored photoprotective mechanisms.
The quickest of the photoprotective processes taking place in natural photosystems relies on the capability of carotenoids to photoprotect the system, either by directly quenching the triplet state of chlorophyll through triplet-triplet energy transfer, or by deactivation of the photosensitized singlet oxygen once it is formed. With the aim of studying this pivotal trait of photosynthetic organisms, in the course of my graduate research I characterized the role of carotenoids in the photoprotection of the two major components of the oxygenic photosynthetic machinery, namely Photosystem I and Photosystem II. Due to the high complexity of the studied samples, consisting of multi-subunit complexes formed by the assembly of Reaction Centers with numerous bound antenna complexes, an Optically Detected Magnetic Resonance (ODMR) approach has been utilized. ODMR, being a double resonance technique, makes possible to disentangle the different triplet state contributions and extract the information regarding the triplet states populated upon illumination in the multi-chromophore complex of interest. A comparative approach, involving either differences in the size of the complexes or mutations, allowed to get insight into the energy transfer pathways and into the differential role that β-carotene and xanthophylls play in the photoprotection of the two photosystems.
With the aim to extend the study of natural photoprotective mechanisms and understand them at a molecular level, we started to work on an unusual chlorophyll binding protein, the Water-Soluble Chlorophyll-binding Protein (WSCP). This research has been conduct in the framework of a joint project between the university of Padova and the university of Mainz. WSCP remarkably differs from the other known chlorophyll-binding proteins, being not involved in the photosynthetic process. WSCP has been shown to be an incredibly stable complex, being able to protect its chlorophylls towards photodamage. Interestingly this protein does not contain carotenoids, in contrast to every other known chlorophyll-binding protein. By combining biochemical and spectroscopic methodologies, we discovered a mechanism for the photoprotection of chlorophylls in WSCP completely new in the landscape of photoprotection We demonstrated that the observed resistance of the WSCP-bound chlorophylls to singlet oxygen damage depends on the localization of the phytyls moieties between the chlorophylls forming a tight dimer in WSCP. We were able to propose a photoprotective mechanism based on the capability of the phytyls to limit the singlet oxygen accessibility to the oxidizable sites of the chlorophylls
