Biophysical Behaviour of Carotenoid and Chlorophyll Pigments in Natural and Artificial Photosynthetic Systems Probed by Time-resolved EPR Spectroscopy

Marilena Di Valentin*, Luca Degli Esposti*, Giancarlo Agostini° and Donatella Carbonera*

* University of Padova, Dipartimento di Scienze Chimiche, Via Marzolo 1, I-35131 Padova, Italy;° CNR-Istituto di Chimica Biomolecolare, Sezione di Padova, Via Marzolo 1, I-35131 Padova, Italy

Absorption of light occurring in the antenna system of photosynthetic organisms and photoinduced multistep electron transfer, involving the reaction center cofactors, constitute the strategy adopted by nature for solar energy conversion. Artificial photosynthetic systems of covalently linked chromophores may be synthesized starting from the same basic principles used by nature. The photophysical processes induced by light in natural and artificial photosynthetic systems lead to the population of excited states of the pigments involved, such as charge separated and triplet states. Being paramagnetic states, some of their physical properties can be studied by electron paramagnetic resonance (EPR) spectroscopy. The non-conventional time-resolved EPR technique allows detection of the electron spin polarization, which is the non-Boltzmann distribution of the spin-level population, and the evolution in time of this observable. The study of the spin-polarization has demonstrated to be a very powerful tool in order to investigate the formation mechanism of the charge separated states in the electron transfer process and of triplet states involved in energy transfer. Simulations of the EPR spectra allow to derive the magnetic interaction parameters which characterize these states and derive the structural information on the pigments. Carotenoid triplets in the light-harvesting complex of Photosystem II have been characterized by EPR in order to elucidate the structural relationship between the chlorophyll and carotene pigments for efficient energy transfer. The electron transfer properties of both Photosystem I reaction centers and biomimetic model compounds, containing covalently linked carotenoid and porphyrin chromophores, have been also investigated, through the corresponding charged separated states. A recombination route of the charge separated state based on the population of the carotenoid triplet state has been proven, in both natural and artificial systems.

Email: m.divalentin@chfi.unipd.it

Address: Dr Marilena Di Valentin, Dipartimento di Chimica, Università degli studi di Padova,, via Marzolo 1,, I-35131, Padova, Italy