Plant Photosystem II and ODMR Spectroscopy of its Pigments

Natural photosystems, that are present in photosynthetic bacterial membranes and in thylakoids of algae and green plants, contain pigment molecules which are necessary for the three primary processes of Photosynthesis: - Light harvesting; - Energy transport to the Reaction Centre; - Primary charge separation. In the so-called Photosystem II (PSII) of plants, this complex pigment system is made up essentially by chlorophylls, pheophytins and carotenoids. There are about 300 chlorophyll molecules (a and b), more than 100 carotenoids (xanthophylls and beta-carotene) and a few molecules of pheophytin-a. The system can be studied by optical spectroscopy in the visible region, a main difficulty being the overlap of the pigment transitions in the red and blue regions where the most informative transitions of chlorophylls and carotenoids are present, In this report it will be shown how the association of optical spectroscopy and magnetic resonance in a double resonance spectroscopic technique known as Optically Detected Magnetic Resonance (ODMR) allows the above-mentioned difficulties to be partially overcome. On the one side, it is possible to assign optical transitions within unresolved bands to specific pigments selected by their magnetic resonances which have an intrinsically larger resolution. On the other hand, it is possible to study paths of excitonic energy within the pigment-protein complexes through the fluorescence response and to obtain interesting information about the function of the system. Some examples will be illustrated taken from recent experiments performed in our laboratory both on the antenna and on the reaction centre of PSII.