CAROTENOID TRIPLET DETECTION BY TIME-RESOLVED EPR SPECTROSCOPY IN CAROTENOPYROPHEOPHORBIDE DYADS

Carotenoid triplets play a photoprotective role in natural photosynthe sis. The main process of carotenoid triplet formation is known to be t riplet-triplet energy transfer from chlorophyll triplets. The structur al requirements for high transfer yields are still a matter of discuss ion and the presence of competitive triplet formation pathways has not been excluded. Transient EPR measurements of triplet states formed by photoexcitation allow detection of the initial spin polarization. Thi s pattern derives from the mechanism of triplet formation. In the case of triplet-triplet energy transfer, if the condition of spin angular momentum conservation is fulfilled, simulation of the EPR spectra give s information about the donor-acceptor mutual orientation. We describe transient EPR experiments on two artificial photosynthetic dyads, con sisting of a carotenoid covalently-linked to a free-base or zinc subst ituted pyropheophorbide moiety and we discuss the results in terms of possible dyad conformations. (C) 1997 Elsevier Science S.A.