ENERGY TRANSFER IN THE PERIDININ CHLOROPHYLL a PROTEIN OF AMPHIDINIUM CARTERAE STUDIED BY POLARIZED ABSORPTION MEASUREMENTS

In this work we report the results of ultrafast transient absorption studies which examined energy transfer between carotenoids and chlorophylls in Peridinin-Chlorophyll-a-Protein (PCP) light harvesting complex from the dinoflagellate Amphidinium carterae. Global analysis of the isotropic decay reveals that the peridinin lifetime in PCP is 2.3±0.2 ps. This time constant is the peridinin- Chl a energy transfer, which is essentially entirely mediated by coupling of the peridinin S 1 and Chl a Q y transitions. Based on this lifetime we estimated an energy transfer timescale of ~2.4 ps and a peridinin- Chl a coupling of 46 cm -1. The magnitude of this coupling suggests a peridinin S 1 transition dipole moment of ~3 D . Global analysis of the depolarization measurements, shows, a small decrease in the depolarization from the initial 0.4 value to 0.36±0.03. This decrease occurs on the same timescale as the S2→S1 internal conversion, suggesting that either the S 1 transition dipole is oriented ~15° away from the S 2 transition dipole, or, that a small amount of peridinin S 2-peridinin S 2 energy transfer occurs in competition with internal conversion, or some combination of the two.