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 dinofiagellate 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-Chla energy transfer, which is essentially entirely mediated by coupling of the peridinin S₁ and Chla Q_y transitions. Based on this lifetime we estimated an energy transfer timescale of ∼ 2.4 ps and a peridinin-Chla coupling of 46 cm^-1. The magnitude of this coupling suggests a peridinin S₁ 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 S₁ transition dipole is oriented ∼ 15° away from the S₂ transition dipole, or, that a small amount of peridinin S₂-peridinin S₂ energy transfer occurs in competition with internal conversion, or some combination of the two.