Energy transfer in the peridinin chlorophyll-a protein of Amphidinium carterae studied by polarized transient absorption and target analysis

The peridinin chlorophyll-a protein (PCP) of dinoflagellates differs from t he well-studied light-harvesting complexes of purple bacteria and green pla nts in its large (4:1) carotenoid to chlorophyll ratio and the unusual prop erties of its primary pigment, the carotenoid peridinin. We utilized ultraf ast polarized transient absorption spectroscopy to examine the flow of ener gy in PCP after initial excitation into the strongly allowed peridinin S-2 state. Global and target analysis of the isotropic and anisotropic decays r eveals that significant excitation (25-50%) is transferred to chlorophyll-a directly from the peridinin S-2 state. Because of overlapping positive and negative features, this pathway was unseen in earlier single-wavelength ex periments. In addition, the anisotropy remains constant and high in the per idinin population, indicating that energy transfer from peridinin to peridi nin represents a minor or negligible pathway. The carotenoids are also coup led directly to chlorophyll-a via a low-lying singlet state S-1 or the rece ntly identified S-CT. We model this energy transfer time scale as 2.3 +/- 0 .2 ps, driven by a coupling of similar to 47 cm(-1). This coupling strength allows us to estimate that the peridinin S-1/S-CT donor state transition m oment is similar to3 D.