Energy Transfer in the Peridinin Chlorophyll-a Protein of Amphidinium carterae Studied by Polarized Transient Absorption and Target Analysis

Abstract

The peridinin chlorophyll-a protein (PCP) of dinoflagellates differs from the well-studied light-harvesting complexes of purple bacteria and green plants in its large (4:1) carotenoid to chlorophyll ratio and the unusual properties of its primary pigment, the carotenoid peridinin. We utilized ultrafast polarized transient absorption spectroscopy to examine the flow of energy in PCP after initial excitation into the strongly allowed peridinin S2 state. Global and target analysis of the isotropic and anisotropic decays reveals that significant excitation (25-50%) is transferred to chlorophyll-a directly from the peridinin S2 state. Because of overlapping positive and negative features, this pathway was unseen in earlier single-wavelength experiments. In addition, the anisotropy remains constant and high in the peridinin population, indicating that energy transfer from peridinin to peridinin represents a minor or negligible pathway. The carotenoids are also coupled directly to chlorophyll-a via a low-lying singlet state S1 or the recently identified SCT. We model this energy transfer time scale as 2.3 +/- 0.2 ps, driven by a coupling of approximately 47 cm(-1). This coupling strength allows us to estimate that the peridinin S1/SCT donor state transition moment is approximately 3 D.

Identification of a single peridinin sensing Chl- a excitation in reconstituted PCP by crystallography and spectroscopy

“…The electrostatic effect of the N89L mutation is predicted to shift the two lowest lying energy levels of Per-614 to higher energies (see Origin of the absorption shift in the N89L 10-K absorption spectrum in the SI Text and Table S4). Consistent with recent studies on PCP (16,24,25), we also observe a small bleaching signal (a downward dip in the line shape) in TA measurements on both RFPCP and the N89L mutant at delay times after Per has relaxed back to the ground state, but while Chl-a is still excited (see Fig. S4 and Fig.…”

“…This effect has been attributed to the presence of an intramolecular charge transfer (ICT) state that acts in conjunction with the lowest excited singlet state S 1 (13). In PCP the dominant energy transfer pathway from peridinin to Chl-a is via this S 1 /ICT state, whose lifetime is Ϸ2.5 ps in the PCP complex, corresponding to an efficiency of 80% for this preferred route (14)(15)(16). The dominance of the S 1 /ICT-mediated pathway may be due to the unique properties of a state that can induce coupling to Chl-a while maintaining an energy high enough for efficient transfer to Chl-a (17).…”

Metallic Nanoparticles Coupled with Photosynthetic Complexes

“…Optical spectroscopy studies of both native and reconstituted PCP complexes have been carried out on the ensemble (Akimoto, 1996;Kleima, 2000;Krueger, 2001) and singlemolecule levels Wormke, 2007a;Wormke, 2008). Using transient absorption in femtosecond timescale main energy transfer pathways have been described, it has also been demonstrated that the two Chl a molecules interact relatively weakly with characteristic transfer time between them to be of the order to 12 ps (Kleima, 2000).…”

Carotenoid to chlorophyll energy transfer in the peridinin–chlorophyll- a –protein complex involves an intramolecular charge transfer state

“…The pigments are arranged as two essentially identical domains of four peridinins and one Chl-a molecule. Structural information of the PCP complex has provided a good basis for both experimental and theoretical studies of energy transfer pathways within PCP (9)(10)(11)(12)(13)(14). These studies demonstrated a high efficiency of peridinin to Chl-a energy transfer, with a characteristic time constant of 2.3-3.2 ps.…”