Low-temperature time-resolved spectroscopic study of the major light-harvesting complex of Amphidinium carterae.

The major light-harvesting complex of Amphidinium (A.) carterae, chlorophyll-a-chlorophyll-c 2-peridinin-protein complex (acpPC), was studied using ultrafast pump-probe spectroscopy at low temperature (60 K). An efficient peridinin-chlorophyll-a energy transfer was observed. The stimulated emission signal monitored in the near-infrared spectral region was stronger when redder part of peridinin pool was excited, indicating that these peridinins have the S1/ICT (intramolecular charge-transfer) state with significant charge-transfer character. This may lead to enhanced energy transfer efficiency from "red" peridinins to chlorophyll-a. Contrary to the water-soluble antenna of A. carterae, peridinin-chlorophyll-a protein, the energy transfer rates in acpPC were slower under low-temperature conditions. This fact underscores the influence of the protein environment on the excited-state dynamics of pigments and/or the specificity of organization of the two pigment-protein complexes.

Structure and function of native and refolded peridinin-chlorophyll-proteins from dinoflagellates.

Peridinin-chlorophyll a-proteins are a class of light-harvesting proteins only found in photosynthetic dinoflagellates. Due to their exceptional stability they are an excellent model system to study carotenoid to chlorophyll energy transfer. We were able to solve structures of these complexes at near atomic resolution, allowing the detailed discussion of pigment-pigment and pigment-protein interactions. Using a refolding system, we also determined structures of complexes with mutated apoproteins and modified pigment compositions. Here we summarize the current understanding of PCP structures, with an emphasis on how the basic dimeric structure may be modified in the oligomeric state of these complexes.

Single molecule fluorescence of native and refolded peridinin-chlorophyll-protein complexes.

Single molecule spectroscopy was applied to study the optical properties of native and refolded peridinin-chlorophyll-protein (PCP) complexes. The native system is a trimer with six chlorophyll a (Chl a) molecules, while the refolded one contains two Chl a and resembles structurally and spectroscopically the PCP monomer. The fluorescence emission of single PCP complexes strongly broadens with increasing excitation power. Simultaneously, the distribution of fluorescence maximum frequencies is also broadened. These spectral changes are attributed to photoinduced conformational changes of the protein that influence the fluorescence of embedded chromophores. Comparison of fluorescence intensities measured for PCP complexes in two different solvents indicates that the native PCP trimers are preserved in EDTA Tris buffer, while in PVA polymer matrix only monomers are stable.