Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters

The initial energy transfer in photosynthesis occurs between the light-harvesting pigments and on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that F\"orster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which leads to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited state as part of the system's Hamiltonian, reproduces all measured quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play a central role in the excited state population transfer to bacteriochlorophyll as the resonance between the donor-acceptor energy gap and vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems

Vertical Distribution of Epibenthic Freshwater Cyanobacterial Synechococcus spp. Strains Depends on Their Ability for Photoprotection

Epibenthic cyanobacteria often grow in environments where the fluctuation of light intensity and quality is extreme and frequent. Different strategies have been developed to cope with this problem depending on the distribution of cyanobacteria in the water column. and either constant or enhanced levels of carotenoids were assayed in phycocyanin-rich strains collected from 1.0 and 0.5 m water depths. Protein analysis revealed that while the amount of biliproteins remained constant in all strains during light stress and recovery, the amount of D1 protein from photosystem II reaction centre was strongly reduced under light stress conditions in strains from 7.0 m and 1.0 m water depth, but not in strains collected from 0.5 m depth. spp. strains, depending on their genetically fixed mechanisms for photoprotection

EU enlargement and the reform of the structural funds The implications for Scotland

SIGLEAvailable from British Library Document Supply Centre- DSC:m03/19286 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Assignment of spectral substructures to pigment-binding sites in higher plant light-harvesting complex LHC-II

The trimeric main light-harvesting complex (LHC-II) is the only antenna complex of higher plants of which a high-resolution 3D structure has been obtained (Kuhlbrandt, W., Wang, D., and Fujiyoshi, Y. (1994) Nature 367, 614-621) and which can be refolded in vitro from its components. Four different recombinant forms of LHC-II, each with a specific chlorophyll (Chl) binding site removed by site-directed mutagenesis, were refolded from heterologously overexpressed apoprotein, purified pigments, and lipid. Absorption spectra of mutant LHC-II were measured in the temperature range from 4 to 300 K and compared to likewise refolded wild-type complex and to native LHC-II isolated from pea chloroplasts. Chls at different binding sites have characteristic, well-defined absorption sub-bands. Mixed occupation of binding sites with Chls a and b is not observed. Temperature-dependent changes of the mutant absorption spectra reveal a consistent shift of the major difference bands but an irregular behavior of minor bands. A model of the spectral substructure of LHC-II is proposed which accounts for the different absorption properties of the 12 individual Chls in the complex, thus establishing a first consistent correlation between the 3D structure of LHC-II and its spectral properties. The spectral substructure is valid for recombinant and native LHC-II, indicating that both have the same spatial arrangement of Chls and that the refolded complex is fully functional

Large plasmonic fluorescence enhancement of cyanobacterial photosystem I coupled to silver island films

A large, two-orders-of-magnitude enhancement of fluorescence emission from cyanobacterial Photosystem I (PSI) coupled to plasmonic excitations in silver island films was observed. Such a high value has not been reported for metal-enhanced fluorescence of photosynthetic pigment-protein complexes before. The dramatic enhancement of the PSI emission occurs when PSI is excited resonantly into the Qx and Qy bands of chlorophyll a. In contrast, excitation in the carotenoid absorption band yields ten times lower enhancement factors. We attribute these large values of enhancement factor to plasmon-induced activation of excitation and emission channels absent for isolated PSI complexes