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The structure of phycobiliproteins of the cyanobacterium Acaryochloris marina was investigated in buffer solution at physiological temperatures, i.e. under the same conditions applied in spectroscopic experiments, using small angle neutron scattering. The scattering data of intact phycobiliproteins in buffer solution containing phosphate can be well described using a cylindrical shape with a length of about 225 A and a diameter of approximately 100 A. This finding is qualitatively consistent with earlier electron microscopy studies reporting a rod-like shape of the phycobiliproteins with a length of about 250 (M. Chen et al., FEBS Letters 583, 2009, 2535) or 300 A (J. Marquart et al., FEBS Letters 410, 1997, 428). In contrast, phycobiliproteins dissolved in buffer lacking phosphate revealed a splitting of the rods into cylindrical subunits with a height of 28 A only, but also a pronounced sample aggregation. Complementary small angle neutron and X-ray scattering experiments on phycocyanin suggest that the cylindrical subunits may represent either trimeric phycocyanin or trimeric allophycocyanin. Our findings are in agreement with the assumption that a phycobiliprotein rod with a total height of about 225 A can accommodate seven trimeric phycocyanin subunits and one trimeric allophycocyanin subunit, each of which having a height of about 28 A. The structural information obtained by small angle neutron and X-ray scattering can be used to interpret variations in the low-energy region of the 4.5 K absorption spectra of phycobiliproteins dissolved in buffer solutions containing and lacking phosphate, respectively.

The unique high spectral resolution resulting from application of nonlinear polarisation spectroscopy in the frequency domain at 77 K directly reveals 6 subbands (at 649, 657, 667, 672, 675 and 679 nm) in the chlorophyll a/b Qy-region of trimeric light-harvesting complex II. A shortening (from about 90 to 35 fs) of the corresponding dephasing times towards the `blue' edge of the Qy-band was observed.

The effects of nonphotochemical quenching were quantified separately applying the Stem-Volmer formalism for dark level (SV 0 ) and maximum chlorophyll fluorescence yield (SV N ) in barley leaves comprising a step-wise altered Photosystem II (PS II) antenna size. Proportions of overall SV N can be attributed to distinct sites of the photosynthetic apparatus: (i) the bulk light-harvesting complex of PS II (LHC II), (ii) the inner LHC II antenna, and (iii) the reaction center/core complex of PS II. The fraction of SV N which exerts an effect on SV 0 appeared to arise almost exclusively from the inner LHC II antenna. A strong linear correlation between SV 0 and violaxanthin de-epoxidation points to an intrinsic relationship of both. The results are in line with the notion of a regulatory function of the inner LHC II antenna, thus controlling excitation energy delivery from the bulk LHCII to the PS II-core complex.

Prochlorococcus marinus strain CCMP 1375 is the sole prokaryote to possess phycoerythrin in addition to (divinyl-)chlorophyll a/b binding antenna complexes. Here we demonstrate, employing a spectrofluorimetric assay, that phycoerythrin serves a light-harvesting antenna function (transfers energy to chlorophylls).

The first excited singlet state (S1) of carotenoids (also termed 2Ag ) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S1 (Qy) level of chlorophylls. The determination of carotenoid 2Ag energies by optical techniques is difficult; transitions from the ground state (S0 ,1 Ag ) to the 2Ag state are forbidden (boptically darkQ) due to parity (g p //Y g) as well as pseudo-parity selection rules ( p //Y ). Of particular interest are S1 energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S1 energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one k* energy level (about 0.5 eV below the lowest 1Bu vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states. D 2005 Elsevier B.V. All rights reserved.

Geranylgeranyl reductase catalyses the reduction of geranylgeranyl pyrophosphate to phytyl pyrophosphate required for synthesis of chlorophylls, phylloquinone and tocopherols. The gene chlP (ORF sll1091) encoding the enzyme has been inactivated in the cyanobacterium Synechocystis sp. PCC 6803. The resulting DeltachlP mutant accumulates exclusively geranylgeranylated chlorophyll a instead of its phytylated analogue as well as low amounts of alpha-tocotrienol instead of alpha-tocopherol. Whereas the contents of chlorophyll and total carotenoids are decreased, abundance of phycobilisomes is increased in DeltachlP cells. The mutant assembles functional photosystems I and II as judged from 77 K fluorescence and electron transport measurements. However, the mutant is unable to grow photoautotrophically due to instability and rapid degradation of the photosystems in the absence of added glucose. We suggest that instability of the photosystems in DeltachlP is directly related to accumulation of geranylgeranylated chlorophyll a. Increased rigidity of the chlorophyll isoprenoid tail moiety due to three additional CC bonds is the likely cause of photooxidative stress and reduced stability of photosynthetic pigment-protein complexes assembled with geranylgeranylated chlorophyll a in the DeltachlP mutant.