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Reaction centers (RCs) of purple bacteria are uniquely suited objects to study the mechanisms of the photosynthetic conversion of light energy into chemical energy. A recently introduced method of higher order derivative spectroscopy [I.K. Mikhailyuk, H. Lokstein, A.P. Razjivin, A method of spectral subband decomposition by simultaneous fitting the initial spectrum and a set of its derivatives, J. Biochem. Biophys. Methods 63 (2005) 10-23] was used to analyze the NIR absorption spectra of RC preparations from Rhodobacter (R.) sphaeroides strain 2R and Blastochloris (B.) viridis strain KH, containing bacteriochlorophyll (BChl) a and b, respectively. Q(y) bands of individual RC porphyrin components (BChls and bacteriopheophytins, BPheo) were identified. The results indicate that the upper exciton level Py+ of the photo-active BChl dimer in RCs of R. sphaeroides has an absorption maximum of 810nm. The blue shift of a complex integral band at approximately 800nm upon oxidation of the RC is caused primarily by bleaching of Py+, rather than by an electrochromic shift of the absorption band(s) of the monomeric BChls. Likewise, the disappearance of a band peaking at 842 nm upon oxidation of RCs from B. viridis indicates that this band has to be assigned to Py+, A blue shift of an absorption band at approximately 830nm upon oxidation of RCs of B. viridis is also essentially caused by the disappearance of Py+, rather than by an electrochromic shift of the absorption bands of monomeric BChls. Absorption maxima of the monomeric BCHls, B-B and B-A are at 802 and 797nm, respectively, in RCs of R. sphaeroides at room temperature. BPheo co-factors H-B and HA peak at 748 and 758 nm, respectively, at room temperature. For B. viridis RCs the spectral positions of HB and HA were found to be 796 and 816nm, respectively, at room temperature.
Peridinin-chlorophyll a-protein (PCP) is a unique antenna complex in dinoflagellates that employs peridinin (a carotenoid) as its main light-harvesting pigment. Strong excitonic interactions between peridinins, as well as between peridinins and chlorophylls (Chls) a, can be expected from the short intermolecular distances revealed by the crystal structure. Different experimental approaches of nonlinear polarization spectroscopy in the frequency domain (NLPF) were used to investigate the various interactions between pigments in PCP of Amphidinium carterae at room temperature. Lineshapes of NLPF spectra indicate strong excitonic interactions between the peridinin's optically allowed S-2 (1Bu(+)) states. A comprehensive subband analysis of the distinct NLPF spectral substructure in the peridinin region allows us to assign peridinin subbands to the two Chls a in PCP having different S-1-state lifetimes. Peridinin subbands at 487, 501, and 535 nm were assigned to the longer-lived Chl, whereas a peridinin subband peaking at 515 nm was detected in both clusters. Certain peridinin(s), obviously corresponding to the subband centered at 487 nm, show(s) specific (possibly Coulombic?) interaction between the optically dark S-1(2A(g)(-)) and/or intramolecular charge- transfer (ICT) state and S-1 of Chl a. The NLPF spectrum, hence, indicates that this peridinin state is approximately isoenergetic or slightly above S-1 of Chl a. A global subband analysis of absorption and NLPF spectra reveals that the Chl a Q(y)-band consists of two subbands ( peaking at 669 and 675 nm and having different lifetimes), confirmed by NLPF spectra recorded at high pump intensities. At the highest applied pump intensities an additional band centered at <= 660 nm appears, suggesting-together with the above results-an assignment to a low-dipole moment S-0-> S-1/ICT transition of peridinin
Peridinin-chlorophyll a protein (PCP) is a unique water soluble antenna complex that employs the carotenoid peridinin as the main light-harvesting pigment. In the present study the near edge X-ray absorption fine structure (NEXAFS) spectrum of PCP was recorded at the carbon Kedge. Additionally, the NEXAFS spectra of the constituent pigments, chlorophyll a and peridinin, were measured. The energies of the lowest unoccupied molecular levels of these pigments appearing in the carbon NEXAFS spectrum were resolved. Individual contributions of the pigments and the protein to the measured NEXAFS spectrum of PCP were determined using a "building block" approach combining NEXAFS spectra of the pigments and the amino acids constituting the PCP apoprotein. The results suggest that absorption changes of the pigments in the carbon near K-edge region can be resolved following excitation using a suitable visible pump laser pulse. Consequently, it may be possible to study excitation energy transfer processes involving "optically dark" states of carotenoids in pigment-protein complexes by soft X-ray probe optical pump double resonance spectroscopy (XODR).
Extremophilic organisms are gaining increasing interest because of their unique metabolic capacities and great biotechnological potential. The unicellular acidophilic and mesothermophilic red alga Galdieria sulphuraria (074G) can grow autotrophically in light as well as heterotrophically in the dark. In this paper, the effects of externally added glucose on primary and secondary photosynthetic reactions are assessed to elucidate mixotrophic capacities of the alga. Photosynthetic O-2 evolution was quantified in an open system with a constant Supply Of CO2 to avoid rapid volatilization of dissolved inorganic carbon at low pH levels. In the presence of glucose, O-2 evolution was repressed even in illuminated cells. Ratios of variable to maximum chlorophyll fluorescence (F-v/F-m) and 77 Kfluorescence spectra indicated a reduced photochemical efficiency of photosystem II. The results were corroborated by strongly reduced levels of the photosystem 11 reaction centre protein D1. The downregulation of primary photosynthetic reactions was accompanied by reduced levels of the Calvin Cycle enzyme ribu lose-1,5-bisphosphate carboxylaselfoxygenase (Rubisco). Both effects depended on functional sugar uptake and are thus initiated by intracellular rather than extracellular glucose. Following glucose depletion, photosynthetic O-2 evolution of illuminated cells commenced after 15 h and Rubisco levels again reached the levels of autotrophic cells. It is concluded that true mixotrophy, involving electron transport across both photosystems, does not occur in G. sulphuraria 074G, and that heterotrophic growth is favoured over autotrophic growth if sufficient organic carbon is available.