Partial characterization of six chlorophyll a-protein complexes isolated from a blue-green alga by a nondetergent method

Picosecond time-resolved fluorescence from detergent-free photosystem I particles

Picosecond time-resolved fluorescence measurements have been taken on a detergent-free P700-enriched complex at room temperature isolated from the blue-green alga Phormidium luridum with a chlorophyll a to reaction center ratio of 100. Emission at greater than 665 nm is characterized by two exponential-decay components. A fast component, which dominates the initial decay with an average lifetime of 16 ps and 87% amplitude, is attributed to excitations in the core antenna chlorophyll-proteins, which are rapidly trapped by the primary electron donor, P700. A second component, with an average lifetime of 106 ps and 13% amplitude, is attributed to the peripheral antenna proteins. For 532-nm, 30-ps pulse excitation the results are virtually independent of fluence in the range of 2 x 10(12) to 4 x 10(16) photons/cm(2) and the oxidation state of P700. Addition of sodium dodecyl sulfate to 0.1% causes the second component's lifetime to increase by an average of a factor of 2.5. Only minor changes are observed in the first component's lifetime and the relative amplitudes of the two components. Two fractions isolated from the detergent-treated samples have also been examined. Our results indicate that excitation energy transfer within photosystem I is very efficient and that the excitation kinetics of the antennae may be limited by the trapping rate of P700 or strongly affected by the heterogeneity of the antennae.

Temperature dependence and polarization of fluorescence from Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803

To determine the fluorescence properties of cyanobacterial Photosystem I (PS I) in relatively intact systems, fluorescence emission from 20 to 295 K and polarization at 77 K have been measured from phycobilisomes-less thylakoids of Synechocystis sp. PCC 6803 and a mutant strain lacking Photosystem II (PS II). At 295 K, the fluorescence maxima are 686 nm in the wild type from PS I and PS II and at 688 nm from PS I in the mutant. This emission is characteristic of bulk antenna chlorophylls (Chls). The 690-nm fluorescence component of PS I is temperature independent. For wild-type and mutant, 725-nm fluorescence increases by a factor of at least 40 from 295 to 20 K. We model this temperature dependence assuming a small number of Chls within PS I, emitting at 725 nm, with an energy level below that of the reaction center, P700. Their excitation transfer rate to P700 decreases with decreasing temperature increasing the yield of 725-nm fluorescence.Fluorescence excitation spectra of polarized emission from low-energy Chls were measured at 77 and 295 K on the mutant lacking PS II. At excitation wavelengths longer than 715 nm, 760-nm emission is highly polarized indicating either direct excitation of the emitting Chls with no participation in excitation transfer or total alignment of the chromophores. Fluorescence at 760 nm is unpolarized for excitation wavelengths shorter than 690 nm, inferring excitation transfer between Chls before 760-nm fluorescence occurs.Our measurements illustrate that: 1) a single group of low-energy Chls (F725) of the core-like PS I complex in cyanobacteria shows a strongly temperature-dependent fluorescence and, when directly excited, nearly complete fluorescence polarization, 2) these properties are not the result of detergent-induced artifacts as we are examining intact PS I within the thylakoid membrane of S. 6803, and 3) the activation energy for excitation transfer from F725 Chls to P700 is less than that of F735 Chls in green plants; F725 Chls may act as a sink to locate excitations near P700 in PS I.

Supramolecular structure of chlorophyll-protein complexes in relation to the chlorophyll a fluorescence of chloroplasts at room or liquid nitrogen temperature

To investigate further the possibility that changes in the organization of the thylakoid pigment-protein complexes are monitored by the chlorophyll a fluorescence yield changes, or by changes in the F685/F730 ratio in the 77 degrees K fluorescence, as earlier proposed (A. Castorinis, G. Akoyunoglou, and J.H. Argyroudi-Akoyunoglou (1982) Photobiochem. Photobiophys. 4, 283-291; J.H. Argyroudi-Akoyunoglou, A. Castorinis, and G. Akoyunoglou (1982) Photobiochem. Photobiophys. 4, 201-210), the effect of pH, Cd2+ or Zn2+ addition, and trypsinization on all parameters was studied. We found that (a) the pH of the medium affects the ratio of oligomeric to monomeric structures of the complexes (monomers predominate at low pH), as well as the ratio F685/F730 of thylakoids or of the isolated CPIa complex (the F685/F730 ratio is high at low pH, and low at high). (b) Zn2+ or Cd2+ addition to thylakoids, suspended in Tricine (N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine), has no effect on the chlorophyll a fluorescence yield nor on the F685/F730 ratio at 77 degrees K; similarly, no effect can be noticed on the F685/F730 ratio at 77 degrees K of the isolated CPIa complex. These cations, contrary to Mg2+, do not affect the oligomer to monomer dissociation, but they instead reverse the cation effect and enhance the oligomeric structures. (c) Trypsinized thylakoids in Tricine do not show the Mg2+ effect on the chlorophyll a fluorescence yield, have a reduced F730/F685 ratio at 77 degrees K, and are deficient in the CPIa complex, which is mainly responsible for the F685/F730 ratio changes in the presence of cations. The results support the proposal that cation- or pH-induced changes in the chlorophyll a fluorescence yield at room temperature, or in the F685/F730 ratio at 77 degrees K, are irrelevant to the excitation energy distribution between the two photosystems. They instead seem to reflect changes in the organization of the supramolecular structure of the pigment-protein complexes in the photosystem I and photosystem II units.