No abstract

Photosystem I (PS-I) contains a small fraction of chlorophylls (Chls) that absorb at wavelengths longer than the primary electron donor P700. The total number of these long wavelength Chls and their spectral distribution are strongly species dependent. In this contribution we present room temperature time-resolved fluorescence data of five PS-I core complexes that contain different amounts of these long wavelength Chls, i.e., monomeric and trimeric photosystem I particles of the cyanobacteria Synechocystis sp. PCC 6803, Synechococcus elongatus, and Spirulina platensis, which were obtained using a synchroscan streak camera. Global analysis of the data reveals considerable differences between the equilibration components (3.4-15 ps) and trapping components (23-50 ps) of the various PS-I complexes. We show that a relatively simple compartmental model can be used to reproduce all of the observed kinetics and demonstrate that the large kinetic differences are purely the result of differences in the long wavelength Chl content. This procedure not only offers rate constants of energy transfer between and of trapping from the compartments, but also well-defined room temperature emission spectra of the individual Chl pools. A pool of red shifted Chls absorbing around 702 nm and emitting around 712 nm was found to be a common feature of all studied PS-I particles. These red shifted Chls were found to be located neither very close to P700 nor very remote from P700. In Synechococcus trimeric and Spirulina monomeric PS-I cores, a second pool of red Chls was present which absorbs around 708 nm, and emits around 721 nm. In Spirulina trimeric PS-I cores an even more red shifted second pool of red Chls was found, absorbing around 715 nm and emitting at 730 nm.

Photosystem I of the cyanobacterium Synechococcus elongatus contains two spectral pools of chlorophylls called C-708 and C-719 that absorb at longer wavelengths than the primary electron donor P700. We investigated the relative quantum yields of photochemical charge separation and fluorescence as a function of excitation wavelength and temperature in trimeric and monomeric photosystem I complexes of this cyanobacterium. The monomeric complexes are characterized by a reduced content of the C-719 spectral form. At room temperature, an analysis of the wavelength dependence of P700 oxidation indicated that all absorbed light, even of wavelengths of up to 750 nm, has the same probability of resulting in a stable P700 photooxidation. Upon cooling from 295 K to 5 K, the nonselectively excited steady-state emission increased by 11- and 16-fold in the trimeric and monomeric complexes, respectively, whereas the quantum yield of P700 oxidation decreased 2.2- and 1.7-fold. Fluorescence excitation spectra at 5 K indicate that the fluorescence quantum yield further increases upon scanning of the excitation wavelength from 690 nm to 710 nm, whereas the quantum yield of P700 oxidation decreases significantly upon excitation at wavelengths longer than 700 nm. Based on these findings, we conclude that at 5 K the excited state is not equilibrated over the antenna before charge separation occurs, and that approximately 50% of the excitations reach P700 before they become irreversibly trapped on one of the long-wavelength antenna pigments. Possible spatial organizations of the long-wavelength antenna pigments in the three-dimensional structure of photosystem I are discussed.

Polarized site-selected fluorescence spectroscopy of isolated Photosystem I particles P ol ari zed site-sel ected fluo resc ence sp ec tro sco py of iso lat ed P hot osy stem I pa rtic les Bas Gobets, Herbert van Amerongen, René Monshouwer, Jochen Kruip, Matthias Rögner, Rienk van Grondelle and Jan P. Dekker Polarized steady-state fluorescence spectra have been obtained from Photosystem I core complexes of the cyanobacterium Synechocystis PCC 6803 and from LHCI containing Photosystem I (PSI-200) complexes of spinach by selective laser excitation at 4 K. Excitation above 702 nm in Synechocystis and 720 nm in PSI-200 results in highly polarized emission, suggesting that pigments absorbing at these and longer wavelengths are not able to transfer excitation energy at 4 K. In both systems the peak wavelength of the emission (λ em ) depends strongly on the excitation wavelength (λ ex ). This indicates that in both systems the long-wavelength bands responsible for the steady-state emission are inhomogeneously broadened. The width of the inhomogeneous distribution is estimated to be about 215 cm -1 in Synechocystis and 400 cm -1 in PSI-200. We conclude that the peaks of the total absorption spectra of the long-wavelength pigments of Synechocystis and PSI-200 are at 708 and 716 nm, respectively, and therefore designate these pigments as C708 and C716. The results further show that C708 and C716 are strongly homogeneously broadened, i.e. carry broad phonon side-bands. The width of these bands is estimated to be about 170 and 200 cm -1 for C708 and C716, respectively. The Stokes' shifts appear to be large: about 200 cm -1 (10 nm) for C708 and about 325 cm -1 (17 nm) for C716. These values are much higher than usually observed for 'normal' antenna pigments, but are in the same order as found previously for a number of dimeric systems. Therefore, we propose that the long-wavelength pigments in Photosystem I are excitonically coupled dimers. Based on fitting with Gaussian bands the presence of one C708 dimer per P700 is suggested in the core antenna of Synechocystis. This chapter has been published in Biochimica etBiophysica Acta 1188, 75-85, reproduced with permission.

Time-resolved fluorescence anisotropy spectroscopy has been used to study the chlorophyll a (Chl a) to Chl a excitation energy transfer in the water-soluble peridinin-chlorophyll a-protein (PCP) of the dinoflagellate Amphidinium carterae. Monomeric PCP binds eight peridinins and two Chl a. The trimeric structure of PCP, resolved at 2 A (, Science. 272:1788-1791), allows accurate calculations of energy transfer times by use of the Förster equation. The anisotropy decay time constants of 6.8 +/- 0.8 ps (tau(1)) and 350 +/- 15 ps (tau(2)) are respectively assigned to intra- and intermonomeric excitation equilibration times. Using the ratio tau(1)/tau(2) and the amplitude of the anisotropy, the best fit of the experimental data is achieved when the Q(y) transition dipole moment is rotated by 2-7 degrees with respect to the y axis in the plane of the Chl a molecule. In contrast to the conclusion of, Biochemistry. 23:1564-1571) that the refractive index (n) in the Förster equation should be equal to that of the solvent, n can be estimated to be 1.6 +/- 0.1, which is larger than that of the solvent (water). Based on our observations we predict that the relatively slow intermonomeric energy transfer in vivo is overruled by faster energy transfer from a PCP monomer to, e.g., the light-harvesting a/c complex.