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• Articles by G. Hastings
• Articles by R.E. Blankenship

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• Ultrafast Primary Processes in Photosystem I of the Cyanobac...

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• Spectral Inhomogeneity of Photosystem I and Its Influence on...

  Spectral Inhomogeneity of Photosystem I and Its Influence on Excitation Equilibration and Trapping in the Cyanobacterium Synechocystis sp. PCC6803 at 77 K Biophysical Journal, Volume 81, Issue 2, 1 August 2001, Pages 1144-1154 Alexander N. Melkozernov, Su Lin, Robert E. Blankenship and Leonas Valkunas Abstract Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77K in the photosystem I (PSI) core antenna from the cyanobacterium PCC 6803. Excitation of the bulk antenna at 670 and 680nm induces a subpicosecond energy transfer process that populates the Chl spectral form at 685–687nm within few transfer steps (300–400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4–6ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30–50-ps process of photochemical trapping of the excitation by P. These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708nm and 714nm (full-width at half-maximum (fwhm) ∼ 200–300cm). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683nm and 678nm under excitation into the red side of the absorption spectrum (>690nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll monomers from the excited-state absorption of the dimeric red pigments. Abstract | Full Text | PDF (218 kb)

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Abstract

Femtosecond transient absorption spectroscopy has been used to investigate the energy transfer and trapping processes in both intact membranes and purified detergent-isolated particles from a photosystem II deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803, which contains only the photosystem I reaction center. Processes with similar lifetimes and spectra are observed in both the membrane fragments and the detergent-isolated particles, suggesting little disruption of the core antenna resulting from the detergent treatment. For the detergent-isolated particles, three different excitation wavelengths were used to excite different distributions of pigments in the spectrally heterogeneous core antenna. Only two lifetimes of 2.7–4.3 ps and 24–28 ps, and a nondecaying component are required to describe all the data. The 24–28 ps component is associated with trapping. The trapping process gives rise to a nondecaying spectrum that is due to oxidation of the primary electron donor. The lifetimes and spectra associated with trapping and radical pair formation are independent of excitation wavelength, suggesting that trapping proceeds from an equilibrated excited state. The 2.7–4.3 ps component characterizes the evolution from the initially excited distribution of pigments to the equilibrated excited state distribution. The spectrum associated with the 2.7–4.3 ps component is therefore strongly excitation wavelength dependent. Comparison of the difference spectra associated with the spectrally equilibrated state and the radical pair state suggests that the pigments in the photosystem I core antenna display some degree of excitonic coupling.