Spectral Inhomogeneity of Photosystem I and Its Influence on Excitation Equilibration and Trapping in the Cyanobacterium Synechocystis sp. PCC6803 at 77 K

AbstractUltrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680nm induces a subpicosecond energy transfer process that populates the Chl a 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 P700. 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−1). 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 a monomers from the excited-state absorption of the dimeric red pigments