Biophys J, August 2001, p. 1144-1154, Vol. 81, No. 2

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

Alexander N. Melkozernov,^* Su Lin,^* Robert E. Blankenship,^* and Leonas Valkunas

 ^*Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1604 USA; and  Institute of Physics, Vilnius LT2600, Lithuania

Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685-687 nm within few transfer steps (300-400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4-6 ps, 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 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) ~ 200-300 cm¹). 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 683 nm and 678 nm under excitation into the red side of the absorption spectrum (>690 nm) 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.

Biophys J, August 2001, p. 1144-1154, Vol. 81, No. 2
© 2001 by the Biophysical Society   0006-3495/01/08/1144/11  $2.00

This article has been cited by other articles:

				M. K. Sener, C. Jolley, A. Ben-Shem, P. Fromme, N. Nelson, R. Croce, and K. Schulten Comparison of the Light-Harvesting Networks of Plant and Cyanobacterial Photosystem I Biophys. J., September 1, 2005; 89(3): 1630 - 1642. [Abstract] [Full Text] [PDF]

				K. Gibasiewicz, V. M. Ramesh, S. Lin, K. Redding, N. W. Woodbury, and A. N. Webber Excitonic Interactions in Wild-Type and Mutant PSI Reaction Centers Biophys. J., October 1, 2003; 85(4): 2547 - 2559. [Abstract] [Full Text] [PDF]

				M. Yang, A. Damjanovic, H. M. Vaswani, and G. R. Fleming Energy Transfer in Photosystem I of Cyanobacteria Synechococcus elongatus: Model Study with Structure-Based Semi-Empirical Hamiltonian and Experimental Spectral Density Biophys. J., July 1, 2003; 85(1): 140 - 158. [Abstract] [Full Text] [PDF]