PARENT SESSION

Symposium S4Aa Chlorophyll and bilin based antenna systems
Tuesday August 31st, 2004 2:40 PM-4:40 PM Room 511D
Chair: Roberto Bassi
Co-Chair: Herbert van Amerongen

Energy transfer pathways of Photosystem I studied by modified Redfield theory and photon echo spectroscopy. Harsha Vaswani^*,1, Jens Stenger¹, Mino Yang², Ana Damjanovic⁴, Petra Fromme³, Graham Fleming¹, ¹ Department of Chemistry, University of California, Berkeley and, Berkeley, CA, U.S.A.² Department of Chemistry and Institute for Basic Sciences Research, Chungbuk National University, Cheongju, South Korea⁴ Biophysics Department, John Hopkins University, Baltimore, Maryland, U.S.A.³ Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, U.S.A.

ABSTRACT- The population dynamics of PSI has been studied extensively in the past yet little is known about the actual pathway the excitation follows through the complex landscape of PSI. We approach this problem by using complementary experimental and theoretical methods. A model based on experimentally determined spectral densities and calculated Qy site energies of the Chls in PSI was developed and theoretical predictions using modified Redfield theory within master equation calculations were made. The disparity between the timescales of direct transitions across domains and the effective transfer times implicate entropy rather than enthalpy as the major driving force. One- and two-color three pulse photon echo peak shift spectroscopy (3PEPS) is used to test the model and study the pathways of excitation in PSI. 3PEPS is an incisive tool in the study of energetically disordered systems, exploiting the disorder to determine energy transfer timescales, electron-phonon coupling and the degree to which the disorder is correlated within individual complexes. Wavelength-dependent one-color 3PEPS suggests slower energy transfer and less sampling of Chls at increasingly lower energies from 675 nm to 700 nm. At 713 nm, the peak shift decays more rapidly and the decay is complete by 800 fs. This suggests the red Chls are sampled within 800 fs in contrast to the bulk Chls that retain their heterogeneity past several picoseconds. Two-color 3PEPS experiments were conducted with the first two pulses at 675 nm and the third at 690 or 700 nm. These measurements are characterized by a quantity called the difference peak shift (DPS). A rapid, almost instantaneous, rise to a large DPS is measured, suggesting large correlation between pump and probe regions. This DPS decays on the timescale of hundreds of fs. The DPS calculated from the inhomogeneous contribution provided by the calculations fit well with the experimental results.

KEY WORDS: spectroscopy, antenna, photosystem I, energy transfer