PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Excited state dynamics in the PSI-LHCI supercomplex from Chlamydomonas reinhardtii: excitation wavelength dependence study. Alexander Melkozernov^*,1, Joanna Kargul², Su Lin¹, James Barber², Robert Blankenship¹, ¹ Department of Chemistry and Biochemistry, Tempe, AZ, USA² Wolfson Laboratories, Department of Biological Sciences, London, UK

ABSTRACT- In photosynthetic apparatus of green algae and higher plants the peripheral chlorophyll a/b binding antennas (LHCI and LHCII) increase light-harvesting capacity of Chl a binding photosystem core reaction centers (PSI and PSII). The PSI-LHCI supercomplex from the green alga Chlamydomonas reinhardtii binds ca. 215 Chls including 95 Chl a in the PSI core antenna and 80 Chl a and 40 Chl b in the LHCI peripheral antenna. Severe overlap of the Chl a spectral forms in the PSI-LHCI supercomplex significantly complicates analysis of the excitation energy transfer processes between the LHCI and the PSI core. Femtosecond transient absorption spectroscopy at 295 K and 77 K upon multicolor excitation of the PSI-LHCI in the 640 - 700 nm region have been used to find kinetic contributions of the peripheral LHCI antenna to the excitation dynamics of the supercomplex on five time scales, from hundreds of femtoseconds to nanoseconds. Excitation wavelength dependent kinetics revealed a subpicosecond Chl b to Chl a energy transfer processes in the peripheral antenna followed by a picosecond excitation equilibration among the Chl a spectral forms in the PSI-LHCI clearly involving red pigments of the LHCI. The overall excitation decay processes in the PSI-LHCI supercomplex include the excitation wavelength independent photochemical trapping (20 - 30 ps) in the PSI core as well as excitation wavelength dependent slow decay process (100-150 ps) indicating presence of an energy transfer pathway from the LHCI to the PSI core, which introduces a diffusion-limited step. The origin of the kinetic heterogeneity in the excitation dynamics in the PSI-LHCI is discussed on the basis of available structural models of the supercomplex.

KEY WORDS: peripheral light-harvesting antenna, photosystem I, energy transfer, time-resolved absorption spectroscopy