Berlin 2005 – wissenschaftliches Programm
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MO: Molekülphysik
MO 12: Biomolecules I
MO 12.1: Hauptvortrag
Freitag, 4. März 2005, 14:00–14:30, HU 2091
Initial electron donor and acceptor in isolated Photosystem II reaction centers identified with femtosecond midinfrared spectroscopy — •M. L. Groot1, N. Pawlowicz-Wereszczynska1, L. J.G.W. van Wilderen1, J. Breton2, I. H.M. van Stokkum1, and R. van Grondelle1 — 1Faculty of Sciences, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands — 2Service de Bioénergétique, Bât. 532, CEA-Saclay, 91191 Gif-sur-Yvette, France
The primary steps in photosynthesis of energy- and electron transfer occur in green plants in two large chlorophyll-containing protein complexes called Photosystem I and Photosystem II. We measured the time-dependent absorption changes of isolated reaction centers of Photosystem II upon excitation at 670 and 681 nm in the mid-infrared, in the region of the ester and keto modes, between 1775 and 1585 cm−1, with 150 fs time resolution. A detailed comparison with steady state P+/P and H−/H spectra reveals that the primary electron donor is not P+, but the accessory chlorophyll on the D1 polypeptide, ChlD1. The charge separated pair ChlD1+H− is formed with a time constant of 0.6-0.8 ps, upon excitation at 681 nm. The hole is transferred from ChlD1+ to P with an intrinsic rate of (4 ps)−1. Slow energy transfer leads to a part of the P+H− state to appear with a time constant of 36 ps. Relaxation of the radical pair, with a drop in free energy of ∼ 60 meV, occurs in ∼ 200 ps.