Low Quantum Yield Electron Transfer Pathways in PSII
At low temperatures (<180 K) the electron transfer in photosystem II (PSII) is inhibited from QA − to QB and from YZ to P680 +. Depending on the initial redox state of Cytb559 different long-lived states are formed upon illumination (QA −P680Cytb559+, QA −P680Car+, QA −P680Chlz+). The formation and decay of these states are investigated by means of Raman and (polarized) absorption difference spectroscopy. Raman bands appearing upon illumination of PSII at 77 K with Cytb559 preoxidized are assigned to the β-carotene cation. DFT-calculations on β-carotene and its radical cation confirm this assignment. The linear dichroism of the 1,000 nm band of the β-carotene cation shows that the oxidized Car is oriented parallel to the membrane plane, which is in agreement with the orientation of the Car on the D2 side. Chl oxidation has been monitored by the bleaching bands in the Soret (433 nm), in the QX (610 and 624 nm) and in the QY region (665 nm).
KeywordsPhotosystem II Raman DFT calculation secondary electron donors linear dichroism
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