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Disputation Sarah Maya Mäusle

Jan 31, 2025 | 05:00 PM
Thema der Dissertation:
Time-Resolved Single-Frequency Infrared Spectroscopy Provides Atomistic Insight into the Functioning of the Two Photosystems of Oxygenic Photosynthesis
Abstract: Photosystems I and II (PSI, PSII) are two large protein complexes involved in the electron transport chain of oxygenic photosynthesis in plants, algae, and cyanobacteria. PSII harbors the light-driven water splitting process, which results in the removal of four protons and four electrons from two water molecules. This process is coupled to the release of molecular oxygen (O2) and is thus also referred to as oxygen evolution reaction (OER). The OER cycle of PSII (S-state cycle) involves four successive photon absorption events, which lead to the stepwise accumulation of four oxidation equivalents at the catalytic site, a protein-bound Mn4CaOx cluster, prior to O2 formation. The light-induced processes are still insufficiently understood regarding the temporal sequence of atomistic events. The focus of this thesis is the investigation of the S-state transitions in PSII by means of time-resolved single-frequency infrared (TRSF-IR) spectroscopy. Measurements on spinach PSII membrane particles in H2O, D2O and at several different pH values allowed for the characterization of the kinetics of three S-state transitions (S1→S2, S2→S3 and S3→S0). Global analysis of time-resolved spectral data sets resulted in decay-associated spectra (DAS), which are essentially spectral finger prints of individual kinetic phases. The DAS of the proton transfer (PT) and electron transfer (ET) phases of the O2-producing S3→S0 transition mostly reproduced the results of a previous step-scan Fourier-transform infrared (FTIR) study (Greife et al., 2023, Nature), but also managed to resolve a crucial deuteration-induced band shift, which was left undetected by the FTIR study.
PSII from the cyanobacterium Synechocystis sp. PCC 6803 carrying either the D1-N298A or the D1-D61A mutation was investigated and compared to wild-type PSII. While the monophasic O2-producing ET step of the S3→S0 transition was found to be drastically slowed in D1-D61A, it was found to be biphasic in D1-N298A. We assign the two observed kinetic phases to (i) O2- evolution occurring at a similar rate as in wildtype PSII and (ii) strongly decelerated substrate water insertion into the Mn4CaOx cluster, which in wild-type PSII occurs faster than the rate-limiting step and is therefore usually not spectroscopically detectable.
All in all, TRSF-IR measurements allowed for the kinetic and spectral characterization of various PSII samples, resulting in a small contribution toward a full understanding of the molecular mechanisms of oxygenic photosynthesis.

Time & Location

Jan 31, 2025 | 05:00 PM

Hörsaal B (0.1.01)
Fachbereich Physik, Arnimallee 14, 14195 Berlin