Low-Frequency Resonance Raman Characterization of the Oxygen-Evolving Complex of Photosystem II

Abstract

The O2-evolving complex (OEC) of photosystem II (PSII) contains a tetramanganese (Mn4) cluster, a redox-active tyrosine, and Ca2+/Cl- ions, but its molecular structure has not been determined. Vibrational spectroscopy has the potential of providing new structural information for the OEC, particularly the Mn4 cluster. Toward this goal, the vibrational characteristics of the OEC of PSII were examined using near-infrared (NIR) excitation Raman spectroscopy. NIR excitation decreases the background contribution from chlorophyll emission/Raman scattering and affords the opportunity of probing selectively low-energy electronic transitions of the Mn4 cluster. The primary emphasis of the Raman study was on the low-frequency range of the spectrum (220−620 cm-1) where Mn−ligand vibrational modes are expected to occur. The low-frequency region was examined for both the S1 and S2 oxidation states of the Mn4 cluster. A particular effort was made to probe a NIR transition of the S2 state that has been reported to mediate photoconversion from the multiline to the g = 4.1 form of the S2 state [Boussac et al. Biochemistry 1996, 35, 6984−6989]. The Raman studies revealed the following: (1) the Raman spectra of Mn-depleted PSII and PSII in the S2 state are nearly identical; (2) the Raman spectrum of PSII in the S1 state displays several unique low-frequency bands not present in the S2 state that can be assigned as Mn−ligand vibrational modes and appear to maximize in intensity at λex ∼ 820 nm; and (3) several of the S1 state Raman bands are shifted by D2O/H2O exchange. Collectively, these results indicate that the S1 state of the Mn4 cluster (1) has a NIR electronic transition from which resonance enhanced Raman scattering can be induced and (2) is coordinated by at least two H2O or OH- groups. The studies reported herein also demonstrate the potential of NIR-excitation Raman techniques for probing selectively the OEC in PSII and, in particular, for characterizing the coordination environment of the Mn4 cluster.