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

The O 2 -evolving complex (OEC) of photosystem II (PSII) contains a tetramanganese (Mn 4 ) cluster, a redox-active tyrosine, and Ca 2+ /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 Mn 4 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 Mn 4 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 S 1 and S 2 oxidation states of the Mn 4 cluster. A particular effort was made to probe a NIR transition of the S 2 state that has been reported to mediate photoconversion from the multiline to the g = 4.1 form of the S 2 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 S 2 state are nearly identical; (2) the Raman spectrum of PSII in the S 1 state displays several unique low-frequency bands not present in the S 2 state that can be assigned as Mn−ligand vibrational modes and appear to maximize in intensity at λ ex ∼ 820 nm; and (3) several of the S 1 state Raman bands are shifted by D 2 O/H 2 O exchange. Collectively, these results indicate that the S 1 state of the Mn 4 cluster (1) has a NIR electronic transition from which resonance enhanced Raman scattering can be induced and (2) is coordinated by at least two H 2 O 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 Mn 4 cluster.