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

Abstract

The O 2 -evolving complex (OEC) of photosystem II (PSII) contains a tetramanganese (Mn 4 ) cluster, a redox-active tyrosine, and Ca 2+ /Clions, 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 OHgroups. 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.

Time-resolved vibrational spectroscopy detects protein-based intermediates in the photosynthetic oxygen-evolving cycle

“…Although cryogenic, rapid-scan FT-IR spectroscopy has been used to study the S 1 -to-S 2 transition, this technique cannot be applied to the other S state transitions, which do not occur at cryogenic temperature (44). Raman spectroscopy has been used to study the S 1 and S 2 states, but these experiments were designed to probe structure on a relatively long time scale under cryogenic conditions (45). Thus, to our knowledge, our measurements are the first to detect these short-lived intermediates with vibrational spectroscopy.…”

18O-Water exchange in photosystem II: Substrate binding and intermediates of the water splitting cycle

“…DFT calculations of water in PSII [12] were used to substantiate these assignments. In contrast, Raman vibrational spectroscopy has been difficult to apply due to the chlorophyll and fluorescence backgrounds, yet shifted-excitation Raman difference spectroscopy (SERDS) was successfully used to identify a number of 1 H/ 2 H sensitive bending modes at low frequencies (∼300-500 cm −1 ) [13], implicating the binding of two terminal water/hydroxide molecules to the water oxidizing complex (WOC). FTIR has also been used to examine metal-oxygen bands below 1000 cm −1 that are sensitive to 16 [14].…”

Understanding the Signatures of Secondary‐Structure Elements in Proteins with Raman Optical Activity Spectroscopy

“…Keywords: ab initio calculations · chirality · helical structures · protein structures · vibrational spectroscopy over two frequency axes such as two-dimensional IR spectroscopy, [5,[9][10][11][12] or that filter specific spectroscopic information such as resonance Raman spectroscopy [13][14][15][16][17] often provide more detailed information on the secondary structure of polypeptides and proteins. Similarly, chiral vibrational spectroscopy filters out information related to the chirality of the investigated molecule.…”