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 tetraman ganese (Mn-4) cluster, a redox-active tyrosine, and Ca2+/Cl- ions, but its molecular structure has not been determined. Vibrational spectroscopy has t he potential of providing new structural information for the OEC, particula rly the Mn4 cluster. Toward this goal, the vibrational characteristics of t he OEC of PSII were examined using near-infrared (NIR) excitation Raman spe ctroscopy. NIR excitation decreases the background contribution from chloro phyll emission/ Raman scattering and affords the opportunity of probing sel ectively 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 st ates of the Mn-4 cluster. A particular effort was made to probe a NIR trans ition of the S-2 State that has been reported to mediate photoconversion fr om the multiline to the g = 4.1 form of the St state [Boussac et al. Bioche mistry 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 St state that can be assigne d as Mn-ligand vibrational modes and appear to maximize in intensity at lam bda(ex) similar to 820 nm; and (3) several of the S-1 state Raman bands are shifted by D2O/H2O exchange. Collectively, these results indicate that the S-1 state of the Mn-4 cluster (1) has a NIR electronic transition from whi ch resonance enhanced Raman scattering can be induced and (2) is coordinate d by at least two H2O or OH- groups. The studies reported herein also demon strate the potential of NIR-excitation Raman techniques for probing selecti vely the OEC in PSII and, in particular, for characterizing the coordinatio n environment of the Mn-4 cluster.