Identification of a Mn-O-Mn cluster vibrational mode of the oxygen-evolving complex in photosystem II by low-frequency FTIR spectroscopy

We have developed conditions for recording the low-frequency S-2/S-1 Fourie r transform infrared difference spectrum of hydrated PSII samples. By excha nging PSII samples with buffered O-18 water, we found that a positive band at 606 cm(-1) in the S-2/S-1 spectrum in O-16 water is clearly downshifted to 596 cm(-1) in O-18 water. By taking double-difference (S-2/S-1 and O-16 minus O-18) spectra, we assign the 606 cm(-1) mode to an St mode and also i dentify a corresponding S-1 mode at about 625 cm(-1). In addition, by Sr an d Ca-44 substitution experiments, we found that the 606 cm(-1) mode is upsh ifted to about 618 cm(-1) by Sr2+ substitution but that this mode is not af fected by substitution with the Ca-44 isotope. On the basis of these result s and also on the basis of studies of Mn model compounds, we assign the 625 cm(-1) mode in the S-1 state and the 606 cm(-1) mode in the S-2 State to a Mn-O-Mn cluster vibration of the oxygen-evolving complex (OEC) in PSII. Th is structure may include additional bridge(s), which could be another ore, carboxylato(s), or atoms derived from an amino acid side chain. Our results indicate that the bridged oxygen atom shown in this Mn-O-Mn cluster is exc hangeable and accessible by water. The downshift in the Mn-O-Mn cluster vib ration as manganese is oxidized during the S-1 --> S-2 transition is counte rintuitive; we discuss possible origins of this behavior. Our results also indicate that Sr2+ substitution in PSII causes a small structural perturbat ion that affects the bond strength of the Mn-O-Mn cluster in the PSII OEC. This suggests that Sr2+ and by inference, Ca2+, communicates with, but is n ot integral to, the manganese core.