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The low-frequency (<1000 cm-1) region of the IR spectrum has the potential to provide detailed structural and mechanistic insight into the photosystem II/oxygen evolving complex (PSII/OEC). A cluster of four manganese ions forms the core of the OEC and diagnostic manganese-ligand and manganese-substrate modes are expected to occur in the 200-900 cm-1 range. However, water also absorbs IR strongly in this region, which has limited previous Fourier transform infrared (FTIR) spectroscopic studies of the OEC to higher frequencies (>1000 cm-1). We have overcome the technical obstacles that have blocked FTIR access to low-frequency substrate, cofactor, and protein vibrational modes by using partially dehydrated samples, appropriate window materials, a wide-range MCT detector, a novel band-pass filter, and a closely regulated temperature control system. With this design, we studied PSII/OEC samples that were prepared by brief illumination of O2 evolving and Tris-washed preparations at 200 K or by a single saturating laser flash applied to O2 evolving and inhibited samples at 250 K. These protocols allowed us to isolate low-frequency modes that are specific to the QA-/QA and S2/S1 states. The high-frequency FTIR spectra recorded for these samples and parallel EPR experiments confirmed the states accessed by the trapping procedures we used. In the S2/S1 spectrum, we detect positive bands at 631 and 602 cm-1 and negative bands at 850, 679, 664, and 650 cm-1 that are specifically associated with these two S states. The possible origins of these IR bands are discussed. For the low-frequency QA-/QA difference spectrum, several modes can be assigned to ring stretching and bending modes from the neutral and anion radical states of the quinone acceptor. These results provide insight into the PSII/OEC and demonstrate the utility of FTIR techniques in accessing low-frequency modes in proteins.

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