Hsiu-An Chu

Vibrational spectroscopy of the oxygen-evolving complex and of manganese model compounds

A number of molecularly specific models for the oxygen-evolving complex in photosystem II (PSII) and of manganese-substrate water intermediates that may occur in this process have been proposed recently. We summarize this work briefly. Fourier transform infrared techniques have emerged as fruitful tools to study the molecular structures of Y(Z) and the manganese complex. We discuss recent work in which mid-IR (1000-2000 cm(-1)) methods have been used in this effort. The low-frequency IR region (<1000 cm(-1)) has been more difficult to access for technical reasons, but good progress has been made in overcoming these obstacles. We update recent low-frequency work on PSII and then present a detailed summary of relevant manganese model compounds that will be of importance in understanding the emerging biological data.

Manganese and tyrosyl radical function in photosynthetic oxygen evolution

Photosystem II catalyzes the photosynthetic oxidation of water to O2. The structural and functional basis for this remarkable process is emerging. The catalytic site contains a tetramanganese cluster, calcium, chloride and a redox-active tyrosine organized so as to promote electroneutral hydrogen atom abstraction from manganese-bound substrate water by the tyrosyl radical. Recent work is assessed within the framework of this model for the water oxidizing process.

Low-frequency fourier transform infrared spectroscopy of the oxygen-evolving complex in Photosystem II*

In this communication, we report our progress on the development of low-frequency Fourier transform infrared (FTIR) spectroscopic techniques to study metal-substrate and metal-ligand vibrational modes in the Photosystem II/oxygen-evolving complex (PS II/OEC). This information will provide important structural and mechanistic insight into the OEC. Strong water absorption in the low-frequency region (below 1000 cm−1), a lack of suitable materials, and temperature control problems have limited previous FTIR spectroscopic studies of the OEC to higher frequencies (>1000 cm−1). We have overcome these technical difficulties that have blocked access to the low-frequency region and have developed successive instruments that allow us to move deeper into the low-frequency region (down to 350 cm−1), while increasing both data accumulation efficiency and S/N ratio. We have detected several low-frequency modes in the S2/S1spectrum that are specifically associated with these two states. Our results demonstrate the utility of FTIR techniques in accessing low-frequency modes in Photosystem II and in proteins generally.

Detection of Low Frequency Modes of S 1 /S 2 and Q A /Q A - in Photosystem II by Light-Induced Fourier Transform Infrared Difference Spectroscopy

Several new structural and mechanistic models for photosynthetic water oxidation predict unique molecular structures for each S state of the OEC that should present characteristic signals in the low frequency region of their vibrational spectra [1,2]. For example, oxo-bridged Mn2(μ-O)2 core vibrations occur at 600-700cm−1, metal-oxo stretches occur in the 700-1000 cm−1 region and Mn-OH stretches might occur around 400-500 cm−1 region [3,4]. Therefore, vibrational spectroscopies should provide critical and potentially conclusive structural and mechanistic insights into the PSII/OEC. Unfortunately, Raman spectroscopy is ineffective in PSII, owing to the presence of the strongly scattering chlorophyll molecules. Additionally, there are formidable technical problems, e.g., extensive water absorbance and limitations in optical materials, that have thwarted attempts to access the low-frequency region by FTIR. Up to now, there is a good deal of valuable information in the literature from the high frequency region of PSII by extensive FTIR studies, but there is no report of the use of FTIR techniques to study low-frequency metal-ligand vibrations in proteins.