Structure of an active water molecule in the water-oxidizing complex of photosystem II as studied by FTIR spectroscopy.
The vibrations of a water molecule in the water-oxidizing complex (WOC) of photosystem II were detected for the first time using Fourier transform infrared (FTIR) spectroscopy. In a flash-induced FTIR difference spectrum upon the S(1)-to-S(2) transition, a pair of positive and negative bands was observed at 3618 and 3585 cm(-1), respectively, and both bands exhibited downshifts by 12 cm(-1) upon replacement of H(2)(16)O by H(2)(18)O. Upon D(2)O substitution, the bands largely shifted down to 2681 and 2652 cm(-1). These observations indicate that the bands at 3618 and 3585 cm(-1) arise from the O-H stretching vibrations of a water molecule, probably substrate water, coupled to the Mn cluster in the S(2) and S(1) states, respectively. The band frequencies indicate that the O-H group forms a weak H-bond and this H-bonding becomes weaker upon S(2) formation. Intramolecular coupling with the other O-H vibration of this water molecule was studied by a decoupling experiment using a H(2)O/D(2)O (1:1) mixture. The downshifts by decoupling were estimated to be 4 and 12 cm(-1) for the 3618 (S(2)) and 3585 cm(-1) (S(1)) bands, both of which were much smaller than 52 cm(-1) of water in vapor, indicating that the observed water has a considerably asymmetric structure; i.e., one of the O-H groups is weakly and the other is strongly H-bonded. The smaller coupling in the S(2) than the S(1) state means that this H-bonding asymmetry becomes more prominent upon S(2) formation. Such a structural change may facilitate the proton release reaction that takes place in the later step by lowering the potential barrier. The present study showed that FTIR detection of the O-H vibrations is a useful and promising method to directly monitor the chemical reactions of substrate water and clarify the molecular mechanism of photosynthetic water oxidation.
Noguchi
T
T
Biophysical Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan.
Sugiura
M
M
D000458:Cyanobacteria; D003903:Deuterium; D006859:Hydrogen; D006860:Hydrogen Bonding; D008345:Manganese / Q000737:chemistry; D010084:Oxidation-Reduction; D010103:Oxygen Isotopes; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D017550:Spectroscopy, Fourier Transform Infrared; D014732:Vibration; D014867:Water / Q000737:chemistry*
D010103:Oxygen Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins; D014867:Water; D008345:Manganese; D006859:Hydrogen; D003903:Deuterium