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 (H2O)-O-16 by (H2O)-O-18. Upon D2O 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 St formation. Intramolecular coupling with the other O-H vibration of this water molecule was studied by a decoupling experiment using a H2O/D2O (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.