Evidence from FTIR Difference Spectroscopy That a Substrate H₂O Molecule for O₂ Formation in Photosystem II Is Provided by the Ca Ion of the Catalytic Mn₄CaO₅ Cluster

The O 2 -producing Mn 4 CaO 5 catalyst in photosystem II oxidizes two water molecules (substrate) to produce one O 2 molecule. Considerable evidence supports the identification of one of the two substrate waters as the Mn 4 CaO 5 cluster’s oxo bridge known as O 5 . The identity of the second substrate water molecule is less clear. In one set of models, the second substrate is the Mn-bound water molecule known as W2. In another set of models, the second substrate is the Ca 2+ -bound water molecule known as W3. In all of these models, a deprotonated form of the second substrate moves to a position next to O5 during the catalytic step immediately prior to O–O bond formation. In this study, FTIR difference spectroscopy was employed to identify the vibrational modes of hydrogen-bonded water molecules that are altered by the substitution of Sr 2+ for Ca 2+ . Our data show that the substitution substantially altered the vibrational modes of only a single water molecule: the water molecule whose D-O–D bending mode is eliminated during the catalytic step immediately prior to O–O bond formation. These data are most consistent with the identification of the Ca 2+ -bound W3 as the second substrate involved in O–O bond formation.