Effect of concomitant oxidation and deprotonation of hydrated Mn centres in rationalising the FTIR difference silence of D1-Asp170 in Photosystem II

The observation of negligible FTIR differences in carboxylate vibrational modes for the D1-Asp170 residue of Photosystem II (PSII) on successive one-electron oxidations of the Mn₄CaO₅ oxygen-evolving complex (OEC) is counterintuitive in light of the apparent ligation of D1-Asp170 to an oxidisable Mn ion in the X-ray crystallographic structures of PSII. Here, we show computational support for the hypothesis that suppression of the FTIR difference spectrum in the 1100 cm^{- 1} to 1700 cm^{- 1} region of D1-Asp170 occurs by concomitant Mn oxidation and deprotonation of water ligands bound to the ligated metal centre. Density functional theory calculations on the model species [Mn^IICa(COOH)(OH)₂(H₂O)₂]⁺ over two successive oxidations of the Mn ion are performed, where those oxidations are accompanied by deprotonation of water and μ-hydroxo ligands coordinated to the Mn ion. In contrast, dramatically increased FTIR difference activity is observed where these oxidations are unaccompanied by proton loss.