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 [(MnCa)-Ca-II(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 mu-hydroxo ligands coordinated to the Mn ion. In contrast, dramatically increased FTIR difference activity is observed where these oxidations are unaccompanied by proton loss. (C) 2015 Elsevier Inc. All rights reserved.