title: Effect of concomitant oxidation and deprotonation of hydrated Mn centres in rationalising the FTIR difference silence of D1-Asp170 in Photosystem II.
abstract: 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 1100cm(-1) to 1700cm(-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(II)Ca(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.
authors.affiliation: Research School of Chemistry, Australian National University, Acton, ACT 2601, Australia.
authors.forename: Richard
authors.initials: R
authors.lastname: Terrett
authors.affiliation: Research School of Chemistry, Australian National University, Acton, ACT 2601, Australia; School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Canberra, ACT 2610, Australia.
authors.forename: Terry
authors.initials: T
authors.lastname: Frankcombe
authors.affiliation: Research School of Chemistry, Australian National University, Acton, ACT 2601, Australia.
authors.forename: Ron
authors.initials: R
authors.lastname: Pace
authors.affiliation: Research School of Chemistry, Australian National University, Acton, ACT 2601, Australia. Electronic address: rob.stranger@anu.edu.au.
authors.forename: Robert
authors.initials: R
authors.lastname: Stranger
mesh_terms: D001224:Aspartic Acid / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry*; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011522:Protons; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*; D014867:Water / Q000737:chemistry
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D001224:Aspartic Acid; D008345:Manganese
keywords: Density functional theory; Photosystem II; Water oxidation