Effect of Ca²⁺/Sr²⁺ Substitution on the Electronic Structure of the Oxygen-Evolving Complex of Photosystem II: A Combined Multifrequency EPR, ⁵⁵Mn-ENDOR, and DFT Study of the S₂ State

Computational Insights on Crystal Structures of the Oxygen-Evolving Complex of Photosystem II with Either Ca2+ or Ca2+ Substituted by Sr2+

“…This is consistent with experimental and computational reports suggesting that the OEC structure does not undergo significant changes upon Ca 2+ depletion 34,35 or Ca 2+ /Sr 2+ substitution. 36 Therefore, the heterocation is not expected to play a dominant role in the stabilization of the OEC core, which is consistent with the low occupancy of Sr 2+ (70%) in the XRD structure. 12 The QM/MM models indicate that Sr 2+ is positioned slightly further away from the OEC relative to the position of Ca 2+ , which is consistent with the larger ionic radius of Sr 2+ versus that of Ca 2+ .…”

S = 3 Ground State for a Tetranuclear MnIV4O4 Complex Mimicking the S3 State of the Oxygen-Evolving Complex

“…Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 rhombically distorted octahedral Mn IV in SnO 2 /TiO 2 . 64,[72][73] Mn IV in SnO 2 /TiO 2 similarly displays ZFS values on the order of a wavenumber 64,[72][73] suggesting that a distortion of the octahedral coordination environment of the individual Mn IV ions in 3 leads to the observed ZFS.…”

Molecular Concepts of Water Splitting: Nature's Approach

“…The oxidation state pattern for the S 2 state is shown in Fig 4B [ 41], which places the only Mn III ion inside the cuboidal unit (Mn D1 ) and requires that the Mn D1 has a particular ligand field. These assignments compare favorably with the additional information from EPR studies [42,43] and complimentary spectroscopic measurements such as X-ray absorption near edge spectroscopy [36,44], K β -fluorescence spectroscopy [44], and resonant inelastic X-ray scattering spectroscopy [45].…”

“…Similarly, they also establish that the two substrate molecules bind at separate sites, as their exchange rates vary independently with S i state. The slower-exchanging substrate water (W S ) can be 55 Mn-ENDOR spectra (Q band) of the WOC of PSII from Thermosynechococcus elongatus native (Ca) and with Ca 2+ replaced by Sr 2+ [41]. Insert: Electronic connectivity (Y-coupling scheme) of the WOC with four exchange couplings J between Mn ions [14].…”

“…Insert: Electronic connectivity (Y-coupling scheme) of the WOC with four exchange couplings J between Mn ions [14]. (B) Structure of the WOC poised in the S 2 state according to Cox et al [41] EPR/ 55 Mn data together with theoretical modeling suggest the Mn III ion is within the cuboidal unit (Mn D1 ) and has a five-coordinate tetragonally elongated ligand field. (C) 17 [46,50,51], as seen in the recent crystal structure (Mn A4 -OH/OH 2 ), but inconsistent with a protonated bridge species, at least in the S 2 state.…”