Comprehensive Evaluation of Models for Ammonia Binding to the Oxygen Evolving Complex of Photosystem II

The identity and insertion pathway of the substrate oxygen atoms that are coupled to dioxygen by the oxygen-evolving complex (OEC) remains a central question toward understanding Nature’s water oxidation mechanism. In several studies, ammonia has been used as a small “water analogue” to elucidate the pathway of substrate access to the OEC and to aid in determining which of the oxygen ligands of the tetramanganese cluster are substrates for O–O bond formation. On the basis of structural and spectroscopic investigations, five first-sphere binding modes of ammonia have been suggested, involving either substitution of an existing H2O/OH–/O2– group or addition as an extra ligand to a metal ion of the Mn4CaO5 cluster. Some of these modes, specifically the ones involving substitution, have already been subject to spectroscopy-oriented quantum chemical investigations, whereas more recent suggestions that postulate the addition of ammonia have not been examined so far with quantum chemistry for their agreement with spectroscopic data. Herein, we use a common structural framework and theoretical methodology to evaluate structural models of the OEC that represent all proposed modes of first-sphere ammonia interaction with the OEC in its S2 state. Criteria include energetic, magnetic, kinetic, and spectroscopic properties compared against available experimental EPR, ENDOR, ESEEM, and EDNMR data. Our results show that models featuring ammonia replacing one of the two terminal water ligands on Mn4 align best with experimental data, while they definitively exclude substitution of a bridging μ-oxo ligand as well as incorporation of ammonia as a sixth ligand on Mn1 or Mn4.

and 3/2 for Mn(IV)], 〈 ) 〉 *+ the total MS of the BS wavefunction (1/2 in all models),  ( the spin projection coefficient of Mni, 1 and 1.78 is a scaling factor used specifically for comparing the computed 55 Mn hyperfine coupling constants with experimental results (see also Computational Details section in the main text).
b A|| is defined as the component of the effective HFC components with the largest difference to Aiso = (Ax + Ay + Az)/3 (Table 2), and A⊥ the average of those components with the smallest difference to Aiso.
c The on-site hyperfine values -ax, ay, az-of Mni are derived from: . ( • 1.78, and αiso = (αx b αaniso is defined as αaniso = α⊥ -α||, where α|| the on-site HFC component with the largest difference to αiso and α⊥ the average of those components with the smallest difference to aiso.•   , where  ( is the site spin of Mni [2 for Mn(III) and 3/2 for Mn(IV)], 〈 ) 〉 *+ the total MS of the BS wavefunction (1/2 in all models), and  ( the spin projection coefficient of Mni.For a bridging ligand coordinated to Mnj and Mnk, such as NHx in models A and O5, the effective HFC components are derived from  != • ( . , • | 3 |), where  4 and  4 are the site spin and the spin projection coefficient, respectively, of Mnj.

Figure S1 .
Figure S1.Complete computational model for the S2 state of the OEC.Mn(III) ions are indicated in pink, Mn(IV) in dark purple, Ca in yellow, N in blue, O in red and Cl in green.Hydrogen atoms connected to carbon atoms are omitted for clarity.All amino-acid residues belong to the D1 subunit, except from Arg357 and Glu354 (shown in italics), which belong to the CP43 subunit.

Figure S2 .
Figure S2.Frontier orbitals of model C1 of the ammonia-bound S2 state of the OEC (left) and of the oxidized C1Yz • (right); hydrogen atoms are omitted for clarity.

Table S2 .
Calculated Mulliken Mn spin populations for the optimized S2 and S2-NH3 models calculated with broken-symmetry (BS) TPSSh using the lowest energy BS determinant.Very similar spin populations are computed with BP86 and with TPSSh using the high-spin determinant.

Table S3 .
Relative energies (kcal mol −1 ) of BP86 optimized S2-state structures with various NH3 binding sites calculated with BP86, TPSSh using the high-spin determinant and broken-symmetry (BS) TPSSh using the lowest energy BS determinant.

Table S5 .
Calculated effective/projected 14 N hyperfine and NQI tensors (in MHz) for the electron-nuclear couplings of the bound NHx-N and of the His332 imino-N, and 17 O hyperfine tensors (in MHz) of the terminal W1 and W2 ligands and the bridging O5 ligand for the S2-NH3 and the S2-state models in the lowspin ( S = 1/2 ) configuration.
a ABS,x, ABS,y, ABS,z are the components of the hyperfine coupling tensor computed from the BS calculation and they follow the convention |ABS,z| > |ABS,y| > |ABS,x|.The effective (observable) HFC components -Ax, Ay, Azof 14 N or 17 O ligand to Mni are derived from:  !=  BS, • 〈  〉 BS

Table S7 .
Comparison of calculated55Mn HFCs (MHz) for the model in which ammonia binds on the W1 position (B1) between this work, Schraut and Kaupp, 4 and Lohmiller et al.5