Fig. 7. Coordination and reduction of SO₃²⁻.

(a-c) DFT-optimized models of the M-cluster coordinated by SO₃²⁻ or NH₃. DFT models of the M-clusters (taken from PDB entry 6VXT)¹⁸ with and without ligands were optimized with TPSS-D3/def2-SV(P). Coordination energies were obtained by calculating single point energies (TPSS-D3/def2-TZVP) for the optimized structures. SO₃²⁻ is bridged between Fe3 and Fe7 either via its S atom and one of its O atoms (a, coordination scenario 1), or via two of its O atoms (b, coordination scenario 2), at the S5A site; whereas NH₃ is coordinated asymmetrically by Fe3 at the same site (c). The M-cluster and the coordinating ligands (SO₃²⁻ or NH₃) are shown in ball-and-stick presentation. The residues used for the model are shown as sticks. The atoms are coloured as described in Fig. 4. (d) Reaction energies of the two coordination scenarios of SO₃²⁻ derived from DFT calculations (TPSS/def2-TZVP, COSMO ε=20, DFT-D3) on the M-cluster models, assuming that coupled e⁻/H⁺ transfer takes place after the initial coordination step. The energies (kcal/mol) for both pathways (coordination scenario 1: steps 1a, 2a, 3a and 4; coordination scenario 2: Steps 1b, 2b, 3b and 4) are indicated in the scheme. (e) Cumulative reaction energies of sulphur incorporation for the two coordination scenarios of SO₃²⁻.