What kinds of ferryl species exist for compound II of chloroperoxidase? A dialog of theory with experiment

QM/MM calculations of Mössbauer parameters lead to assignments of the three isomeric species of CPO-II, "major", "minor", and unassigned "6% species", which were recently observed in experiment and posed two puzzles (Stone, K. L.; Hoffart, L. M.; Behan, R. K.; Krebs, C.; Green, M. T. J. Am. Chem. Soc. 2006, 128, 6147). Both the minor and major species were found to be iron(IV)-hydroxides, thus accounting for the observed ratio of their relative yield that is pH-independent. The difference between the minor and major species is a single water molecule that acts as a H-bond acceptor from the ferryl in the minor species (2b) and it is essential to get a good match of the calculated Mössbauer parameters to the experimentally observed ones for the minor species. The major species (2c-2e, 2e-NW) may or may not have a water molecule. The calculations reveal also two candidates for the unassigned 6% species, which are a Por^+,Fe^IIIOH species 2e-Fe(III), without or with a water molecule, or the corresponding aqua complex POr^+.Fe^IIIOH₂ 3c formed by adding an additional proton to the system. These species have △E_Q parameters of the same magnitude but with opposite signs: negative (-2.30 mm/s) for the two 2e-Fe(III) species and positive (2.39 mm/s) for 3c. The above assignments were further consolidated by an extended correlation (Figure 2) between the iron spin density and the △E_Q parameters of the species calculated in the present study and by relating △E_Q to the d-electronic configuration on iron. A bonding model of the FeO(H) moiety (Figure 3) was used to account for the variation of the spin density and provided further support for the correlation in Figure 2 and the assignment. Experimental determination of the sign of the quadruple parameter will finally confirm the identity of this species. In addition, since 3c possesses an additional proton, its identity can be revealed by pHdependent yield. All in all, the present paper shows that QM/MM calculations can conduct a useful dialogue with experiment in this complex field.