Rationales for the Choice of Metals for Super-Reduced Biological Metal Centers: Cobalt in Cobalamin Versus Nickel in...

Reported here are DFT calculations on porphyrin, corrin, and corphin complexes with formally metal (I) states of cobalt, rhodium, iron, manganese, or nickel, complemented by QM/MM calculations on enzyme models, and exploring the available spin states, with the goal to understand factors controlling the availably of super-reduced metal(I) states on such biologically-relevant systems. In the porphyrin systems, the preferred spin states offer no clear case of metal(I) electronic description-but rather metal(II) coupled with porphyrin anion radicals; this is in line with the fact that porphyrin is not used in biology for mechanisms/reactions entailing super-reduced metals. The corrin, somewhat less conjugated than the porphyrin, affords a clear-cut ground-state S = 0 Co(I) complex, with essentially no delocalization of the extra electron onto the macrocycle. None of the neighbors of cobalt in the third period afford clear super-reduced ground states with corrin-in line with cobalamin's choice of metal in biology, cobalt. The least conjugated system, the corphin (as seen in F₄₃₀), now affords a metal(I) state even in the case of Ni. Moreover, in fact, only Ni affords a ground state with clear super-reduced character-in line with F₄₃₀'s choice of metal in vivo.