The structure and reactivity of metallohydroporphyrins

Metallohydroporphyrins are metal-containing porphyrins that have been reduced at one or more of the porphyrin macrocycle positions. Such complexes have been identified as being essential in a variety of biological systems including nitrite and sulfite reductases and S-methyl coenzyme M reductase. Of particular interest is the highly reduced nickel-containing macrocyclic tetrapyrrole F$\sb{430}$, found in the latter enzyme of methanogenic bacteria. This cofactor possesses the most highly reduced porphyrin macrocycle yet encountered in nature.The relative rigidity of the macrocycle is expected to be important in metallo-hydroporphyrin enzymes containing metal ions which undergo changes in either spin state or oxidation state during the course of catalytic activity. It has been argued that ring reduction gives the macrocycle greater flexibility: the expected reduction in aromaticity and observed S$\sb4$-ruffling both in solids and in solution lend support to these arguments. Through ligand binding experiments with a series of metallo(hydro)porphyrins we have determined whether reactivity varies as a function of macrocycle reduction level. In addition, through the binding of sterically hindered ligands which perturb the macrocycle plane, we have determined that macrocycle flexibility does not increase when the metalloporphyrin is reduced at $\beta$-pyrrole positions.Molecular mechanics calculations have been performed on a variety of metallo-(hydro)porphyrins in order to further examine the thermodynamic and structural properties of these molecules. It is shown that molecular mechanics does a good job in predicting the properties of metallo(hydro)porphyrins and metallo(hydro)porphyrin-ligand complexes provided that the bonded axial ligand is not sterically bulky. The molecular mechanics technique is used to determine how cavity size and flexibility vary as a function of tetrapyrrole reduction level and position. It is shown that macrocycle hole sizes change significantly upon reduction. In addition, molecular mechanics predicts that while reduction of a tetrapyrrole at $\beta$-pyrrole positions do not increase the macrocycle's flexibility, reduction at meso-positions does. Finally, the molecular mechanics technique has been extended to study the epimers of F$\sb{430}$. The technique has accounted for experimentally determined F$\sb{430}$ reactivity properties and has identified important structural features of the molecule.U of I OnlyETDs are only available to UIUC Users without author permissio