Michael D. Ryan, Ph.D.
Professor, Analytical Chemistry
(414) 288-1625
E-mail: michael.ryan@mu.edu

Professor Ryan obtained his B.S. degree in 1969 from the University of Notre Dame and his Ph.D. in 1973 from the University of Wisconsin. His interest in electroanalytical chemistry led him to a postdoc at the University of Arizona before joining the faculty here in 1974. He also took a sabbatical leave at the University of Iowa with D. Coucouvanis (1981-82), and the University of Paris VII with J.-M. Saveant (1995-96).

Research Fields
The primary aim of our research has been to investigate multielectron reactions that occur either in solution or at the electrode surface. The system that has been studied most intensely by us is the reduction of nitrite to ammonia. In nature, this reaction is catalyzed by a siroheme containing enzyme, nitrite reductase, which converts nitrite to ammonia without any intermediates being released in the process. Only nitric oxide has been observed as an intermediate.Several avenues are being investigated in modeling this redox process. First, models of siroheme (tetrahydroporphine) have been synthesized to determine if the porphyrin macrocycle has any effect on the redox chemistry of the iron complex. Second, the electrochemical reduction of the iron-nitrosyl complex in the presence of weak acids is being investigated in order to elucidate the reduction mechanism and to identify intermediates in the reaction. Stable intermediates can be characterized by spectroscopic techniques such as UV/visible, IR and NMR spectroscopy. Unstable intermediates can be identified by combining an optical method with a thin-layer electrolysis cell (spectroelectrochemistry). Third, work has begun on the isolation and study of the enzyme itself.The electrochemical reduction of iron-porphyrins and hydroporphyrins has been studied by other workers and ourselves. These studies have involved the reaction of the iron complexes with nitrite and other ligands in order to identify chemical differences between the porphyrins and hydroporphyrins. The iron(III) reduction is essentially unaffected by the macrocycle, but differences have been observed for the iron(II) and iron(I) reduction. The electrochemical reduction of Fe(TPP)(NO) leads to a stable product, Fe(TPP)(NO)-, which has been characterized by NMR, UV/visible and resonance Raman spectroscopy. The first reduction of Fe(P)(NO) leads to a significant weakening of the NO band and a strengthening of the Fe-N bond. This is consistent with the addition of the electron into a pi* NO orbital. Surprisingly, Fe(TPP)(NO) is not a good base, and reacts only slowly with weak acids such as phenol. In coulometry, the two electron reduced product, Fe(TPP)(NO)2-, reacts rapidly with phenol and substituted phenols to form ammonia. Reaction of phenols with Fe(TPP)(NO)- itself, leads to regeneration of Fe(TPP)(NO), while the dianion reacts with phenol with further reduction of NO.Lastly, we have isolated a sulfite reductase from E. coli which is capable of reducing nitrite to ammonia. The prosthetic groups of the sulfite reductase are identical to the nitrite reductase. Interestingly, this enzyme can be reduced directly at a gold electrode, albeit slowly. The reduction was monitored using spectroelectrochemistry.

Selected Publications

For complete publications text: e-publications


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