MCAA: New Models of Hydrogen Bonding in Iron-Sulfur Proteins

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This award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports research by Professor Marc Walters of New York University to address hydrogen bonding effects in the Fe-S proteins rubredoxin (Rd) and ferredoxin (Fd) through the use of model compounds. The modulation of hydrogen bonding to the iron sulfur electron-transfer sites in these proteins is thought to have a major influence on the redox potential of the clusters. The influence of N-H...S hydrogen bonds to iron-sulfur clusters will be examined with models of high electronic and structural fidelity and through the use of surface modified electrodes. Three sets of iron sulfide cluster complexes will be synthesized as models of Fd for X-ray crystallographic and X-ray absorption edge (XAS) studies. The first will consist of non-polar counterions. The second will be paired with polar non-hydrogen bonding counterions. A third set will have hydrogen bonds that link the complex (acceptor) to a counterion (donor). The hydrogen bond donors will include secondary or tertiary ammonium groups or amides bearing a pendant quaternary ammonium group. These should allow the formation of N-H...S hydrogen bonds to the bridging sulfide ligands. Chemically modified surfaces of gold electrodes will be employed as a new approach to study the redox effect of N-H.S hydrogen bonding in anionic electroactive metal thiolate model compounds. The electrode surface will be modified by a self-assembled monolayer (SAM) of long chain alkyl thiolate molecules anchored to the surface through sulfur and with amide groups at the terminal positions of the attached molecule. In this arrangement, redox cycling will occur for complexes that make contact with the amide-rich SAM, which is designed for hydrogen bond formation with the analyte. The use of amide terminal SAMs will obviate the need to synthesize unique hydrogen bonding donor ligands for each electrochemical study. Success in these experiments would greatly expand our ability to determine hydrogen bond induced redox potential shifts in a variety of Fe-S model compounds and possibly lead to tools for the selective detection of redox active species in solution. This work probes the effect of the surrounding protein on metal clusters involved in critical biological electron transport steps. The educational component includes participation by undergraduate students, a high school teacher and a high school student.