Photochemical Sciences Ph.D. Dissertations

Design, Characterization, and Electron Transfer Properties of Synthetic Metalloproteins

Date of Award

2006

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Advisor

Michael Ogawa

Abstract

The binding of Cu(I) to the random coil peptide C16C19-GGY produces a self-organized metal-peptide assembly which displays an intense room-temperature luminescence at 600 nm. It was shown that this synthetic metalloprotein exists as a 4-helix bundle which contains a cyclic Cu 4 S 4 cofactor in which each Cu(I) atom is bridged by two cysteine residues and has a terminal N/O ligand. The strong luminescence of the Cu(I) protein suggests that it might function as a photoinduced electron-transfer agent. The emission follows biexponential decay kinetics with t 1 = 1.0 µsec and t 2 = 7.5 µsec. These components have approximately equal amplitudes and the results indicate that the Cu 4 S 4 cofactor contains two independent lumophores. Both lifetime components are quenched by a series of [Ru(NH 3 ) 5 L] 3+ (L = chloro, amine, lutidine, pyridine, nicotinamide, and 3,5-dimethyl pyridine dicarboxylate). The quenching mechanism is assigned to a photoinduced electron-transfer event by transient spectroscopy. The results show the occurrence of bimolecular forward electron-transfer in the inverted Marcus regime.

Electron-transfer (ET) reactions occur between a negatively charged cyclic metallopeptide [Ru(bpy) 2 (phen-am)-cyclo(Cys-Glu-D-Glu-Glu-Pro-Glu-D-Glu)] 3- = Rucyclic, and ferricytochrome c, in which an acetamido linker was used to attach the ruthenium polypyridyl complex to the cysteine side chain of a head to tail cyclic peptide. In the presence of cyt c, the triplet state of ruthenium metallopeptide decays via parallel pathways that involve two different encounter peptide-protein complexes. That the electron transfer rate constants of both encounter complexes decrease with increasing viscosity demonstrates that the kinetics are gated by rate-limiting configurational changes occurring within the complexes. NMR experiments confirm that two separate conformations exist for Rucyclic.

Metallopeptides, 5-Chloro-PhenRuCE5G with a different redox potential but a similar conformation with those of RuCE5G give different driving forces of the excited-state ET reaction. The reorganization energy (λ) and donor-acceptor separation (r) of the preformed complex between metallopeptides and cyt c are determined to be 1.25 eV and 16.4 Å by measuring the actual electron transfer rates of these four metallopeptides at their most stable configurations. These values are comparable with those of some protein-protein systems reported previously.

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