Bidirectional Photoinduced Electron Transfer in Ruthenium(II)-Tris-bipyridyl-Modified PpcA, a Multi-heme c-Type Cytochrome from Geobacter sulfurreducens

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• 作者：Oleksandr Kokhan ; Nina S. Ponomarenko ; P. Raj Pokkuluri ; Marianne Schiffer ; Karen L. Mulfort ; David. M. Tiede
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：June 18, 2015
• 年：2015
• 卷：119
• 期：24
• 页码：7612-7624
• 全文大小：526K
• ISSN：1520-5207

文摘

PpcA, a tri-heme cytochrome c₇ from Geobacter sulfurreducens, was investigated as a model for photosensitizer-initiated electron transfer within a multi-heme 鈥渕olecular wire鈥?protein architecture. Escherichia coli expression of PpcA was found to be tolerant of cysteine site-directed mutagenesis, demonstrated by the successful expression of natively folded proteins bearing cysteine mutations at a series of sites selected to vary characteristically with respect to the three -CXXCH- heme binding domains. The introduced cysteines readily reacted with Ru(II)-(2,2鈥?bpy)₂(4-bromomethyl-4鈥?methyl-2,2鈥?bipyridine) to form covalently linked constructs that support both photo-oxidative and photo-reductive quenching of the photosensitizer excited state, depending upon the initial heme redox state. Excited-state electron-transfer times were found to vary from 6 脳 10^鈥?2 to 4 脳 10^鈥? s, correlated with the distance and pathways for electron transfer. The fastest rate is more than 10³-fold faster than previously reported for photosensitizer鈥搑edox protein constructs using amino acid residue linking. Clear evidence for inter-heme electron transfer within the multi-heme protein is not detected within the lifetimes of the charge-separated states. These results demonstrate an opportunity to develop multi-heme c-cytochromes for investigation of electron transfer in protein 鈥渕olecular wires鈥?and to serve as frameworks for metalloprotein designs that support multiple-electron-transfer redox chemistry.