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Nanosecond Photoreduction of Cytochrome P450cam by Channel-Specific Ru-diimine Electron Tunneling Wires

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journal contribution
posted on 23.09.2003, 00:00 by Alexander R. Dunn, Ivan J. Dmochowski, Jay R. Winkler, Harry B. Gray
We report the synthesis and characterization of Ru-diimine complexes designed to bind to cytochrome P450cam (CYP101). The sensitizer core has the structure [Ru(L2)L‘]2+, where L‘ is a perfluorinated biphenyl bridge (F8bp) connecting 4,4‘-dimethylbipyridine to an enzyme substrate (adamantane, F8bp-Ad), a heme ligand (imidazole, F8bp-Im), or F (F9bp). The electron-transfer (ET) driving force (−ΔG°) is varied by replacing the ancillary 2,2‘-bipyridine ligands with 4,4‘,5,5‘-tetramethylbipyridine (tmRu). The four complexes all bind P450cam tightly:  Ru−F8bp-Ad (1, Kd = 0.077 μM); Ru−F8bp-Im (2, Kd = 3.7 μM); tmRu−F9bp (3, Kd = 2.1 μM); and tmRu−F8bp-Im (4, Kd = 0.48 μM). Binding is predominantly driven by hydrophobic interactions between the Ru-diimine wires and the substrate access channel. With Ru−F8bp wires, redox reactions can be triggered on the nanosecond time scale. Ru-wire 2, which ligates the heme iron, shows a small amount of transient heme photoreduction (ca. 30%), whereas the transient photoreduction yield for 4 is 76%. Forward ET with 4 occurs in roughly 40 ns (kf = 2.8 × 107 s-1), and back ET (FeII → RuIII, kb ≈ 1.7 × 108 s-1) is near the coupling-limited rate (kmax). Direct photoreduction was not observed for 1 or 3. The large variation in ET rates among the Ru-diimine:P450 conjugates strongly supports a through-bond model of Ru−heme electronic coupling.