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Ruthenium Bisbipyridine Complexes of Horse Heart Cytochrome c: Characterization and Comparative Intramolecular Electron-Transfer Rates Determined by Pulse Radiolysis and Flash Photolysis
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Section:Abstract
The reaction of [Ru(bpy)2L(H2O)]2+ (bpy = 2,2‘-bipyridine, L = imidazole, water) with reduced horse heart cytochrome c results in coordination of [RuII(bpy)2L] at the His 33 and His 26 sites. Coordination at the His 33 site gave a diastereomeric [RuII(bpy)2L]-His-cyt c(II) mixture favoring the Λ-Ru form regardless of the substituent on the bipyridine ligands, while substitution at the more buried His 26 site gave an isomeric distribution that varies according to the substituent on the bipyridine ligands. The diastereomeric aquoproteins (L = H2O) are distinguished by their redox potentials and their conversion to the corresponding fluorescent imidazole proteins. Intramolecular electron transfer between the reduced ruthenium bipyridine and cyt c(III) in [RuII(bpy•)(bpy)L]-His33-cyt c(III) was determined by reductive pulse radiolysis using the aqueous electron as a reducing agent, kret = (2.0 ± 0.3) × 105 s-1, and kret is independent of the sixth ligand L = H2O, imidazole. In addition, the rate constant for intramolecular electron transfer from cyt c(II) to the ruthenium(III) center in [RuIII(bpy)2L]-His33-cyt c(II) was determined by oxidative pulse radiolysis using azide and carbonate radicals. This rate is very sensitive to the nature of the sixth ligand. When L = H2O, the intramolecular electron-transfer rate for the major diastereomer Λ-cis-[RuIII (bpy)2(H2O)]-His33-cyt c(II) is k = 1.1 × 104 s-1 and is independent of pH between 5.6 and 8.3. The minor Δ-cis-[RuIII(bpy)2(H2O)]-His33-cyt c(II) isomer has pH-dependent electrochemistry and a lower rate of intramolecular electron transfer. Complete conversion from L = H2O to L = imidazole is slow, requiring more than 7 days in 1 M imidazole. A lower limit (k > 2 × 106 s-1) for the intramolecular electron-transfer rate constant in [RuIII(bpy)2(L)]-His33-cyt c(II), L = imidazole, could be obtained by pulse radiolysis in the absence of the slower reacting aquo species. This observation is in agreement with the value of 3 × 106 s-1 measured by flash photolysis. Earlier pulse radiolysis experiments primarily measured the aquoligated ruthenium protein, while the flash photolysis experiments measured the imidazole-ligated fraction because it is the only species oxidatively quenched in the photoinduced reactions. Intramolecular electron-transfer reactions for a new series of ruthenium bipyridine complexes, [Ru(dabpy)2L]-His33-cyt c proteins (dabpy = 4,4‘-diamino-2,2‘-bipyridine) (L = imidazole, pyridine, isonicotinamide and pyrazine), proceed with lower driving force, resulting in slower rate constants amenable to measurement by oxidative pulse radiolysis. The electron-transfer rate constants for this series spanned a wide range of the Marcus log k vs ΔG plot.
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This article has been cited by 5 ACS Journal articles (5 most recent appear below).
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Positive Activation Volume for a Cytochrome C Electrode Process: Evidence for a “Protein Friction” Mechanism from High-Pressure Studies
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Charge-Transfer Mechanism for Cytochrome c Adsorbed on Nanometer Thick Films. Distinguishing Frictional Control from Conformational Gating
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Efficient Generation of the Ligand Field Excited State of Tris-(2,2‘-bipyridine)-ruthenium(II) through Sequential Two-Photon Capture by [Ru(bpy)3]2+ or Electron Capture by [Ru(bpy)3]3+
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David W. Thompson, James F. Wishart, Bruce S. Brunschwig, and Norman SutinThe Journal of Physical Chemistry A2001 105 (35), 8117-8122The relaxation dynamics and product distribution resulting from the decay of high lying excited states generated via sequential two-photon capture by [Ru(bpy)3]2+ or electron capture by [Ru(bpy)3]3+ have been investigated by flash photolysis and pulse ...
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- Published In Issue May 29, 2000
- Received November 18, 1999
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O)(PR3)2 or the Cationic Hydride Complex [trans,trans-ReH(CO)2(NO)(PPh3)2+][SO3CF3-]
