Long-Range Electron Transfer in Ruthenium-Labelled Myoglobin

Author: Karas, Jennifer Lynn

Year: 1989

Degree: Dissertation (Ph.D.)

Advisors: Chan, Sunney I.; Gray, Harry B.

Committee Members: Chan, Sunney I.; Bercaw, John E.; Marcus, Rudolph A.; Gray, Harry B.

Option: Chemistry

DOI: 10.7907/pc6d-xe62

Abstract

The driving force dependence of electron-transfer (ET) reactions has been investigated in protein derivatives of the type trans-Ru(NH₃)₄L-myoglobin (L=NH₃, pyridine, isonicotinamide). In these species, the labelling sites of the ruthenium moiety were determined by proton NMR spectroscopy. ET rates have been measured from the iron(II)-heme in myoglobin to both a₅Ru (a=NH₃) and a₄pyRu (py=pyridine) acceptors bound to histidine 48 (kET=0.04 and 2.5 s⁻¹; ΔE°=0.02 and 0.275, respectively). These kinetic results demonstrate for the ruthenium-labelled Mb system that long-range ET is reversible and that the dependence of the ET rates on reaction free energy closely follows Marcus theory. ET rates were also determined for high driving-force systems that were prepared by substitution of palladium and zinc(mesoporphyrin IX), (PdP, ZnP) into the a₄LRu(His-48) derivatives of Mb. ET from the excited state MbMP* (M=Pd or Zn) to the ruthenium acceptor was measured by transient spectroscopy. Reverse ET rates from the surface RuII to the ZnP radical cation, (RuII→ZnP⁺) were also determined. The ET rates range from 0.04 s⁻¹ for FeII→RuIII ET in a₅Ru(His-48)MbFe (ΔE°=0.02 V), to 3x10⁵ s⁻¹ for ZnP*→RuIII in a₄isnRu(His48)MbZn (ΔE°=1.2 V). The driving force dependence of the ET rates in the ruthenium-labelled Fe, Zn and Pd(His-48)Mb derivatives has provided information on the reorganization energy. Using Marcus theory, a reorganization energy of 1.5(2) eV has been estimated.

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