Electron paramagnetic resonance of the excited triplet state of metal-free and metal-substituted cytochrome c.
The photoactivated metastable triplate states of the porphyrin (free-base, i.e., metal-free) zinc and tin derivatives of horse cytochrome c were investigated using electron paramagnetic resonance. Zero-field splitting parameters, line shape, and Jahn-Teller distortion in the temperature range 3.8-150 K are discussed in terms of porphyrin-protein interactions. The zero-field splitting parameters D for the free-base, Zn and Sn derivatives are 465 x 10(-4), 342 x 10(-4) and 353 x 10(-4) cm-1, respectively, and are temperature invariant over the temperature ranges studied. AN E value at 4 K of 73 x 10(-4) cm-1 was obtained for Zn cytochrome c, larger than any previously found for Zn porphyrins derivatives of hemeproteins, showing that the heme site of cytochrome c imposes an asymmetric field. Though the E value for Zn cytochrome c is large, the geometry of the site appears quite constrained, as indicated by a spectral line shape showing a single species. Intersystem crossing occurred predominantly to the T2 > zero-field spin sublevel. EPR line shape changes with respect to temperature of Zn cyt c are interpreted in terms of vibronic coupling, and a maximum Jahn-Teller crystal-field splitting of approximately 180 cm-1 is obtained. Sn cytochrome c in comparison with the Zn protein exhibits a photoactivated triplet line shape that is less well resolved in the X-Y region. The magnitude of E value is approximately 60 x 10(-4) cm-1 at 4 K; its value rapidly tends toward zero with increasing temperature, from which a value for the Jahn-Teller crystal-field splitting of > or = 40 cm-1 is estimated. In contrast to those for the metal cytochromes, the magnitude of E value for the free-base derivative was essentially zero at all temperatures studied. This finding is discussed as a consequence of an excited-state tautomerization process that occurs even at 4 K.
Angiolillo
P J
PJ
Johnson Research Foundation, Department of Biochemistry and Biophysics, School of Medicine, University of Pennsylvania, Philadelphia 19104, USA.
Vanderkooi
J M
JM
D000818:Animals; D003574:Cytochrome c Group / Q000737:chemistry* / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D006736:Horses; D007501:Iron; D007700:Kinetics; D049449:Luminescence; D008433:Mathematics; D008665:Metalloporphyrins; D008962:Models, Theoretical; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D013816:Thermodynamics; D014001:Tin; D015032:Zinc
D003574:Cytochrome c Group; D008665:Metalloporphyrins; D014001:Tin; D007501:Iron; D015032:Zinc