Intramolecular electron transfer in cytochrome c has

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Intramolecular electron transfer in cytochrome c has been investigated by attaching photoactive Ru complexes to the protein surface. 98 ,161 Ru(bpyh(C0 3 ) (bpy = 2,2' -bipyridine) has been shown to react with surface His residues to yield, after addition of excess imidazole (im), Ru(bpyh(im)(His)2+. The pro- tein-bound Ru complexes are luminescent, but the excited states (*Ru 2 +) are rather short lived (1":5 100 ns). When direct electron transfer from *Ru 2 + to the heme cannot compete with excited-state decay, electron-transfer quenchers (e.g., Ru(NH 3 )63+) are added to the solution to intercept a small fraction (I-I 0%) of the excited molecules, yielding (with oxidative quenchers) Ru3+. If, before laser excitation of the Ru site, the heme is reduced, then the Fe 2 + to Ru3+ reaction (k et ) can be monitored by transient absorption spectroscopy. The k et values for five different modified cytochromes have been reported: (Ru(His- 33), 2.6(3) x 10 6 ; Ru(His-39), 3.2(4) x 10 6 ; Ru(His-62), 1.0(2) x 10 4 ; Ru(His- 72),9.0(3) x 10 5 ; and Ru(His-79), > 10 8 S-I).162,163 According to Equation (6.27), rates become activationless when the reaction driving force (- ~GO) equals the reorganization energy (A), The driving force (0.74 eV) is approximately equal to the reorganization energy (0.8 eV) esti- mated for the Ru(bpyh(im)(His)-cyt c reactions. 161 The activationless (maxi- mum) rates (k max ) are limited by Hh, where H AB is the electronic matrix ele- ment that couples the reactants and products at the transition state. Values of k max and H AB for the Fe 2 + to Ru3+ reactions are given in Table 6.7. Calculations that explicitly include the structure of the intervening medium 81- 86,164-169 have been particularly helpful in developing an understanding of distant electronic couplings. As discussed in Section IV.A, the couplings in proteins can be interpreted in terms of pathways comprised of covalent, H-bond, and through-space contacts. An algorithm has been developed 85,170 that searches a Table 6.7 Electron-transfer parameters 163 for Ru(bpYb(im)(His-X)- cytochromes c. H AB (cm-') X k max (5-') [Fe2+ _Ru 3t ] d (A) nef/ crl(A) 79 > 1.0 x 10 8 >0.6 4.5 8 (8C) 11.2 39 3.3 x 10 6 0.11 12.3 14.0 (l1C) 19.6 (lH) 33 2.7 x 10 6 0.097 11.1 13.9 (11C) 19.5 (lH) 72 9,4x 10 5 0.057 8,4 17.6 (7C) 24.6 (lS) 62 1.0 x 10 4 0.006 14.8 20.6 (l6C) 28.8 (2H) ac covalent bond, H hydrogen bond, S = space jump.
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357 33~ , (A) 62 \-e 72 (B) b 10 x 1 79 7J '" E 39 39 -'<: 33_- 33 0> 72- -72 .Q 5 62- 62 5 10 (d-3), A 15 40 Figure 6.35 (A) Electronic coupling pathways to the heme from Ru-modified residues in native (His-33 horse heart, His-39 yeast), genetically engineered (His-62 yeast), and semisynthetic (His-n, His-79 horse heart) cytochromes c. Solid lines are covalent bonds; dashed lines are hydrogen bonds; and the dotted line (His-n pathway) is a space jump. (B) The left half of the diagram (a) shows maximum electron-transfer rate vs. d minus 3 A (van der Waals contact). Exponential-decay line with I x 10 13 S - 1 intercept and 1.4 A-I slope. The right half of the diagram (b) shows maximum rate vs.
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