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Hoffman and co-workers have observed a number of intermediates before P450-I formation but no spectroscopic evidence for the buitdup of compound I (11).
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15
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2942674083
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-1) antiferromagnetic coupling between its S = 1 iron(IV)-oxo unit and S = 1/2 radical. Calculations On ferryl radicals indicate that axialtigand spin density favors antiferromagnetic coupling in these intermediates (16).
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17
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2942654392
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With the use of Gaussian 98, we performed density functional calculations (B3LYP/6-311G) on a ferryl porphine with a methyl thiotate ligand and the appropriate distal ligand.
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18
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2942647716
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cam was expressed in Escherichia coli and purified by standard procedures. The camphor-bound (HS) sample was ≈ 4 mM In P450 (Rz = 1.56). The substrate-free (LS) sample was ≈ 2mM (Rz = 1.35). Both samples were in potassium phosphate buffer (40 mM and pH = 7.5). The HS sample also contained 100 mM KCI and 10 mM camphor. Horseradish peroxidase (Sigma) was purified according to standard methods. HRP (Rz - 3.2) and stock solutions of peracetic acid and ascorbate were used to generate HRP-II and HRP-I according to known procedures. XAS samples (≈ 2 mM in 0.3 M potassium phosphate) were at pH = 6.
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22
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2942634858
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With the use of a stopped flow apparatus, we examined the visible absorption spectrum of CPO-II under steady state conditions (using peracetic acid and ascorbate) at pH values of 3.5 and 6.9. No difference in the absorption maxima of the Soret (437 nm) or Q bands (541 and 571 nm) of CPO-II was observed. Minor variations in the bluest peak (372 nm) are attributable to changes in the relative steady state concentrations of compounds I and II as a function of pH (23). CPO-I absorbs at 367 and 688 nm. CPO undergoes an irreversible alkaline transition above pH = 7.
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2942652190
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Recent crystallographic investigations indicate that other protein ferryl species may be protonated as well. Reported Fe-O bond lengths in HRP-II, myoglobin-II (Mb-II), and cytochrome c peroxidase-I (CCP-I) are 1.8, 1.92, and 1.87 A, respectively (30-32). However, it is difficult to reconcile the observed ferryl stretching frequencies with the long Fe-O bonds found in these crystal structures. Much weaker stretches would be expected for these intermediates, if their Fe-O bonds were truly 8, 16, and 13% longer than that of HRP-I.
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2942691212
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CPO-I oxidizes chloride at pH = 3 (40). It must have a two-electron reduction potential in excess of 1.18 V at pH = 6. The two-electron reduction potential of peroxide (1.41 V at pH. = 6) provides an upper limit for the CPO-I reduction potential.
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40
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2942674082
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note
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We thank M. Bollinger, I. Dmochowski, J. Labinger, M. Machczynski, M. McGuirl, A. Tezcan, and J. Winkler for helpful discussions: G. George, I. Pickering, M. Latimer, B. Butler, A. Soo Hoo, S. Debeer George, D. Durkin, and other SSRL staff members for assistance with XAS measurements; and L Hager for a CPO sample used in the initial experiments. Supported by NIH (GM26730 to J.H.D. and DK19038 to H.B.G.), NSF, and the Arnold and Mabel Beckman Foundation. 18 February 2004; accepted 21 April 2004
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