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XAS Structural Comparisons of Reversibly Interconvertible Oxo- and Hydroxo-Bridged Heme-Copper Oxidase Model Compounds

1996; American Chemical Society; Volume: 118; Issue: 1 Linguagem: Inglês

10.1021/ja951686b

1943-2984

S. Fox, Alaganandan Nanthakumar, Mårten Wikström, Kenneth D. Karlin, Ninian J. Blackburn,

Porphyrin and Phthalocyanine Chemistry

In this study on model compounds for the iron−copper dinuclear center in heme-copper oxidases, we (i) detail the synthesis and reversible acid−base interconversion of μ-oxo and μ-hydroxo complexes [(F8-TPP)FeIII−(O2-)−CuII(TMPA)]+ (1) and [(F8-TPP)FeIII−(OH-)−CuII(TMPA)]2+ (2) [F8-TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2−), TMPA = tris[(2-pyridylmethyl)amine]; (ii) compare their physical properties; (iii) establish the structure of 2 using XAS (X-ray absorption spectroscopy), a novel application of a three-body two-edge multiple-scattering (MS) analysis of ligand connectivity; and (iv) compare the XAS of 2 with those of 1 and an enzyme preparation. Complex 1 was prepared by reaction of [(TMPA)CuII(CH3CN)]2+ (3) and [(F8-TPP)FeIII−OH] (4) with triethylamine in acetonitrile (>70% yield). Salts 2-(ClO4)2 and 2-(CF3SO3)2 were synthesized (>60% yield) by addition of 3 with 4 in dichloroethane or by protonation of 1 with triflic acid. In a 1H-NMR spectroscopic titration (298 K) with triflic acid, the pyrrole 65 ppm resonance for 1 progressively converts to one near 70 ppm (71.5 for triflate, 68.5 for perchlorate), diagnostic of 2. The protonation−deprotonation rate is slow on the NMR time scale, the 1H-NMR spectral properties are consistent with antiferromagnetically coupled high-spin iron(III) and Cu(II) ions (S = 2 ground state), and the interaction is weaker in 2 (2, 5.5 ± 0.1 μB; 1, 5.1 ± 0.1 μB, Evans method). UV−vis spectroscopy was also used to monitor the conversion of 2 (Soret, 410 nm) to 1 (434 nm) using Et3N. The aqueous pKa for deprotonation of 2 is estimated as 8 ± 2.5. Both Fe and Cu K-edge XAS was performed on 1, 2, and μ-peroxo complex [{(TMPA)Cu}2(O2)]2+ (5). The strong MS interaction observed in the EXAFS of 1 is due to the nearly linear Fe−O−Cu moiety. Least-squares refinement of the Cu K-EXAFS of 1 gives Cu···Fe = 3.56 ± 0.03 Å, ∠Cu−O−Fe = 176 ± 5°, Cu−O = 1.83 ± 0.02 Å; the Fe K-EXAFS analysis gives Fe−O = 1.72 ± 0.02 Å, Fe···Cu = 3.54 ± 0.05 Å, ∠Fe−O−Cu = 172 ± 10°. The intense Fe−Cu (or Cu−Fe) feature is lacking in 2, but the iron-edge spectra do reveal a weaker MS ascribed to the Fe−Cu interaction. The Cu−O(H) and Fe−O(H) bonds are elongated in 2 (1.89 ± 0.02 Å and 1.87 ± 0.02 Å, respectively), with Fe···Cu = 3.66 ± 0.03 Å. This protonated complex is bent; ∠Fe−O(H)−Cu = 157 ± 5°. An EXAFS comparison with an enzyme preparation of the quinol oxidase aa3-600 from Bacillus subtilis supports the notion that μ-OH- complex 2 may be a good heme-Cu enzyme model for the resting state and/or turnover intermediate.