XAS structural comparisons of reversibly interconvertible oxo- and hydroxo-bridged heme-copper oxidase model compounds

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 mu-oxo and mu-hydroxo complexes [(F-8-TPP)Fe-III-O2-)-Cu-II(TMPA)](+) (1) and [(F-8-TPP)Fe-III-(OH-)-Cu-II(TMPA)](2+) (2) [F-8-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)Cu-II(CH3CN)](2+) (3) and [(F-8-TPP)Fe-III-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 H-1-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 H-1-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 mu(B); 1, 5.1 +/- 0.1 mu(B), Evans method). W-vis spectroscopy was also used to monitor the conversion of 2 (Soret, 410 nm) to 1 (434 nn) using Et(3)N. The aqueous pK(a) for deprotonation of 2 is estimated as 8 +/- 2.5. Both Fe and Cu K-edge XAS was performed on 1, 2, and mu-peroxo complex [{(TMPA)Cu}(2)(O-2)](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 Angstrom, angle Cu-O-Fe = 176 +/- 5 degrees, Cu-O = 1.83 +/- 0.02 Angstrom; the Fe K-EXAFS analysis gives Fe-O = 1.72 +/- 0.02 Angstrom, Fe ... Cu = 3.54 +/- 0.05 Angstrom, angle Fe-O-Cu = 172 +/- 10 degrees. 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 Angstrom and 1.87 +/- 0.02 Angstrom, respectively), with Fe ... Cu = 3.66 +/- 0.03 Angstrom. This protonated complex is bent; angle Fe-O(H)-Cu = 157 +/- 5 degrees. An EXAFS comparison with an enzyme preparation of the quinol oxidase aa(3)-600 from Bacillus subtilis supports the notion that mu-OH- complex 2 may be a good heme-Cu enzyme model for the resting state and/or turnover intermediate.