1H-NMR and rosonance Raman spectra of octaethylporphyrinatoiron(III) perchlorate and its mono imidazole adduct

New pinch-porphyrin complexes with quantum mixed spin ground state S=, of iron (III) and their catalytic activity as peroxidase

New complexes of the pinch-porphyrin family were obtained from the dimethylester of (proto-, meso-, and deutero-porphyrinato)iron(III) with the ligand [N,N'-bis-pyridin-2-ylmethyl-propane-1,3-diamine] 1-3 and with the ligand [N-pyridin-2-ylmethyl-N'-[3-[(pyridin-2-ylmethyl)-amino]-propyl]-propane-1,3-diamine] 4-6. The UV/VIS studies of 1-6 indicate an increase in the distortion of the ligand field excited state. The 1H NMR spectra of 1-6 at RT and over the range 223-328 K show iron(III)-complexes with quantum mixed spin state (qms) S=5/2, S=3/2. The chemical shifts of the meso protons are consistent with qms state S=3/2, S=5/2, where the S=3/2 spin state is lowest in energy. For methyl-heme the chemical shifts are also consistent with a qms state but now the S=5/2 ground state is lowest in energy. ESR spectra of 1-6 show two different species, B and C, of iron(III) with qms, S=5/2, S=3/2 consistent with the 1H NMR results. Species B with 70% of S=5/2 and species C with 72.5% of S=3/2. The catalytic activity as peroxidase of 1-6 was quantified by guaiacol test; their theoretical maximum rate constants were k(cat) approximately 10(2)-10(3) M(-1) s(-1). A quantitative empirical correlation is found: the higher the 32 spin contribution to the qms state and the higher proportion of this species into the samples, the higher the peroxidase activity. Such a correlation was also obtained for pinch-porphyrins already reported.

Models for cytochromes c': spin states of mono(imidazole)-ligated (meso-tetramesitylporphyrinato)iron(III) complexes as studied by UV-Vis, 13C NMR, 1H NMR, and EPR spectroscopy

A number of mono(imidazole)-ligated complexes of perchloro(meso-tetramesitylporphyrinato)iron(III), [Fe(TMP)L]ClO(4), have been prepared, and their spin states have been examined by (1)H NMR, (13)C NMR, and EPR spectroscopy as well as solution magnetic moments. All the complexes examined have shown a quantum mechanical spin admixed state of high and intermediate-spin (S = 5/2 and 3/2) states though the contribution of the S = 3/2 state varies depending on the nature of axial ligands. While the complex with extremely bulky 2-tert-butylimidazole (2-(t)()BuIm) has exhibited an essentially pure S = 5/2 state, the complex with electron-deficient 4,5-dichloroimidazole (4,5-Cl(2)Im) adopts an S = 3/2 state with 30% of the S = 5/2 spin admixture. On the basis of the (1)H and (13)C NMR results, we have concluded that the S = 3/2 contribution at ambient temperature increases according to the following order: 2-(t)BuIm < 2-(1-EtPr)Im < 2-MeIm <or= 2-EtIm <or= 2-(i)PrIm < 4,5-Cl(2)Im. The effective magnetic moments determined by the Evans method in CH(2)Cl(2) solution are 5.9 and 5.0 mu(B) at 25 degrees C for [Fe(TMP)(2-(t)BuIm)]ClO(4) and [Fe(TMP)(2-MeIm)]ClO(4), respectively, which further verify the order given above. Comparison of the NMR and EPR data has revealed that the S = 3/2 contribution changes sensitively by the temperature; the S = 3/2 contribution decreases as the temperature is lowered for all the mono(imidazole) complexes examined in this study. The solvent polarity also affects the spin state; polar solvents such as methanol and acetonitrile increase the S = 3/2 contribution while nonpolar solvents such as benzene decrease it. These results are explained in terms of the structurally flexible nature of the mono(imidazole) complexes; structural parameters such as the Fe(III)-N(axial) bond length, displacement of the iron from the N4 core, tilting of the Fe(III)-N(axial) bond to the heme normal, orientation of the coordinated imidazole ligand, etc., could be altered by the nature of the axial ligands as well as by the solvent polarity and temperature. Some mysteries on the spin states of cytochromes c' isolated from various bacterial sources are possibly explained in terms of the flexible nature of the mono(imidazole)-ligated structure.

Cytochromes c' in their reaction with ethyl isocyanide

The binding of ethyl isocyanide (EIC) to a representative number of cytochromes c' is demonstrated. Spectroscopic and equilibrium constants have been measured and compared for the binding of EIC to cytochromes c' from the photosynthetic bacteria Chromatium vinosum, Rhodopseudomonas palustris, Rhodospirillum rubrum, and Rhodopseudomonas sphaeroides. While the absorption spectra of the EIC complexes resemble those of EIC complexes of other high-spin hemoproteins, the Soret half band widths and extinction coefficients per heme exhibit more than a 2-fold difference with the values of C. vinosum being most similar to those of Rh. sphaeroides and of Rh. palustris similar to those of Rs. rubrum. The cytochromes exhibit binding equilibria consistent with the ligation of one molecule of EIC per heme in contrast to the reported binding of more than one molecule of CO per heme. The binding constants exhibit more than a 1000-fold difference with the values of C. vinosum being closely similar to those of Rh. sphaeroides and of Rh. palustris similar to those of Rs. rubrum. The lack of correlation between EIC and CO binding properties indicates that electronic factors do not determine the difference in EIC binding properties. The observed correlation between the extinction coefficients, half band widths, and equilibrium constants for EIC complex formation provides the first spectroscopic evidence that the differences in binding properties are associated with sterically hindered ligation to the heme. Although the differences in binding properties provide evidence of steric hindrance, the EIC binding constants for particular cytochromes c' indicate that the distal heme binding site is more accessible than previously indicated.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of imidazoles to manganese (III) protoporphyrin in water

Equilibrium constants for the addition of water to manganese(III) protoporphyrin in acetone were measured at 21 degrees C by the visible absorption method. The binding constants of the first and second water molecules were determined to be 209 +/- 3 M-1 and 22 +/- 1 M-1, respectively. This observation suggests that in water the fifth and sixth coordination positions of manganese(III) protoporphyrin are coordinated by two water molecules and that manganese(III) protoporphyrin is monomeric. Addition of 1-methylimidazole and 2-methylimidazole to manganese(III) protoporphyrin in water was also investigated spectrophotometrically, and the respective monoimidazole adducts were found to be born in appreciable amounts. The binding constants of the first imidazole were 24.7 +/- 1.0 M-1 for 1-methylimidazole and 16.5 +/- 0.9 M-1 for 2-methylimidazole in 0.1 M tris(hydroxymethyl)aminomethane buffer at pH 8.0 and 21 degrees C. The steric effect of the 2-methyl group in te aquomanganese(III) (2-methylimidazole) species is expected to mimic the proximal strain in the alpha subunits of human adult manganese(III) hemoglobin. The energy associated with the strain in the Mn(III)-imidazole bond of the model complex was estimated to be 240 cal/mol. The small strain energy in the Mn-(III)-imidazole bond is easily tensed by a slight energy and implies that the major part of the quaternary transition energy in manganese(III) hemoglobin by inositol hexaphosphate binding is widely delocalized around the globin moiety, as has been proposed for hemoglobin.