L. B. Dugad

Magnetic properties and electronic structure of manganese(III) porphyrins

Abstract The average magnetic susceptibility (1.2-100 K) and magnetisation (100–15000 Oe at 4.2 K) of two perchlorato manganese(III) porphyrins establish them to be high-spin, in contrast to the “anomalous” behaviour of analogous iron(III) porphyrins. An explanation of the origin of the zero-field splitting in high-spin manganese(III) porphyrins is presented.

1H-NMR characterization of cucumber peroxidases

Two peroxidase isoenzymes from Cucumber seedlings, one acidic (pI = 4) and one basic (pI = 9), were characterized by 1H-NMR spectroscopy. The NMR spectra were obtained in the native (ferric high-spin) and cyanide ligated (ferric low-spin) forms of both isoenzymes. The NMR spectral comparison of paramagnetically shifted resonances with those of the well characterized horseradish peroxidase C, HRP(C), isoenzyme indicates that both cucumber peroxidases have a protohemin IX prosthetic group with proximal histidine coordinated to the heme iron. The downfield heme 1H-NMR shift pattern is distinct for each isoenzyme, and this reflects presumably dissimilar heme active site environments. The basic isoenzyme shows less asymmetry in heme 1H-NMR signals as compared to the acidic isoenzyme or HRP(C) isoenzyme. It was also found that the acidic cucumber peroxidase exists predominantly as a monomeric species in solution with 30 kDa molecular mass as opposed to its earlier characterization as a 60 kDa dimeric protein.

Electronic structure of synthetic iron(III) porphyrins in pyridine and pyridinewater solutions: A proton magnetic resonance study

Abstract The results of variable temperature proton nuclear magnetic resonance, ESR and magnetic moment measurements on tetraphenylporphyrin iron(III) Cl and octaethylporphyrin iron(III) Cl in pure dry pyridine and in pyridine water mixtures are reported. In pure dry pyridine both Fe(TPP)Cl and Fe(OEP)Cl exist as undissociated adducts. The temperature dependence of the isotropic shifts and magnetic moments in pyridine water solutions is explained in terms of thermal equilibrium between high spin S = 5/2 and low spin S = 1/2 states with low spin state lying lower.

Electronic structure of planar cobalt(II) porphyrins: A proton magnetic resonance study

Abstract Variable-temperature proton NMR studies on planar cobalt(II) tetraaryl porphyrin complexes in solution have been used to deduce the electronic structure of the cobalt(II) ion. The analysis provides further evidence for the existence of a low-lying spin quartet 4 A 2 about 400 cm −1 above the ground state 2 A 1 .

Proton nuclear magnetic resonance studies on haemin chloride in pyridine–water solution

The effect of adding water to a pyridine (py) solution of protoporphyrin IX iron(III) chloride (haemin chloride) has been investigated by high-resolution (500 MHz) proton n.m.r. spectroscopy. When the amount of water is small (50% or less by weight), several iron(III) porphyrin species exist in solution in equilibrium; the solution contains six-co-ordinated high-spin [FeIII(pp)(py)Cl], ‘low-spin’[FeIII(pp)(py)2]Cl, and the aqua-haemin complex [FeIII(pp)(py)(H2O)]Cl (H2pp = protoporphyrin IX). At higher concentrations of water, only [Fe(pp)(py)(H2O)]Cl exists in the solution. The dependence of proton chemical shifts on pH was investigated and the results interpreted on the basis of a rapid exchange mechanism between the aqua and hydroxo complexes of the haemin. The aqua complex shows anomalous temperature dependence of the isotropic proton shifts for the ring methyl protons. These results, together with the temperature dependent magnetic moments in solution, indicate the existence of a spin equilibrium between the high-spin and low-spin states of the iron(III) ion in [Fe(pp)(py)(H2O)]Cl in solution.

Electronic structure of spin-mixed iron(III) porphyrins: A proton magnetic resonance study

The proton magnetic resonance studies on the perchlorato iron(III) porphyrins in solution have been described. The isotropic proton shifts in these complexes show anomalous temperature dependence, consistent with its unusual properties in solid state. TheNMR data have been analysed on the basis of a crystal field theory which includes lower asymmetric field and spin-orbit interaction. The analysis brings out that the ground state of the ferric ion in these porphyrin complexes exhibits the novel spin-mixed behaviour with spin-mixing betweenS = 3/2 andS = 5/2. The ground state is predominantly a spin quartet with the spin sextet being a very close lying excited state. Such a spin situation and spin-mixing have been speculated for the ferric ion in some ferricytochromec’. The present paper also highlights that the isotropic proton shift is very sensitive to the electronic structure of the metal ion and hence can be used to determine the electronic structure of the metal ion in heme systemsin solution.

NUCLEAR MAGNETIC RESONANCE OF PARAMAGNETIC METALLOPORPHYRINS

The present article reviews and discusses proton magnetic resonance studies on metalloporphyrins which provide good models for heme proteins in their physical and chemical properties. Emphasis is given on the discussion of the1Hnmr work done in our laboratory.