Okhil K. Medhi

On the nature of protoporphyrin (IX) iron(III) in aqueous solution at and below the pH of precipitation

Breve presentation des travaux concernant la caracterisation spectrometrique des especes complexees du fer(III) avec une porphyrine substituee

A six-coordinate high spin protoporphyrin IX iron(III) complex as a model for ferric haemproteins: Mössbauer spectrum of bisaquo(protoporphyrinato IX)iron(III) encapsulated in aqueous detergent micelles

Abstract The Mossbauer spectrum is reported for a six- coordinate high spin iron(III) protoporphyrin complex, [(PPIX)Fe(H 2 O) 2 ] + , which is monodispersed in an aqueous frozen solution of sodium dodecyl sulphate micelles. The isomer shift of 0.46 mm −1 and the quadrupole splitting of 1.19 mm 1 are similar to those of aquo met myoglobin. These results are compared with the literature results on other high spin six-coordinated models and with those of some proteins. This is the first report of a Mossbauer study on a haem encapsulated in a detergent micelle.

A Mössbauer study on the (protoporphyrinato IX)iron(II) complexes of imidazole and substituted imidazoles as axial ligands in Frozen aqueous solutions

Abstract A Mossbauer study on (protoporphyrinato IX)- iron(II) complexes of the type [(PPIX)Fe(II)(ImR)n] where ImR=imidazole (ImH), 1-methylimidazole (1-MeIm) or 2-methylimidazole (2-MeImH) indicates that the ImH and 1 MeIm complexes are low spin S = O and monomeric in 1:1 ethanol:water whereas the 2 MeImH complex in this solvent is present as a five-coordinated high spin (S = 2) aggregate. The quadrupole splitting (ΔEQ) of the bis(1-MeIm) compound was found to be similar to that of reduced cytochrome b5·. In a frozen aqueous solution of a detergent, ethyltrimethylammonium bromide (CTAB), the five-coordinated complex [(PPIX)Fe(II)- (2-MeImH)] was obtained as a monomeric high spin compound with a ΔEQ similar to that of deoxy myoglobin. A low spin bis(2-MeImH) complex was also found in the frozen (78 K) solution of the detergent with ΔEQ = 1.26 which is similar to that found for the low spin [(PPIXFe(II)] complexes with sterically hindered amines. Analysis of the isomer shifts and the quadrupole splittings of the low spin bis(ImR) complexes indicate that the dominant mechanism in the covalent bonding is the σ donation of imidazole → iron and the observed trend of σ basicity of the coordinated ligands found is: 2-MeImH

The Mössbauer spectrum of an intermediate spin (S= 1) four-co-ordinated (protoporphyrinato IX)iron(II) complex in a frozen aqueous solution of cetyltrimethylammonium bromide

The Mossbauer spectrum of the four-co-ordinated haem, (protoporphyrinato IX)iron(II) monodispersed in an aqueous detergent solution of 5% cetyltrimethylammounium bromide (CTAB), is that of a typical intermediate spin (S= 1) system [the isomer shift is 0.57(2) mm s–1 and the quadrupole splitting (ΔEQ) is 1.44(2) mm s–1]; the results are comparable to those found in synthetic (porphyrinato)iron(II) analogues, the ΔEQ values follows the σ donating ability (basicity) of the porphyrin ligands and the two lines of the quadrupole doublet are of unequal intensity at 78 K.

Mössbauer studies on cytochrome b models: bis ligated complexes of iron(III) protoporphyrinate IX with imidazole and substituted imidazoles

Iron-57 Mossbauer spectra at 80 K have been recorded for a series of bis ligated complexes of iron(III) protoporphyrinate IX (3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionate)[FeL]+ with imidazole (Him), 1-methylimidazole (1Me-im), and 2-methylimidazole (2Me-im). The spectra were recorded in frozen solutions of dimethyl sulphoxide (dmso), acetone, and 50%(v/v) ethanol–water and also in the presence of hydrogen-bonding agents such as trifluoroethanol or 1,10-phenanthroline in acetone. This is the first report of Mossbauer spectroscopic studies on the solvent dependence of a series of substituted imidazole complexes of [FeLCl]. Significant new results are obtained which are interpreted in the light of reported e.s.r., n.m.r., and electronic absorption data. A value of 2.43 mm s–1 for the quadrupole splitting (ΔEQ) of the imidazolate complex [FeL(Him)(im)] is assigned to a parallel orientation of the planar axial ligands, whereas a value of ΔEQ= 1.87 mm s–1 for the [FeL(2Me-im)2]+ is assigned to a perpendicular orientation of the imidazole planes. Large linewidths of about 0.6–0.98 mm s–1 are attributed to the presence of a statistical distribution of imidazole planes around the thermodynamically stable arrangement. The influences of hydrogen bonding on the Mossbauer parameters are discussed. The 2Me-im complex and the 1Me-im complex in dmso show slow spin–lattice relaxation at 80 K.

Mössbauer spectroscopic evidence for molecular complex formation between histidine and μ-oxo-bis(protoporphyrinato IX)iron(III)

Abstract Compelling Mossbauer spectroscopic evidence is presented here for the formation of molecular complexes of μ-oxo-bis(protoporphyrinato IX)iron(III) ([(PPIXFe) 2 O]) with histidine. The quadrupole splitting of [(PPIXFe) 2 O] (Δ E Q =0.56±0.01 mm s −1 ) is modified considerably in the molecular complexes to give Δ E Q =0.43±0.01 mm s −1 in the histidine complex. Histamine and N -α-acetyl histidine also form similar molecular complexes. Mossbauer studies and a molecular scale model suggest that the donor-acceptor complex is formed via π-π charge transfer interaction between the imidazole of histidine with one of the pyrrole rings of the porphyrin. The role of the amino group in the histidine side chain is to form a NH··· − OOCR hydrogen bond with the propionic carboxylate group of protoporphyrin IX. This aligns the imidazole ring of histidine parallel to the pyrrole ring of the porphyrin. It is suggested that the apoprotein conformation in a haem protein may provide such a favourable alignment of an aromatic amino acid residue in the protein pocket so as to favour such a π-π interaction. This is to our knowledge the first Mossbauer spectroscopic study on a molecular π complex involving a porphyrin moiety.

Mössbauer spectroscopic studies on the bis(histidine) adducts of (protoporphyrinato IX)iron(II)

Abstract Mossbauer spectra in frozen aqueous ethanolic solution of complexes of the type [(PPIX)Fe(II)-(HisR)2] are reported where HisR = histidine, N-α-acetyl histidine, pilocarpate or histamine. At alkaline pH 12.0, the complexes are found to be monomeric and low spin with quadrupole splittings (ΔEQ) similar to that of reduced cytochrome b5. The ΔEQ values are typical of bis(imidazole) coordination to (protoporphyrinato IX)iron(II) (ΔEQ = 0.9–1.04 mm s−1). The results indicate that hydrogen bonding to the NH of the coordinated imidazole considerably decreases the ΔEQ values.

Models of cytochrome b: Mössbauer studies on bis-ligated complexes of (protoporphyrinato IX)iron(III) with histidine and its substituted derivatives

Mossbauer spectroscopic studies have been carried out on a series of complexes of the type [FeL(L′)2]+, where H2L = 3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionic acid, L′= histidine, Nα-acetylhistidine, histamine, or pilocarpate. Measurements were made at various pH values in the range 7.5–12.0 in 20%(v/v) ethanol–water solution frozen at 80 K. When the pH is 8.0–8.5, the major species are low-spin bis complexes [δ= 0.25(2) mm s–1 and ΔEQ= 2.0–2.2 mm s–1]. These are rapidly converted into a molecular complex of histidine and [(FeL)2O] at high pH (10.1–12.0). The results for the bis(histidine) and related complexes show that histidine binds as a sterically hindered imidazole and that the iron–imidazole bonds are weak. The ΔEQ value of 2.14 mm s–1 and large linewidths (0.48–1.22 mm s–1) of the complexes indicate that the two imidazole planes in the bis(histidine) complex are non-parallel with a large angle between the planes. The observation of asymmetric quadrupole doublets and broad lines are typical of slow spin–lattice relaxation of iron similar to that observed for cytochrome C. The present Mossbauer spectroscopic results for the bis(histidine) complexes are similar to those found for low-spin iron(III)-cytochromes and cytochrome b5. Steric strain due to the histidine side-chains and electrostatic interactions between the charged groups and the porphyrin propionate carboxylates are found to influence the iron electronic structure and the imidazole plane orientations.

A frozen solution mössbauer spectroscopic study of six-coordinated high spin (S = 2)bis(tetrahydrofuran)(protoporphyrinato IX)iron(II)

Abstract The Mossbauer spectrum of six-coordinated iron-(II) bis(tetrahydrofuran)(protoporphyrinato IX), [(PPIX)Fe(II)(THF)2], was recorded in frozen solutions of tetrahydrofuran, 5% aqueous cetyltrimethylammonium bromide (CTAB) and 5% aqeuous sodium dodecylsuphate (SDS) detergents. The bis(THF) adduct of [(PPIX)Fe(II)] was found to be high spin (S = 2) with a quadrupole splitting of 2.49 mm s−1. In the absence of the detergents the formation of the compound was considerably suppressed by [(PPIX)Fe(II)] aggregation. The basicity of the porphyrin has an influence on the quadrupole splitting of the compound.

Low-spin iron(III) porphyrins encapsulated in aqueous detergent micelles: proton- and nitrogen-15 nuclear magnetic resonance studies

Variable temperature 1H and 15N n.m.r. studies have been made on low-spin iron(III) protoporphyrin IX (3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionic acid) complexes, [FeL(CN)2]– and [FeL(py)(CN)], encapsulated in aqueous detergent micelles. The haem is shown to be monodispersed in the micellar solution, which ensures the paramagnetic shift is free from the effect of aggregation. The effects of the hydrophobic interactions of the micellar cavity on the linewidth, and the magnitude and spread of the chemical shift, are discussed and compared with those in absence of micelles and in haemoproteins. The temperature dependence of the paramagnetic shift of the haem methyl proton shows deviation from Curies law, which is discussed in terms of the electronic structure of haem. The effect of the size and hydrophobicity of the micelles is further shown in the 15N n.m.r. signal of the bound CN–, where the downfield shift follows the trend: absence of detergent < sodium dodecyl sulphate < Triton X-100 < hexadecyltrimethylammonium bromide.