IX Iron Research Articles

Studies on the binding of nitrogenous bases to protoporphyrin IX iron(II) in aqueous solution at high pH values

Studies are reported on the formation of low-spin six-coordinate [Fe(PPIX)L2] complexes from iron(II) protoporphyrin where L is one of a series of nitrogenous ligands (aliphatic, aromatic or heterocyclic). The bonding constants have been determined by titration of the metal complex with these ligands and are compared in relation to previous studies. The adduct formation was monitored utilising optical spectroscopy. In addition, Mӧssbauer spectroscopic experiments were conducted to monitor the electronic environment around the central iron atom in these complexes. The two complementary spectroscopic methods indicated that all nitrogen ligands formed low-spin octahedral complexes. The magnitude of the overall binding constants (β2 values) are discussed and related to (a) the pKa values of the free ligands and (b) the Mössbauer parameter ΔEQ, which represents the quadrupole splitting of the haem iron. The β2 and ΔEQ values are also discussed in terms of the structure of the ligand. Cooperative binding was observed for nearly all the ligands with Hill coefficients close to 2 for iron(II) protoporphyrin; one of these ligands displayed a much greater affinity than any we previously studied, and this was a direct consequence of the structure of the ligand. Overall conclusions on these and previous studies are drawn in terms of aliphatic ligands versus aromatic ring structures and the absence or presence of sterically hindered nitrogen atoms. The implications of the work for the greater understanding of haem proteins in general and in particular how the nitrogenous ligand binding results are relevant to and aid the understanding of the binding of inhibitor molecules to the cytochrome P450 mono-oxygenases (for therapeutic purposes) are also discussed.Graphical abstractChanges in the electronic absorption spectra of five-coordinate [Fe(II)(PPIX)(2-MeIm)] that occurred as the temperature was lowered from room temperature to 78° K

Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes

ABSTRACTSix iron complexes (FeCs) with various ligands have been designed and synthesized. In combination with additives (e.g., iodonium salt, N‐vinylcarbazole, amine, or chloro triazine), the FeC‐based systems are able to efficiently generate radicals, cations, and radical cations on a near UV or visible light‐emitting diode (LED) exposure. These systems are characterized by an unprecedented reactivity, that is, for very low content 0.02% FeC‐based systems is still highly efficient in photopolymerization contrary to the most famous reference systems (Bisacylphosphine oxide) illustrating the performance of the proposed catalytic approach. This work paves the way for polymerization in soft conditions (e.g., on LED irradiation). These FeC‐based systems exhibit photocatalytic properties, undergo the formation of radicals, radical cations, and cations and can operate through oxidation or/and reduction cycles. The photochemical mechanisms for the formation of the initiating species are studied using steady state photolysis, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 42–49

CHARACTERIZATION OF MODEL NITROSYLHEME PIGMENTS WITH VISIBLE, INFRARED AND15N FOURIER TRANSFORM NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Synthetic pigments that model the organic-soluble pigment in heat-processed cured ham were characterized with infrared (IR) and 15N nuclear magnetic resonance (NMR) spectroscopy. The two pigments synthesized were 15N nitrosyl (protoporphyrinato- IX) iron (II), or FeII PP15 NO, and 15N dinitrosyl (protoporphyrinato-IX), or FeII PP(15NO)2. An NMR spectrum was obtained only for the FeII PP15 NO. The NMR spectrum consisted of a single peak + 1.78 ppm from the Na15 NO3 and –6.8 from the CH3NO2 reference. The IR spectrum included a single NO symmetric stretch at 1801 cm−1. The visible spectrum of FeII PPNO did not match the visible spectrum of ham extracts, while it did with FEII PP(15NO)2. An unequivocal structural assignment of FeII PP15 NO was made, but longer NMR runs may be necessary to obtain a spectrum and unequivocal structural assignment of FeII PP(15 NO)2.

Studies on the binding of nitrogenous bases to protoporphyrin IX iron(II) in aqueous solution at high pH values part II. Aliphatic nitrogen ligands

The first complimentary studies using Mössbauer spectroscopy and ligand titrations on protoporphyrin IX iron(II) with a series of aliphatic nitrogenous ligands are reported. Binding constants have been determined by titration of the metal complex with these ligands. The adduct formation was monitored through optical spectroscopy. Parallel Mössbauer spectroscopic experiments were conducted to monitor the electronic environment around the central iron atom in these adducts. Both spectroscopic methods indicated that all the aliphatic nitrogen bases yielded low spin octahedral complexes. The magnitude of the overall binding constants (logβ 2 values which varied from 1.7 for ethanolamine to 4.26 for n-octylamine) are discussed and related to (i) the p K a values of the free ligands and (ii) to the quadrupole splitting, Δ E Q, of the haem iron. The aliphatic nitrogenous ligands studied here exhibited cooperative binding (except ethanolamine) with Hill coefficients close to 2 for protoporphyrin IX iron(II), but with significantly lower (∼ 1000-fold) affinity than pyridine and imidazole ligands (which we have studied previously). However, these aliphatic and aromatic ligands displayed comparable Δ EQ values (∼ 1.00 to 1.20 mm s −1). The β and Δ E Q values are also discussed in terms of the structure of the ligand.

Studies on the binding of nitrogenous bases to protoporphyrin IX iron(II) in aqueous solution at high pH values. Part I. Pyridine and imidazole ligands

The binding constants between protoporphyrin IX iron(II) and a series of pyridine ligands and imidazole have been determined by titration of the metal complex with these ligands. The adduct formation was monitored through optical spectroscopy. Parallel Mössbauer spectroscopic experiments were conducted to monitor the electronic environment around the central iron atom in these adducts. Both spectroscopic methods indicated that, with the exception of pyridine- N-oxide, all nitrogen bases yielded low spin octahedral complexes. The magnitude of the overall binding constants (β 2 values) are discussed and related to (i) the p K a values of the free ligands and (ii) to the quadrupole splitting, Δ E o, of the haem iron. The β 2 and Δ E o values are also discussed in terms of the structure of the ligand.

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

Mössbauer 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 (Δ E Q) similar to that of reduced cytochrome b 5. The Δ E Q values are typical of bis(imidazole) coordination to (protoporphyrinato IX)iron(II) (Δ E Q = 0.9–1.04 mm s −1). The results indicate that hydrogen bonding to the NH of the coordinated imidazole considerably decreases the Δ E Q values.

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

A Mössbauer 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 (Δ E Q) of the bis(1-MeIm) compound was found to be similar to that of reduced cytochrome b 5·. 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 Δ E Q 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 Δ E Q = 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 < 1-MeIm < ImH. Hydrogen bonding of coordinated imidazoles and steric hindrance of axial ligands are shown to influence the trend in basicity of the free ligands.

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 Mössbauer 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.

Model complexes for the high spin iron(II) state in the catalytic cycle of cytochrome P450

Studies using 57Fe Mössbauer spectroscopy on frozen concentrated solutions of two iron(II) porphyrins in the presence of a large excess of thiophenol or 2-mercaptoethanol are reported. The iron(II) porphyrins used were photoporphyrin IX iron(II) (PPIXFe(II)) and tetra( p-sulphophenyl)porphinato-iron(II) (TTPPSFe(II)). Evidence for high-spin five-coordinate iron(II) complexes were found for both iron(II) porphyrins with thiophenol, but no reaction was found to occur with 2-mecaptoethanol. In contrast to these findings in solutions dilute in TPPSFe(II) evidence was found (from electronic absorption spectroscopy and spectrophotometric titrations) for both thiophenol and 2-mercaptoethanol acting as axial ligands in high-spin five-coordinate TPPSFe(II) species. Mössbauer data for frozen solutions containing TPPSFe(II), carbon monoxide (CO) and either thiol, are consistent with the presence of only low-spin six-coordinate iron(II) complexes. These latter complexes are deduced to contain both a thiol and a CO molecule as axial ligands> These results are discussed in relation to earlier work on PPIXFe(II)-thiol solutions and also in relation to the high-spin iron(II) state in the catalytic cycle of cytochrome P450. The reactions of TPPSFe(II) with excess of either 2-mercaptoethanol or ethyl 2-mercaptoacetate in air are also reported and their complex nature discussed.

Mössbauer spectroscopic studies on tetra(sulphonaphthyl)porphine iron(II) solutions

Mössbauer (78 K) and electronic absorption spectra (room temperature) of tetra(sulphonaphthyl)- porphine iron(II) solutions are reported and discussed. Evidence for only two iron(II) electronic environments, a low spin and a high spin site is found. The nature of each iron(II) environment is deduced with reference to previous work. The influence of the steric bulk of the meso substituents is discussed in comparison to similar studies on protoporphyrin IX iron(II) solutions and tetra( p-sulphophenyl)iron(II) solutions. The presence of the napthyl substituents on the methine carbons stabilises the low spin iron(II) species containing two axial water ligands.

Mössbauer spectroscopic studies on concentrated protoporphyrin IX iron(II) solutions

57Fe Mössbauer parameters are presented for protoporphyrin IX iron(II) frozen solutions These are interpreted as evidence for changes in the electronic environments around the iron(II) centres when the metalloporphyrin molecules aggregate in solution.

Mössbauer studies on oxygen binding in protoporphyrin IX iron(II) solutions in the presence of other ligands

Mössbauer parameters of frozen solutions of protoporphyrin IX iron(II) (containing either 2- methyl-piperidine or mercaptoethanol as the fifth iron ligand) that were exposed to oxygen before freezing are similar to those of oxyhaemoglobin. These results are discussed in relation to known porphyrin iron(II) chemistry.

Mössbauer studies on protoporphyrin IX iron(II) solutions containing sulphur ligands and their carbonyl adducts. Models for the active site of cytochromes P-450

A number of sulphur ligands in solution with protoporphyrin IX iron(II) give Mössbauer data similar to that recorded for cytochromes P-450 in the deoxy-high-spin ferrous stage. Addition of carbonyl to the solution results in Mössbauer spectra that have wider quadrupole splittings than that of P-450 CO. This suggest that the sulphur to iron bond in the cytochromes P-450 is unusually strong, probably due to other bonding forces in these proteins.

Mössbauer studies on protoporphyrin IX iron(II) frozen solutions containing ligands that cause the iron to be in a five coordinate high spin iron(II) environment

Mössbauer parameters are presented for a number of protoporphyrin IX iron(II) complexes containing ligands that allow the iron to be in a five coordinate high spin iron(II) electronic environment. Such environments are characterised by large quadrupole splittings in the range 4.0 to 4.4 mm s −1. These compounds have characteristic electronic spectra. The implications of catechol type ligands binding protoporphyrin IX iron II/III moieties are discussed.

Mössbauer studies on protoporphyrin IX iron(II) solutions

Mössbauer spectroscopic studies on frozen solutions of protoporphyrin IX iron(II) in the pH range 7–14 reveal evidence for three different species. The species dominant over the pH range 7–13 is a polymeric species (A) made up of a bare protoporphyrin IX iron(II) moieties, the number of moieties in the polymer is discussed relative to concentration and pH. This species is assigned an S = 1 spin state. Above pH 13 a high spin iron(II) species (C) is present. The quadrupole splitting associated with this species is discussed and is the largest yet reported for an iron(II) propoporphyrin. This species (C) is a five coordinate protoporphyrin IX iron(II) with a hydroxide ion as the fifth ligand. The third species (B) is a minor constituent of this system and also has a high spin iron(II) structure, though the Mössbauer parameters for this site are more in keeping with a distorted octahedral environment.