NMR and EPR studies of chloroiron(III) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities

The photodynamic activity properties of a series of structurally analogous tetraarylporphyrin, chlorin and N-confused porphyrin dyes and their Sn(IV) complexes

A series of tetraarylporphyrin, -chlorin and N-confused porphyrin dyes with 4‑methoxy‑meso-aryl rings (1-Por, 1-Chl and 1-NCP) and their Sn(IV) complexes (1-SnPor, 1-SnChl and 1-SnNCP) have been synthesized and characterized. The heavy atom effect of the Sn(IV) ion results in relatively high singlet oxygen quantum yield values of 0.67, 0.71 and 0.85 for 1-SnPor, 1-SnChl and 1-SnNCP, respectively. The photodynamic activities of 1-Por, 1-Chl, 1-NCP, 1-SnPor, 1-SnChl and 1-SnNCP were determined against MCF-7 breast cancer cells through illumination with Thorlabs 625 or 660 nm (240 or 280 mW.cm-2) light emitting diodes (LEDs) for 20 min. The IC50 values for 1-SnChl and 1-SnNCP lie between 1.4 - 6.1 and 1.6 - 4.8 µM upon photoirradiation with the 660 and 625 nm LEDs, respectively, while higher values of >10 µM were obtained for 1-SnPor and the free base dyes. In a similar manner, 1-SnChl and 1-SnNCP were found to also have significantly higher photodynamic antimicrobial activity against planktonic Gram-(+) Staphylococcus aureus and Gram-(-) Escherichia coli bacteria than the other dyes studied. Upon illumination with Thorlabs 625 and 660 nm LEDs for 75 min, Log10 reduction values of 7.62 and > 2.40-3.69 were obtained with 1 and 5 µM solutions, respectively.

The Photodynamic Anticancer and Antibacterial Activity Properties of a Series of meso-Tetraarylchlorin Dyes and Their Sn(IV) Complexes

A series of tetraarylchlorins with 3-methoxy-, 4-hydroxy- and 3-methoxy-4-hydroxyphenyl meso-aryl rings (1-3-Chl) and their Sn(IV) complexes (1-3-SnChl) were synthesized and characterized so that their potential utility as photosensitizer dyes for use in photodynamic therapy (PDT) and photodynamic antimicrobial chemotherapy (PACT) can be assessed. The photophysicochemical properties of the dyes were assessed prior to in vitro PDT activity studies against MCF-7 breast cancer cells through irradiation with Thorlabs 625 or 660 nm LED for 20 min (240 or 280 mW·cm^-2). PACT activity studies were performed against both planktonic bacteria and biofilms of Gram-(+) S. aureus and Gram-(-) E. coli upon irradiation with Thorlabs 625 and 660 nm LEDs for 75 min. The heavy atom effect of the Sn(IV) ion results in relatively high singlet oxygen quantum yield values of 0.69-0.71 for 1-3-SnChl. Relatively low IC₅₀ values between 1.1-4.1 and 3.8-9.4 µM were obtained for the 1-3-SnChl series with the Thorlabs 660 and 625 nm LEDs, respectively, during the PDT activity studies. 1-3-SnChl were also found to exhibit significant PACT activity against planktonic S. aureus and E. coli with Log₁₀ reduction values of 7.65 and >3.0, respectively. The results demonstrate that the Sn(IV) complexes of tetraarylchlorins merit further in depth study as photosensitizers in biomedical applications.

Photodynamic Antitumor and Antimicrobial Activities of Free-Base Tetra(4-methylthiolphenyl)chlorin and Its Tin(IV) Complex

Meso-tetra(4-methylthiolphenyl)chlorin (3) and its Sn(IV) complex (3-Sn) have been synthesized and characterized. The heavy atom effects of the Sn(IV) ion and sulfur atoms result in relatively high singlet oxygen quantum yield values of 0.40 and 0.48. The photodynamic activities against MCF-7 breast cancer cells were determined through irradiation with a Thorlabs 660 nm LED for 30 min (280 mW.cm-2 ). IC50 values of 7.8 and 3.9 μM were obtained, respectively. 3-Sn was found to have significant photodynamic antimicrobial activity against both gram-(+) S. aureus and gram-(-) E. coli bacteria upon irradiation with a Thorlabs 660 nm LED for 75 min.

Ruthenium(II) and Ruthenium(III) Complexes of p-Benziporphyrin: Merging Equatorial and Axial Organometallic Coordination

A diamagnetic ruthenium(II) complex of 5,10,15,20-tetraphenyl-p-benziporphyrin [Ru^II(p-BzP)(CO)Cl] was obtained via the insertion of ruthenium into p-benziporphyrin using triruthenium(0) dodecacarbonyl [Ru₃(CO)₁₂] as the metal source. The procedure applying dichloro(cycloocta-1,5-diene)ruthenium(II) (polymer, [Ru(COD)Cl₂]_n) afforded the paramagnetic six-coordinate ruthenium(III) p-benziporphyrin [Ru^III(p-BzP)Cl₂]. As shown by X-ray crystallography, the p-phenylene ring in both complexes is sharply tilted out of the N₃ plane, as reflected by the respective N₃ (pyrrole)-C₆ (p-phenylene) dihedral angle [Ru^II(p-BzP)(CO)Cl, 52.5°; Ru^III(p-BzP)Cl₂, 53.7°]. p-Phenylene is bound to the ruthenium cation in an η² fashion, revealing the shortest ever Ru-C distance in the series of p-benziporphyrin complexes [Ru^II(p-BzP)(CO)Cl, 2.275(2) Å; Ru^III(p-BzP)Cl₂, 2.324(5) Å]. The reaction of Ru^II(p-BzP)(CO)Cl with ArMgCl or AlkMgCl results in the formation of diamagnetic six-coordinate ruthenium(II) p-benziporphyrin complexes containing the apically coordinated σ-alkyl or σ-aryl ligands, where the metal ion simultaneously coordinates to three carbon centers respectively accommodating η² (phenylene) and σ (aryl and alkyl) modes. Reactions of σ-aryl (alkyl) carbanions with paramagnetic Ru^III(p-BzP)Cl₂ have been followed by ¹H NMR spectroscopy. The procedure afforded the six-coordinate paramagnetic ruthenium(III) p-benziporphyrin [Ru^III(p-BzP)(Ph)Cl], which binds one σ-aryl ligand, as reflected by the characteristic ¹H NMR spectra spread within the +120 to -120 ppm range. Both paramagnetic complexes Ru^III(p-BzP)(Ph)Cl and Ru^III(p-BzP)(p-Tol)Cl are formed as a mixture of two stereoisomers differentiated by two nonequivalent locations of σ-aryl with respect to the puckered macrocyclic ring. The paramagnetic shifts of σ-aryls are indicative of π-spin delocalization patterns. Analysis of the contact shifts and parallel density functional theory calculations of the spin density distribution in Ru^III(p-BzP)Cl₂, Ru^III(p-BzP)(Ar)Cl, and Ru^III(p-BzP)(Alk)Cl reflect the features of the d_xy²(d_xzd_yz)³ electronic ground state.

Electronic structure of low-spin six-coordinate iron(III) meso-tetrapropylchlorin complexes

Low-spin iron(III) tetrapropylchlorins [ Fe ( T n PrC ) L 2]± (L = HIm, 1-MeIm, DMAP, CN-, 4-CNPy, tBuNC) adopt the dxy-type ground state regardless of the nature of axial ligands. Among the complexes examined, [ Fe ( T n PrC )( t BuNC )2]+ has shown quite unique spectroscopic properties as described below. (1) 1 H NMR signals were extremely broad as compared with those of other complexes. In particular, 5,20- CH 2(α) signal was too broad to detect. (2) No signals except C γ were observed in 13 C NMR spectra. (3) Tetragonal splitting parameter (|Δ|) estimated by the EPR g values at 4.2 K reached as much as 12.4 λ, which is the largest |Δ| value among all the low-spin iron(III) porphyrins and porphyrinoids reported previously. On the basis of these results, we have concluded that [ Fe ( T n PrC )( t BuNC )2]+ adopts the low-spin iron(III) with (dxz, dyz)4(dxy)1 electronic ground state at 4.2–30 K where the EPR spectra are taken, while it should be expressed as the low-spin Fe ( II ) chlorin π-radical cation [ Fe II ( T n PrC .)( t BuNC )2]+ at ambient temperature where the NMR spectra are taken.

Microbial sulfite respiration

Despite its reactivity and hence toxicity to living cells, sulfite is readily converted by various microorganisms using distinct assimilatory and dissimilatory metabolic routes. In respiratory pathways, sulfite either serves as a primary electron donor or terminal electron acceptor (yielding sulfate or sulfide, respectively), and its conversion drives electron transport chains that are coupled to chemiosmotic ATP synthesis. Notably, such processes are also seen to play a general role in sulfite detoxification, which is assumed to have an evolutionary ancient origin. The diversity of sulfite conversion is reflected by the fact that the range of microbial sulfite-converting enzymes displays different cofactors such as siroheme, heme c, or molybdopterin. This chapter aims to summarize the current knowledge of microbial sulfite metabolism and focuses on sulfite catabolism. The structure and function of sulfite-converting enzymes and the emerging picture of the modular architecture of the corresponding respiratory/detoxifying electron transport chains is emphasized.

Inactivation of the heme degrading enzyme IsdI by an active site substitution that diminishes heme ruffling

IsdG and IsdI are paralogous heme degrading enzymes from the bacterium Staphylococcus aureus. Heme bound by these enzymes is extensively ruffled such that the meso-carbons at the sites of oxidation are distorted toward bound oxygen. In contrast, the canonical heme oxygenase family degrades heme that is bound with minimal distortion. Trp-66 is a conserved heme pocket residue in IsdI implicated in heme ruffling. IsdI variants with Trp-66 replaced with residues having less bulky aromatic and alkyl side chains were characterized with respect to catalytic activity, heme ruffling, and electrochemical properties. The heme degradation activity of the W66Y and W66F variants was approximately half that of the wild-type enzyme, whereas the W66L and W66A variants were inactive. A crystal structure and NMR spectroscopic analysis of the W66Y variant reveals that heme binds to this enzyme with less heme ruffling than observed for wild-type IsdI. The reduction potential of this variant (-96 ± 7 mV versus standard hydrogen electrode) is similar to that of wild-type IsdI (-89 ± 7 mV), so we attribute the diminished activity of this variant to the diminished heme ruffling observed for heme bound to this enzyme and conclude that Trp-66 is required for optimal catalytic activity.

Electronic properties of the highly ruffled heme bound to the heme degrading enzyme IsdI

IsdI, a heme-degrading protein from Staphylococcus aureus, binds heme in a manner that distorts the normally planar heme prosthetic group to an extent greater than that observed so far for any other heme-binding protein. To understand better the relationship between this distinct structural characteristic and the functional properties of IsdI, spectroscopic, electrochemical, and crystallographic results are reported that provide evidence that this heme ruffling is essential to the catalytic activity of the protein and eliminates the need for the water cluster in the distal heme pocket that is essential for the activity of classical heme oxygenases. The lack of heme orientational disorder in (1)H-NMR spectra of the protein argues that the catalytic formation of β- and δ-biliverdin in nearly equal yield results from the ability of the protein to attack opposite sides of the heme ring rather than from binding of the heme substrate in two alternative orientations.

NMR and DFT investigation of heme ruffling: functional implications for cytochrome c

Out-of-plane (OOP) deformations of the heme cofactor are found in numerous heme-containing proteins and the type of deformation tends to be conserved within functionally related classes of heme proteins. We demonstrate correlations between the heme ruffling OOP deformation and the (13)C and (1)H nuclear magnetic resonance (NMR) hyperfine shifts of heme aided by density functional theory (DFT) calculations. The degree of ruffling in the heme cofactor of Hydrogenobacter thermophilus cytochrome c(552) has been modified by a single amino acid mutation in the second coordination sphere of the cofactor. The (13)C and (1)H resonances of the cofactor have been assigned using one- and two-dimensional NMR spectroscopy aided by selective (13)C-enrichment of the heme. DFT has been used to predict the NMR hyperfine shifts and electron paramagnetic resonance (EPR) g-tensor at several points along the ruffling deformation coordinate. The DFT-predicted NMR and EPR parameters agree with the experimental observations, confirming that an accurate theoretical model of the electronic structure and its response to ruffling has been established. As the degree of ruffling increases, the heme methyl (1)H resonances move upfield while the heme methyl and meso (13)C resonances move downfield. These changes are a consequence of altered overlap of the Fe 3d and porphyrin pi orbitals, which destabilizes all three occupied Fe 3d-based molecular orbitals and decreases the positive and negative spin density on the beta-pyrrole and meso carbons, respectively. Consequently, the heme ruffling deformation decreases the electronic coupling of the cofactor with external redox partners and lowers the reduction potential of heme.

Electronic structure of highly ruffled low-spin iron(III) porphyrinates with electron withdrawing heptafluoropropyl groups at the meso positions

Bis(pyridine)[meso-tetrakis(heptafluoropropyl)porphyrinato]iron(III), [Fe(THFPrP)Py(2)](+), was reported to be the low-spin complex that adopts the purest (d(xz), d(yz))(4)(d(xy))(1) ground state where the energy gap between the iron d(xy) and d(π)(d(xz), d(yz)) orbitals is larger than the corresponding energy gaps of any other complexes reported previously (Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999, 121, 5196-5209). Although the highly ruffled porphyrin core expected for this complex contributes to the stabilization of the (d(xz), d(yz))(4)(d(xy))(1) ground state, the strongly electron withdrawing C(3)F(7) groups at the meso positions should stabilize the (d(xy))(2)(d(xz), d(yz))(3) ground state. Thus, we have reexamined the electronic structure of [Fe(THFPrP)Py(2)](+) by means of (1)H NMR, (19)F NMR, and electron paramagnetic resonance (EPR) spectroscopy. The CD(2)Cl(2) solution of [Fe(THFPrP)Py(2)](+) shows the pyrrole-H signal at -10.25 ppm (298 K) in (1)H NMR, the CF(2)(α) signal at -74.6 ppm (298 K) in (19)F NMR, and the large g(max) type signal at g = 3.16 (4.2 K) in the EPR. Thus, contrary to the previous report, the complex is unambiguously shown to adopt the (d(xy))(2)(d(xz), d(yz))(3) ground state. Comparison of the spectroscopic data of a series of [Fe(THFPrP)L(2)](+) with those of the corresponding meso-tetrapropylporphyrin complexes [Fe(TPrP)L(2)](+) with various axial ligands (L) has shown that the meso-C(3)F(7) groups stabilize the (d(xy))(2)(d(xz), d(yz))(3) ground state. Therefore, it is clear that the less common (d(xz), d(yz))(4)(d(xy))(1) ground state can be stabilized by the three major factors: (i) axial ligand with low-lying π* orbitals, (ii) ruffled porphyrin ring, and (iii) electron donating substituent at the meso position.

Iron corrolates: Unambiguous chloroiron(III) (corrolate)2− π-cation radicals

The structures, electron configurations, magnetic susceptibilities, spectroscopic properties, molecular orbital energies and spin density distributions, redox properties and reactivities of iron corrolates having chloride, phenyl, pyridine, NO and other ligands are reviewed. It is shown that with one very strong donor ligand such as phenyl anion the electron configuration of the metal is d(4)S=1 Fe(IV) coordinated to a (corrolate)(3-) anion, while with one weaker donor ligand such as chloride or other halide, the electron configuration is d(5)S=3/2 Fe(III) coordinated to a (corrolate)(2-.) pi-cation radical, with antiferromagnetic coupling between the metal and corrolate radical electron. Many of these complexes have been studied by electrochemical techniques and have rich redox reactivity, in most cases involving two 1-electron oxidations and two 1-electron reductions, and it is not possible to tell, from the shapes of cyclic voltammetric waves, whether the electron is added or removed from the metal or the macrocycle; often infrared, UV-Vis, or EPR spectroscopy can provide this information. (1)H and (13)C NMR spectroscopic methods are most useful in delineating the spin state and pattern of spin density distribution of the complexes listed above, as would also be expected to be the case for the recently-reported formal Fe(V)O corrolate, if this complex were stable enough for characterization by NMR spectroscopy. Iron, manganese and chromium corrolates can be oxidized by iodosylbenzene and other common oxidants used previously with metalloporphyrinates to effect efficient oxidation of substrates. Whether the "resting state" form of these complexes, most generally in the case of iron [FeCl(Corr)], actually has the electron configuration Fe(IV)(Corr)(3-) or Fe(III)(Corr)(2-.) is not relevant to the high-valent reactivity of the complex.