Magnetic circular dichroism studies. 59. Substituent-induced sign variation in the magnetic circular dichroism spectra of chlorins

Keegan, Joseph D.; Stolzenberg, Alan M.; Lu, Yucheng; Linder, Robert E.; Barth, Guenter; Bunnenberg, E.; Djerassi, Carl; Moscowitz, Albert

doi:10.1021/ja00401a045

Cited by 18 publications

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“…For example, the iron-chlorin, known as heme d , occurs as one of three hemes in the quinol-oxidase cytochrome bd and as the only prosthetic group in hydroperoxidase II (HPII). Limited MCD data available for these heme d systems reveal spectroscopic properties distinct from those of protohemes ( − ). It is clear that low-spin ferric heme d gives rise to CT spectra in the near-infrared (NIR) region of unusually low intensity.…”

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confidence: 99%

Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd₁, from Pseudomonas stutzeri and Thiosphaera pantotropha

et al. 1997

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It is shown that, in the oxidized state, heme c of Pseudomonas stutzeri (ZoBell strain) cytochrome cd1 has histidine-methionine ligation as observed for cytochrome cd1 from Pseudomonas aeruginosa [Sutherland, J., Greenwood, C., Peterson, J., and Thomson, A. J. (1986) Biochem. J. 233, 893-898]. However, the X-ray structure of Thiosphaera pantotropha cytochrome cd1 reveals bis-histidine ligation for heme c. It is confirmed by EPR and near-infrared (NIR) MCD measurements that the bis-histidine coordination remains unaltered in the solution phase. Hence, the difference between the heme c ligation states defines two distinct classes of oxidized cytochromes cd1 as isolated. A weak feature in the T. pantotropha NIR MCD at 1900 nm suggests that a small population of heme c has histidine-methionine coordination. The ligation state of heme d1 cannot be defined with the same level of confidence, because the porphyrin-to-Fe(III) charge-transfer (CT) bands are less well characterized for this class of partially reduced porphyrin ring. However, variable temperature absorption and MCD spectra show that, in the T. pantotropha enzyme, heme d1 exists in a thermal low-spin/high-spin mixture with the low-spin as the ground state, whereas in P. stutzeri cytochrome cd1, and d1 heme is low-spin at all temperatures. A weak band, assigned as the heme d1 porphyrin-pi(a1u,a2u)-to-ferric(d) charge-transfer transition has been identified for the first time at 2170 nm. Its magnetic properties show the heme d1 to have an unusual (dxz,yz)4(dxy)1 electronic ground state as is found for low-spin Fe(III) chlorins [Cheesman, M. R., and Walker, F. A. (1996) J. Am. Chem. Soc. 118, 7373-7380]. It is proposed that the localization of the Fe(III) unpaired d-electron in an orbital lying in the heme plane may decrease the affinity of the Fe(III) heme for unsaturated ligands such as NO. Although heme d1 in the enzymes from P. stutzeri and T. pantotropha shows different temperature-dependent spin properties, the positions of the low-spin Fe(III) alpha-absorption band, at approximately 640 nm, are very similar to those observed for cytochromes cd1 from eight other sources, suggesting that all have similar strength fields from the axial ligands and, hence, that all have the same coordination, namely histidine-tyrosine or possibly histidine-hydroxide at the heme.

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Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd₁, from Pseudomonas stutzeri and Thiosphaera pantotropha

et al. 1997

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“…As expected, the DFT calculations yield ΔLUMO = 0, but the splitting of the HOMO pair is not that small: ΔHOMO = 0.605 eV. In the literature, the two HOMOs of the anion are referred to as accidentally nearly degenerate. ,,,, This is based on very weak intensity of the absorption intensity of the Q(0–0) transition. , We note, however, that the absorption spectra have been obtained for solutions containing very high concentration of sodium hydroxide. Under such conditions, it is appropriate to assume that the sodium atoms can be present in the vicinity of the dianion.…”

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confidence: 52%

“…In the literature, the two HOMOs of the anion are referred to as accidentally nearly degenerate. 28 , 30 , 31 , 33 , 35 This is based on very weak intensity of the absorption intensity of the Q(0–0) transition. 33 , 72 We note, however, that the absorption spectra have been obtained for solutions containing very high concentration of sodium hydroxide.…”

Section: Resultsmentioning

confidence: 99%

“…It has been well established that alkyl and aryl substituents can strongly affect absorption and, in particular, MCD spectra of porphyrins. ,− Moreover, substituents of the same type (e.g., electron-donating) can act in different directions, depending on whether they are located at the meso or β positions. Another factor is the nature of the central atom.…”

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confidence: 99%

“…Porphyrins belong to this category. The power of the perimeter model for predicting their special properties has been demonstrated in numerous papers. − ,,− …”

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Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins

et al. 2020

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Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups ( n = 0–4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0–0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

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Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl‐β,β′‐dioxo‐bacterio‐ and ‐isobacteriochlorin Series

Brückner

Chaudhri

Nevonen

et al. 2021

Chemistry A European J

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It is well‐known that treatment of β‐octaethylporphyrin with H2O2/conc. H2SO4 converts it to a β‐oxochlorin as well as all five constitutional isomers of the corresponding β,β’‐dioxo‐derivatives: two bacteriochlorin‐type isomers (β‐oxo groups at opposite pyrrolic building blocks) and three isobacteriochlorin‐type isomers (β‐oxo‐groups at adjacent pyrrolic building blocks). By virtue of the presence of the strongly electronically coupled β‐oxo auxochromes, none of the chromophores are archetypical chlorins, bacteriochlorins, or isobacteriochlorins. Here the authors present, inter alia, the single crystal X‐ray structures of all free‐base diketone isomers and a comparative description of their UV‐vis absorption spectra in neutral and acidic solutions, and fluorescence emission and singlet oxygen photosensitization properties, Magnetic Circular Dichroism (MCD) spectra, and singlet excited state lifetimes. DFT computations uncover underlying tautomeric equilibria and electronic interactions controlling their electronic properties, adding to the understanding of porphyrinoids carrying β‐oxo functionalities. This comparative study lays the basis for their further study and utilization.

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Magnetic circular dichroism studies. 59. Substituent-induced sign variation in the magnetic circular dichroism spectra of chlorins

Cited by 18 publications

References 0 publications

Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd₁, from Pseudomonas stutzeri and Thiosphaera pantotropha

Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd₁, from Pseudomonas stutzeri and Thiosphaera pantotropha

Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl‐β,β′‐dioxo‐bacterio‐ and ‐isobacteriochlorin Series

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Magnetic circular dichroism studies. 59. Substituent-induced sign variation in the magnetic circular dichroism spectra of chlorins

Cited by 18 publications

References 0 publications

Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd1, from Pseudomonas stutzeri and Thiosphaera pantotropha

Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd1, from Pseudomonas stutzeri and Thiosphaera pantotropha

Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl‐β,β′‐dioxo‐bacterio‐ and ‐isobacteriochlorin Series

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Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd₁, from Pseudomonas stutzeri and Thiosphaera pantotropha

Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd₁, from Pseudomonas stutzeri and Thiosphaera pantotropha