Definitive evidence for the existence of the “sitting-atop” porphyrin complexes in nonaqueous solutions

Fleischer, Everly B.; Dixon, F.L.

doi:10.1016/s0006-3061(00)80063-0

Cited by 37 publications

(16 citation statements)

References 19 publications

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“…Several reports proved the existence of SAT complexes in similar reactions, [55,56] and even if some question the existence of such type of intermediate, [57] it still is most appealing structure of intermediate common to the metalation pathways, predicted also by theoretical calculations. [58,59] Therefore this model should also be applicable to considerations for the metalation of Pheo, although the assignment of the SAT intermediate is ambiguous as no clear evidence for its formation was found.…”

Section: Mechanistic Considerationsmentioning

confidence: 96%

Mechanistic Information on Cu^II Metalation and Transmetalation of Chlorophylls

et al. 2009

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Chlorophyll a and pheophytin a have been treated with various metal salts in solution. As a result, metallo derivatives of photosynthetic pigments were formed, or the chlorine ring was oxidized. These studies showed that both effects can be caused by Cu II ions depending on the redox potential of its complexes formed in specific media. In this report the results of studies on metal insertion and exchange reactions are presented. According to the common use of the "acetate method" in the synthesis of metallotetrapyrroles, and the fact that acetates coordinated to Cu II ions protect chlorophylls from oxidative degradation, Cu II acetate monohydrate was selected as main reagent. The metalation and transmetalation processes were investigated with spectroscopic (UV/ Vis absorption and emission) and kinetic (conventional and high-pressure) techniques. It turned out that differences in solvent properties can significantly affect not only the rates (metalation), but also the course (transmetalation) of the reaction. The most pronounced effect was found for the trans-

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Section: Mechanistic Considerationsmentioning

confidence: 96%

Mechanistic Information on Cu^II Metalation and Transmetalation of Chlorophylls

et al. 2009

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“…31 After this report, the reports on the detection of the SAT complex have been very limited, only in peculiar systems. [32][33][34][35] We think that a crucial point to form the SAT complex is to guarantee the thermodynamic and kinetic retardation of the dissociation of protons on the pyrrole rings in the SAT complex. Therefore, we must select solvents with a very low Brønsted basicity; water as a solvent is not adequate.…”

Section: Kinetics and Mechanism For Sitting-atop (Sat) Complex Formationmentioning

confidence: 99%

“…Because the two pyrrolenine groups can potentially bind to the metal ion, the overall metalation reaction will consist of at least two steps, i.e., the coordination step of two pyrrolenine nitrogens to form an intermediate, the so-called sitting-atop (SAT) complex, M(H2por) n+ , 31 in which two protons on the pyrrole nitrogens still remain, [32][33][34][35] and the deprotonation step of the SAT complex to form the metalloporphyrin, as shown by Eqs. (2) and (3), respectively.…”

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confidence: 99%

Dynamic Study of Metal-Ion Incorporation into Porphyrins Based on the Dynamic Characterization of Metal Ions and on Sitting-Atop Complex Formation

2001

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The metalloporphyrin formation reaction is one of the important processes from both analytical and bioinorganic points of view.The large molar absorption coefficient and very high stability of the metalloporphyrins are very useful for the highly sensitive analysis of trace amounts of metal ions, and the size selectivity of the porphyrins is valuable for the separation of various kinds of metal ions. 1-7 A variety of metalloporphyrin formation rates are also applicable for the kinetic analysis of metal ions. We succeeded in the detection of the sitting-atop (SAT) copper(II) complex of TPP (5,10,15,20-tetraphenylporphyrin) in acetonitrile (AN) as a solvent with a very low Brønsted basicity, where two pyrrolenine nitrogens in the Cu(II)-SAT complex coordinate to the metal ion and two protons still remain on the pyrrole nitrogens. The structure parameters around the copper(II) ion in the Cu(II)-SAT complex, as determined by a fluorescent EXAFS method, suggest an axially elongated and equatorially distorted six-coordinate geometry. We measured the rates of the formation reaction of the SAT complexes for a series of transition metal(II) ions in AN using the stopped-flow technique. We propose the mechanism where there is a rapid deformation equilibrium of the porphyrin ring prior to the rate-determining step of the bond rupture of a coordinated solvent molecule on the metal(II) ion. Furthermore, we measured the rates of the deprotonation reaction of the Cu(II)-SAT complex by some Brønsted bases and indicated that the rate-determining step is the attack of the base on the proton of the pyrrole nitrogen in the SAT complex. Finally, a unified mechanism relevant to the porphyrin metalation mechanism has been proposed.

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“…Although a model of the general reaction mechanism of porphyrin metalation has emerged [10][11][12][13][14][15], how chelatases bestow metal ion selectivity and specificity on porphyrin metalation remains unresolved.…”

Section: Introductionmentioning

confidence: 99%

Chelatases: distort to select?

Al-Karadaghi

Franco

Hansson

et al. 2006

Trends in Biochemical Sciences

108

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Chelatases catalyze the insertion of a specific metal ion into porphyrins, a key step in the synthesis of metalated tetrapyrroles that are essential for many cellular processes. Despite apparent common structural features among chelatases, no general reaction mechanism accounting for metal ion specificity has been established. We propose that chelatase-induced distortion of the porphyrin substrate not only enhances the reaction rate by decreasing the activation energy of the reaction but also modulates which divalent metal ion is incorporated into the porphyrin ring. We evaluate the recently recognized interaction between ferrochelatase and frataxin as a way to regulate iron delivery to ferrochelatase, and thus iron and heme metabolism. We postulate that the ferrochelatase-frataxin interaction controls the type of metal ion that is delivered to ferrochelatase.

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Definitive evidence for the existence of the “sitting-atop” porphyrin complexes in nonaqueous solutions

Cited by 37 publications

References 19 publications

Mechanistic Information on Cu^II Metalation and Transmetalation of Chlorophylls

Mechanistic Information on Cu^II Metalation and Transmetalation of Chlorophylls

Dynamic Study of Metal-Ion Incorporation into Porphyrins Based on the Dynamic Characterization of Metal Ions and on Sitting-Atop Complex Formation

Chelatases: distort to select?

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Definitive evidence for the existence of the “sitting-atop” porphyrin complexes in nonaqueous solutions

Cited by 37 publications

References 19 publications

Mechanistic Information on CuII Metalation and Transmetalation of Chlorophylls

Mechanistic Information on CuII Metalation and Transmetalation of Chlorophylls

Dynamic Study of Metal-Ion Incorporation into Porphyrins Based on the Dynamic Characterization of Metal Ions and on Sitting-Atop Complex Formation

Chelatases: distort to select?

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Mechanistic Information on Cu^II Metalation and Transmetalation of Chlorophylls

Mechanistic Information on Cu^II Metalation and Transmetalation of Chlorophylls