Sitting-atop Research Articles

The structure of sitting-atop complexes of metalloporphyrins studied by theoretical methods

The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are still protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and extended X-ray absorption fine structure (EXAFS) data has been reported for Cu2+ in acetonitrile. We have used density functional calculations to obtain reasonable models for SAT complexes of porphyrins with Mg2+, Fe2+, and Cu2+. The results show that there are many possible SAT complexes with 1–5 solvent molecules, one or two metal ions, and cis or trans protonation of the porphyrin ring. Many of these have similar energies and their relative stabilities vary with the metal ion. A complex with two cis pyrrolenine nitrogens atoms and 2–4 solvent molecules coordinated to Cu2+ fits the NMR and EXAFS data best. However, we cannot fully exclude the possibility that what is observed is rather a mixture of a doubly protonated porphyrin and the copper porphyrin. Mg2+ has a lower affinity for porphyrin and stronger affinity for water, so a complex with five water molecules and only one bond to porphyrin seems to be most stable. For Fe2+, a cis structure with two first-sphere water molecules and four interactions to the porphyrin seems to be most likely.

Metal binding to Bacillus subtilis ferrochelatase and interaction between metal sites.

Ferrochelatase, the terminal enzyme in heme biosynthesis, catalyses metal insertion into protoporphyrin IX. The location of the metal binding site with respect to the bound porphyrin substrate and the mode of metal binding are of central importance for understanding the mechanism of porphyrin metallation. In this work we demonstrate that Zn(2+), which is commonly used as substrate in assays of the ferrochelatase reaction, and Cd(2+), an inhibitor of the enzyme, bind to the invariant amino acids His183 and Glu264 and water molecules, all located within the porphyrin binding cleft. On the other hand, Mg(2+), which has been shown to bind close to the surface at 7 A from His183, was largely absent from its site. Activity measurements demonstrate that Mg(2+) has a stimulatory effect on the enzyme, lowering K(M) for Zn(2+) from 55 to 24 micro M. Changing one of the Mg(2+) binding residues, Glu272, to serine abolishes the effect of Mg(2+). It is proposed that prior to metal insertion the metal may form a sitting-atop (SAT) complex with the invariant His-Glu couple and the porphyrin. Metal binding to the Mg(2+) site may stimulate metal release from the protein ligands and its insertion into the porphyrin.

Unusual transformations of a manganese(III) porphyrin

A superstable SAT (sitting atop) complex was synthesised by the incomplete dissociation of a chelate of manganese(III) with meso-monophenyloctamethylporphine in a solid state.

Structural characterization and formation kinetics of sitting-atop (SAT) complexes of some porphyrins with copper(II) ion in aqueous acetonitrile relevant to porphyrin metalation mechanism. Structures of aquacopper(II) and cu(II)-SAT complexes as determined by XAFS spectroscopy.

The formation of the sitting-atop (SAT) complexes of 5,10,15,20-tetraphenylporphyrin (H(2)tpp), 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (H(2)t(4-Clp)p), 5,10,15,20-tetramesitylporphyrin (H(2)tmp), and 2,3,7,8,12,13,17,18-octaethylporphyrin (H(2)oep) with the Cu(II) ion was spectrophotometrically confirmed in aqueous acetonitrile (AN), and the formation rates were determined as a function of the water concentration (C(W)). The decrease in the conditional first-order rate constants with the increasing C(W) was reproduced by taking into consideration the contribution of [Cu(H(2)O)(an)(5)](2+) in addition to [Cu(an)(6)](2+) to form the Cu(II)-SAT complexes. The second-order rate constants for the reaction of [Cu(an)(6)](2+) and [Cu(H(2)O)(an)(5)](2+) at 298 K were respectively determined as follows: (4.1 +/- 0.2) x 10(5) and (3.6 +/- 0.2) x 10(4) M(-1) s(-1) for H(2)tpp, (1.15 +/- 0.06) x 10(5) M(-1) s(-1) and negligible for H(2)t(4-Clp)p, and (4.8 +/- 0.3) x 10(3) and (1.3 +/- 0.3) x 10(2) M(-1) s(-1) for H(2)tmp. Since the reaction of H(2)oep was too fast to observe the reaction trace due to the dead time of 2 ms for the present stopped-flow technique, the rate constant was estimated to be greater than 1.5 x 10(6) M(-1) s(-1). According to the structure of the Cu(II)-SAT complexes determined by the fluorescent XAFS measurements, two pyrrolenine nitrogens of the meso-substituted porphyrins (H(2)tpp and H(2)tmp) bind to the Cu(II) ion with a Cu-N(pyr) distance of ca. 2.04 A, while those of the beta-pyrrole-substituted porphyrin (H(2)oep) coordinate with the corresponding bond distance of 1.97 A. The shorter distance of H(2)oep is ascribed to the flexibility of the porphyrin ring, and the much greater rate for the formation of the Cu(II)-SAT complex of H(2)oep than those for the meso-substituted porphyrins is interpreted as due to a small energetic loss at the porphyrin deformation step during the formation of the Cu(II)-SAT complex. The overall formation constants, beta(n), of [Cu(H(2)O)(n)()(an)(6)(-)(n)](2+) for the water addition in aqueous AN were spectrophotometrically determined at 298 K as follows: log(beta(1)/M(-1)) = 1.19 +/- 0.18, log(beta(2)/M(-2)) = 1.86 +/- 0.35, and log(beta(3)/M(-3)) = 2.12 +/- 0.57. The structure parameters around the Cu(II) ion in [Cu(H(2)O)(n)(an)(6-n)](2+) were determined using XAFS spectroscopy.

Dynamic study of metal-ion incorporation into porphyrins based on the dynamic characterization of metal ions and on sitting-atop complex formation.

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.

Structural characterization and formation mechanism of sitting-atop (SAT) complexes of 5,10,15,20-tetraphenylporphyrin with divalent metal ions. Structure of the Cu(II)-SAT complex as determined by fluorescent extended X-ray absorption fine structure.

The UV-vis absorption spectra and the formation kinetics of sitting-atop (SAT) complexes (M(H2tpp)2+) of 5,10,15,20-tetraphenylporphyrin (H2tpp) with a series of divalent metal ions (M2+ = Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) in acetonitrile have been investigated. The structural characteristics of the SAT complexes for a series of M2+ ions are discussed on the basis of the UV-vis absorption spectra. The structure parameters around Cu2+ in the Cu(II)-SAT complex were determined by a fluorescent EXAFS method: the coordination number is 6 with three kinds of Cu-N interactions having bond lengths of 2.05, 1.98, and 2.32 A for pyrrolenine nitrogens of H2tpp, acetonitrile nitrogens at equatorial sites, and acetonitrile nitrogens at axial sites, respectively. The rate constants at 25 degrees C for the formation of the SAT complex in acetonitrile are as follows: kf/mol-1 kg s-1 = 3.4 x 10(2) for Mn2+, 0.18 for Co2+, 1.6 x 10(-3) for Ni2+, and 61 for Zn2+. The finding that the variation trends in the rate constants for a series of M2+ ions for the SAT complex formation and the solvent exchange reaction in various solvents are very similar indicates that the rate-determining step for the SAT complex formation is the interchange between the coordinating nitrogen of a bound acetonitrile and the pyrrolenine nitrogen of H2tpp. The fact that the values of kf are smaller by 4-6 orders of magnitude in comparison with those of the solvent exchange suggests that there is a large energetic loss due to the fast preequilibria prior to the rate-determining step, such as deformation of a porphyrin ring and outer-sphere encounter formation due to an electrostatic interaction between M2+ and the local negative charge on the pyrrolenine nitrogens. We observed the subsequent formation of the corresponding metalloporphyrins by deprotonation of the SAT complex and oxidation of the M2+ center, of which the dynamic behavior was significantly different for the different M2+ ions.

Crystal of meso- p-tolyl-porphyrinato copper(II) Cu(tptp) and di-cation ion-pair complex [H 4tptp] 2+[CF 3SO 3] 2− formation during the reaction of Cu(CF 3SO 3) 2 with meso- p-tolyl-porphyrin in CDCl 3

In the NMR and ESR time-scale of this experiment, an attempt was made to prepare the copper sitting-atop (SAT) species [Cu(H 2tptp)] 2+ by the reaction of Cu(CF 3SO 3) 2 with meso- p-tolyl-porphyrin (H 2tptp) in CDCl 3; however, this led to the formation of meso- p-tolyl-porphyrinatocopper(II) Cu(tptp) and the protonated porphyrin [H 4tptp] 2+[CF 3SO 3] 2 − ( 2a). Previous researchers might have misinterpreted the SAT [Cu(H 2tpp)] 2+ complex as being a single component instead of a mixture of [H 4tpp] 2+[CF 3SO 3] 2 − ( 1) and Cu(tpp) for a similar reaction between 5,10,15,20-tetraphenylporphyrin H 2tpp and Cu(CF 3SO 3) 2 in CH 3CN. This work determines the crystal structure of Cu(tptp) and its hyperfine constants a Cu=94.0 G and a N=15.8 G using ESR. The crystal structure of [H 4tptp] 2+[CH 3SO 3] 2 − ( 3) is reported and employed to simulate the groupings of [H 4tptp] 2+ and CF 3SO 3 − in the di-cation ion-pair complex 2a.

Formation and Deprotonation Kinetics of the Sitting-Atop Complex of Copper(II) Ion with 5,10,15,20-Tetraphenylporphyrin Relevant to the Porphyrin Metalation Mechanism. Structure of Copper(II)-Pyridine Complexes in Acetonitrile As Determined by EXAFS Spectroscopy.

The formation of a sitting-atop (SAT) complex of Cu(II) ion with 5,10,15,20-tetraphenylporphyrin (H(2)tpp) in acetonitrile has been observed, and the kinetic parameters for the formation were determined as follows: k(S0) = (3.6 +/- 0.1) x 10(5) mol(-)(1) dm(3) s(-)(1) at 25.0 degrees C, DeltaH(S0)() = 56 +/- 5 kJ mol(-)(1), and DeltaS(S0)() = 46 +/- 19 J mol(-)(1) K(-)(1). The (1)H NMR spectrum of the SAT complex (Cu(H(2)tpp)(2+)) indicated that two pyrrolenine nitrogens coordinate to the Cu(II) ion and that two protons bound to the pyrrole nitrogens remain. The protons were abstracted by the addition of pyridine (py) as the Brønsted base to give the Cu(tpp) metalloporphyrin. In the presence of py, the product for the reaction of the Cu(II) ion with H(2)tpp was Cu(tpp) instead of the SAT complex. The observed conditional rates for the formation of Cu(H(2)tpp)(2+) and Cu(tpp) were interpreted by the contribution of Cu(2+), Cu(py)(2+), and Cu(py)(2)(2+) species, and the second-order rate constants of the SAT complex formation were k(S1) = (3.5 +/- 0.3) x 10(4) mol(-)(1) dm(3) s(-)(1) for Cu(py)(2+) and k(S2) = 90 +/- 2 mol(-)(1) dm(3) s(-)(1) for Cu(py)(2)(2+). Deprotonation rates were measured by following the reaction between the SAT complex and py as a function of the py concentration, and the second-order rate constant was determined to be (2.3 +/- 0.1) x 10(2) mol(-)(1) dm(3) s(-)(1). The present kinetic results have indicated that the SAT complex exists during the course of the metalation process and that the SAT complex formation is a rate-determining step.

Spectral Evidence and Kinetics for Formation of the Sitting-Atop Complex of Copper(II) Ion with 5,10,15,20-Tetraphenylporphyrin in Acetonitrile

Abstract The kinetic parameters for the formation of the sitting-atop (SAT) complex of the copper(II) ion with 5,10,15,20-tetraphenylporphyrin (H2tpp) were directly determined in acetonitrile. The large ΔH≠ and ΔS≠ values imply that deformation of the porphyrin ring is required during the activation process. In the SAT complex, the protons bound to the pyrrole nitrogens remain on the nitrogens and by the addition of a base such as pyridine, they are abstracted to form the Cu(tpp) complex, in which the copper(II) ion is incorporated into the porphyrin core.