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

Abstract

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.

Chelatases: distort to select?

“…Heme, chlorophyll, cobalamin (vitamin B 12 ), siroheme and coenzyme F 430 belong to a family of prosthetic groups that are characterized by their tetrapyrrole-derived nature and contain a central, complexed metal ion: Fe 2+ in heme and siroheme, Mg 2+ in chlorophyll and bacterio-chlorophyll, Co 2+ in cobalamin, and Ni 2+ in coenzyme F 430 [5,6,8,9]. All of these molecules are derived from a common tetrapyrrole, uroporphyrinogen III ( Figure I), and require the function of specific enzymes or chelatases for the insertion of their metal ion ( Figure I).…”

“…Subsequent studies in solution and with enzymatic systems led to the formulation of a general reaction mechanism for the metalation of porphyrins [12][13][14][15]. The steps for the mechanism include, first, deformation of the porphyrin ring; second, outer-sphere association of the solvated metal ion and the porphyrin; third, exchange of a coordinated solvent molecule with the first pyrrolenine nitrogen atom; fourth, chelate-ring closure with liberation of additional solvent molecules to form the 'sitting-atop complex'; fifth, first deprotonation of a pyrrole nitrogen atom; and last, deprotonation of the second pyrrole nitrogen with formation of the metalated porphyrin ( Figure Ib).…”

“…The resulting metalated tetrapyrroles constitute the centerpiece for proteins involved in cellular processes ranging from oxygen and electron transport, signal transduction, photosynthesis, and sulfur and nitrogen metabolism to molecular rearrangements, methyl-group transfer reactions and regulation of transcription and translation [1][2][3][4]. Metalated tetrapyrroles such as the cofactors and prosthetic groups heme, chlorophyll, cobalamin (vitamin B 12 ) and coenzyme F 430 have a centrally chelated metal ion and arise from a common tetrapyrrole, uroporphyrinogen III. The chemical nature of the chelated metal ion varies; for example, Fe 2+ is present in heme, Mg 2+ in chlorophyll, Co 2+ in cobalamin and Ni 2+ in coenzyme F 430 [5][6][7][8][9].…”

“…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.…”

A Novel Copper(II) Sensor Based on a Unique Water-soluble Porphyrin

“…S1 in Supporting Information). This demonstrated that the sitting-atop (SAT) complex [Cu(H2TEHPPS)] 2+ had been produced as an intermediate, [29][30][31] and then the protons on two of the pyrrole nitrogen atoms could be lost to form Cu-TEHPPS upon subjecting the complexes to heat (95 C) for 10 min. 32 In order to validate this mechanism, an NMR titration experiment was used.…”