Porphyrins in analytical chemistry. A review.

Porphyrin assemblies as chemical sensors

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

Equilibrium study of ion-pair extraction of a cationic water-soluble porphyrin [5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, H2tmpyp4+] and its metalloporphyrins (MP) into the acetonitrile layer, separated by addition of sodium chloride (4.00 mol dm-3) to a 1:1 (v/v) acetonitrile−water mixed solvent, was carried out to develop a new and useful method for the determination of a subnanogram amount of copper(II). M denotes Zn2+, Cu2+, Co3+, Fe3+, and Mn3+, and P2- is porphyrinate ion. The extraction and dissociation constants of the ion-pair complexes, defined by Kex = [MP(ClO4)4]org[MP4+]aq-1[ClO4-]aq-4, Kdis,1 = [MP(ClO4)3+]org[ClO4-]org[MP(ClO4)4]org-1, and Kdis,2 = [MP(ClO4)22+]org[ClO4-]org[MP(ClO4)3+]org-1, were determined by taking into account the partition constant of sodium perchlorate (KD = 1.82 ± 0.01). The equilibrium constants were found to be KexKdis,1 = (7.2 ± 1.3) × 104, (6.4 ± 0.9) × 104, (1.35 ± 0.13) × 105, (4.8 ± 0.6) × 103, (1.23 ± 0.05) × 104, and (1.42 ± 0.07) × 103 at 25 °C...

Ion-Pair Extraction of Metalloporphyrins into Acetonitrile for Determination of Copper(II).

Developing biosensors for Pb2+ is an important analytical topic. DNA-based Pb2+ sensors have been designed mainly based on RNA-cleaving DNAzymes and Pb2+-induced folding of G-quadruplex (G4) DNA. Porphyrin metalation is a key reaction in biology and catalysis. Many enzyme mimics have been developed to catalyze this reaction, and some metalation DNAzymes were reported with a G4 structure. Inspired by the excellent G4 binding properties of certain divalent metal ions, we herein screened a few metals and G-rich DNA sequences. The metalation activity of a DNA named T30695 (sequence: (G3T)4) was significantly accelerated by Pb2+. The reaction of Cu2+ insertion into the mesoporphyrin IX had a kcat of 0.89 min-1 and a Km of 9.8 μM, representing a catalytic efficiency similar to that of human ferrochelatase. The reason for the acceleration was attributed to Pb2+ binding of the G4 DNA and the catalytic activity of the large Pb2+ ion for this reaction. A ratiometric sensor for Pb2+ was developed by inserting Zn2+ with a detection limit of 23.5 nM Pb2+. This work has established a new DNA-based reaction that can be used for Pb2+ detection, and it also provides a highly efficient new DNAzyme for porphyrin metalation, which might be used for signal production for other biosensors.

Efficient DNA-Catalyzed Porphyrin Metalation for Fluorescent Ratiometric Pb2+ Detection.

Several procedures have been proposed to accelerate the slow rate of metalloporphyrin formation. This article describes kinetic and mechanistic aspects of the incorporation of metal ions into porphyrins and their application to the kinetic analysis of traces of metal ions.

Masaaki Tabata

Papers

Porphyrins as reagents for trace-metal analysis