Metal Ion Promoted Reactions of Phosphate Derivatives. In: Lippard SJ, editor. Progress in Inorganic Chemistry. Hoboken: John Wiley & Sons, Inc.; 1990. pp. 201-58. [DOI: 10.1002/9780470166390.ch4]

A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions

From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [Ni^II₂(L)(H₂O)₂(CH₃OH)][ClO₄]·CH₃OH (1) (H₃L = 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N',N″-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH₃CN/CH₃OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [Ni^II₅(H₂L)₂(μ₃-OH)₂(μ-O₂CCH₃)₄][ClO₄]₂·CH₃CO₂C₂H₅ (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [Ni^II₂(HL)(μ-O₂CCH₃)(H₂O)₂][ClO₄]·4H₂O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H₃L with 2 equiv of Ni^II(O₂CCH₃)₂·4H₂O, followed by the usual workup, and recrystallization from CH₂Cl₂ led to the isolation of [Ni^II₂(H₂L)(μ-O₂CCH₃)₂][ClO₄]·CH₂Cl₂·2H₂O (4). Complex 4 is structurally different from 3, confirming that the reaction of Ni^II(O₂CCH₃)₂·4H₂O with H₃L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by Ni^II-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [Ni^II₂(HL)(μ-O₂CCH₂CH₃)(H₂O)₂][ClO₄] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = -25.0(1) cm^-1 for 1, J = -20.0(1) cm^-1 for 4, and J = -18.80(8) cm^-1 for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (J_a = +16.02 cm^-1, and J_b = +9.02 cm^-1) and an antiferromagnetic (J_c = -49.7 cm^-1), have been identified.

Lead ion induced chemodosimeter approach of a tripodal hydroxyl-quinoline based phospho-ester through P–O bond cleavage

Pb²⁺ induced phosphoester hydrolysis followed by the formation of a penta coordinated chelate complex of lead.

Progress in artificial metallonucleases

The development of synthetic agents able to hydrolytically cleave DNA with high efficiency and selectivity is still a fascinating challenge. Over the years, many examples have been reported reproducing part of the behaviour of the corresponding natural enzymes. Eventually, even the possibility to apply such systems to the manipulation of DNA of higher organisms has been demonstrated. However, efficiency of enzymes is still unrivalled. This feature article discusses the progress reported toward the realization of synthetic nucleases with particular attention to the comprehension of the reaction mechanisms and to the strategies that need to be addressed to obtain more efficient systems.

Synthesis, DNA binding and cleavage activities of copper (II) thiocyanate complex with 4-(N,N-dimethylamino)pyridine and N,N-dimethylformamide

Two novel copper(II) thiocyanate complexes with 4-(N,N-dimethylamino) pyridine and N,N-dimethylformamide (1) and with 4-(N,N-dimethylamino) pyridine (2) have been synthesized and characterized. The crystal and molecular structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction. Antioxidative activity tests in vitro showed that complex 1 has significant antioxidative activity against hydroxyl free radicals from the Fenton reaction and also oxygen free radicals, which is better than standard antioxidants like vitamin C and mannitol. The interaction of complex 1 with calf thymus DNA was investigated by spectroscopic, cyclic voltammetry, and viscosity measurements. Results suggest that complex 1 can bind to DNA via partial intercalation mode. Moreover, complex 1 has been found to cleavage of plasmid DNA pBR322.

Cleavage of phosphate diesters mediated by Zn(II) complex in Gemini surfactant micelles

The cleavage of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) catalyzed by the Zn(II)-biap (biap: N,N-bis(2-ethyl-5-methylimidazole-4-ylmethyl)aminopropane) complex has been investigated spectrophotometrically in a micellar solution of cationic Gemini surfactant 16-2-16 [bis(hexadecyldimethylammonium)ethane bromide] and CTAB (hexadecyltrimethylammonium bromide) at 25+/-0.1 degrees C. The experimental results reveal that a higher rate of acceleration (about 2016-fold) of HPNP cleavage promoted by the Zn(II)-biap complex has been observed in the 16-2-16 micellar solution in comparison with the background rate (k(0)) of HPNP spontaneous cleavage at 25 degrees C. Reaction rates of HPNP cleavage in CTAB micellar solutions are only about 40% of that in Gemini 16-2-16 micelles under comparable conditions. In addition, the cleavage rates of HPNP in Gemini micelles and in CTAB micelles are respectively 29.5 times and 12 times faster than that in aqueous buffer. Especially, a "sandwich absorptive mode" has been proposed to explain the acceleration of HPNP cleavage in a cationic micellar solution.

Artificial dinuclear phosphoesterases

Many elegant dinuclear transition metal and lanthanide complexes that efficiently hydrolyze phosphate esters including RNA and DNA have been reported recently. In most cases, the dinuclear complexes are much more reactive than the corresponding mononuclear metal complexes. Furthermore, structural and kinetic data indicate that substantial rate acceleration for phosphate diester cleavage is obtained by bridging the dinuclear metal centers with the two phosphoryl oxygens. Interestingly, such bridging structures have recently been implicated in several metalloenzyme catalyzed phosphate hydrolyses.

The sequence-specific cleavage of RNA by artificial chemical ribonucleases

Based on work spanning 50 years, several groups have recently achieved the specific cleavage of RNA by attaching RNA-cleaving chemical moieties to antisense oligonucleotides. Such artificial chemical ribonucleases have potential as a possible next generation of antisense compounds and also as probes for structural and functional investigations of RNA. Different chemical moieties, such as polyamines, imidazoles, and metal complexes, have been used as the catalytic part of the artificial nucleases. To be of practical use as therapeutics, however, the conjugates must fulfil a number of strict requirements, such as ease of preparation, chemical stability, selectivity, nontoxicity, and, for metal complexes, inertness to loss of cation from the ligand. In addition, high cleavage efficiency is essential to overcome short lifetimes of cellular mRNA targets, and the reaction should not depend on additional cofactors. Based on these criteria, we believe that metal complexes, in particular macrocyclic lanthanide complexes, have the best chance of success for said purpose.