Apurba Kalita

Fluorescence-based detection of nitric oxide in aqueous and methanol media using a copper(II) complex

The quenched fluorescent intensity of a copper(II) complex, 1, of a fluorescent ligand, in degassed methanol or aqueous (buffered at pH 7.2) solution, was found to reappear on exposure to nitric oxide. Thus, it can function as a fluorescence based nitric oxide sensor. It has been found that the present complex can be used to sense nanomolar quantities of nitric oxide in both methanol and pH 7.2 buffered-water medium.

Reduction of copper(II) complexes of tripodal ligands by nitric oxide and trinitrosation of the ligands.

The copper(II) centers in two copper(II) complexes of tripodal amine ligands, in acetonitrile solvent, upon exposure to nitric oxide have been found to produce a thermally unstable [Cu(II)-NO] intermediate followed by the reduction of copper(II) to copper(I). This reduction resulted in the concomitant trinitrosation of the ligands at their terminal amine centers.

Role of ligand to control the mechanism of nitric oxide reduction of copper(II) complexes and ligand nitrosation.

The nitric oxide reactivity of two copper(II) complexes, 1 and 2 with ligands L(1) and L(2), respectively, [L(1) = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, L(2) = 5,5,7-trimethyl-[1,4]-diazepane] have been studied. The copper(II) center in complex 1 was found to be unreactive toward nitric oxide in pure acetonitrile; however, it displayed reduction in methanol solvent in presence of base. The copper(II) center in 2, in acetonitrile solvent, on exposure to nitric oxide has been found to be reduced to copper(I). The same reduction was observed in methanol, also, in case of complex 2. In case of complex 1, presumably, the attack of nitric oxide on the deprotonated amine is the first step, followed by electron transfer to the copper(II) center to afford the reduction. Alternatively, first NO coordination to the Cu(II) followed by NO(+) migration to the secondary amine is the most probable in case of complex 2. The observation of the transient intermediate in UV-visible and FT-IR spectroscopy prior to reduction in case of complex 2 also supports this possibility. In both cases, the reduction resulted into N-nitrosation; in 1, only mononitrosation was observed whereas complex 2 afforded dinitrosation as major product along with a minor amount of mononitrosation. Thus, it is evident from the present study that the macrocyclic ligands prefer the deprotonation pathway leading to mononitrosation; whereas nonmacrocyclic ones prefer the [Cu(II)-NO] intermediate pathway resulting into nitrosation at all the available sites of the ligand as major product.

Reaction of a copper(II)–nitrosyl complex with hydrogen peroxide: putative formation of a copper(I)–peroxynitrite intermediate

The reaction of a Cu(II)-nitrosyl complex (1) with hydrogen peroxide at -20 °C in acetonitrile results in the formation of the corresponding Cu(I)-peroxynitrite intermediate. The reduction of the Cu(II) center was monitored by UV-visible spectroscopic studies. Formation of the peroxynitrite intermediate has been confirmed by its characteristic phenol ring nitration reaction as well as isolation of corresponding Cu(I)-nitrate (2). On air oxidation, 2 resulted in the corresponding Cu(II)-nitrate (3). Thus, these results demonstrate a possible decomposition pathway for H(2)O(2) and NO through the formation of a peroxynitrite intermediate in biological systems.

Reduction of copper(II) complexes of tridentate ligands by nitric oxide and fluorescent detection of NO in methanol and water media.

Two copper complexes, 1 and 2, with tridentate N-donor ligands, L(1) and L(2) [L(1)= (1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)amine, L(2) = (2-pyridin-2-yl-ethyl)-pyridin-2 yl-methylamine] respectively, have been synthesized and characterized. On exposure to nitric oxide, the copper(II) centers in complexes 1 and 2 were found to undergo reduction in various solvents. In acetonitrile solvent the reduction was accompanied by a simultaneous N-nitrosation on the secondary amine center on the ligand frameworks. Complexes 3 and 4 were prepared with ligands L(3) and L(4), respectively. L(3) and L(4) [L(3) = 5-dimethylamino-naphthalene-1-sulfonic acid (1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)-amide; L(4) = 5-dimethylamino-naphthalene-1-sulfonic acid(2-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl-amide] are the dansyl derivatives of L(1) and L(2), respectively. Complex 4, due to paramagnetic quenching, does not display any fluorescence; however, on addition of nitric oxide to a methanol or water solution of complex 4, the fluorescence intensity of the fluorophore has been found to be restored. This is attributed to the reduction of the Cu(II) center by nitric oxide to diamagnetic Cu(I). The turn-on of quenched fluorescence intensity has been observed both in methanol and water media.

Reaction of a Copper(II)–Nitrosyl Complex with Hydrogen Peroxide: Phenol Ring Nitration through a Putative Peroxynitrite Intermediate

Copper(II) complex, 1, with the histidine-derived ligand L (L = methyl 2-(2-hydroxybenzylamino)-3-(1H-imidazol-5-yl)propanoate) has been synthesized and characterized. Single-crystal structure determination reveals a diphenolato-bridged dicopper(II) core in 1. Addition of (•)NO to an acetonitrile solution of 1 affords the corresponding mononuclear copper(II)-nitrosyl complex, 2. In the presence of H2O2, 2 results in formation of the corresponding copper(I)-peroxynitrite. Formation of peroxynitrite ((-)OONO) intermediate is evident from its characteristic phenol ring nitration reaction which resembles the tyrosine nitration in biological systems. Further, isolation of nitrate (NO3(-)) as the decomposition product from 2 at room temperature also supports the involvement of (-)OONO intermediate.