Type Of Donor Atoms Research Articles

Spectroscopic and voltammetric studies of copper(II) complexes of bis(pyrid-2-yl)-di/trithia ligands bound to calf thymus DNA

Absorption and circular dichroic (CD) spectral, viscometric and electrochemical studies have been carried out on the interaction of [Cu(pdto)(ClO 4)] (ClO 4) (pdto = bis(pyrid-2-yl-3,6-dithiaoctane), [Cu(pttu)] (ClO 4) 2 (pttu = bis(pyrid-2-yl-3,6,9-trithiaundecane) and [Cu(pttn)] (ClO 4) 2 (pttn = bis(pyrid-2,yl)-3,6,9-trithianonane) with calf thymus DNA. The intense S( σ) → Cu(II) charge transfer band of almost all these complexes exhibits a large hypochromism in the presence of DNA suggesting that all the complexes undergo ligand displacement in the presence of DNA leading to stabilisation of the Cu 2+-DNA interaction. Spectral, viscosity and electrochemical studies suggest that the [Cu(pdto)] 2+ complex binds to DNA presaturated with copper(II) perchlorate (Cu 2+-DNA, [NP]/[Cu(ClO 4) 2] ∼-2.2). via partial intercalation of the pyridine ring of the ligand. On the other hand, [Cu(pttu)] 2+ and [Cu(pttn)] 2+ engage in relatively weak binding with Cu 2+-DNA via simple surface contact. CD spectral studies suggest negligible transconformational changes for Cu 2-DNA from a more B-like to a more C-like structure on binding to all the complexes. The diaza analogue of [Cu(pdto)] 2+ is stable in the presence of DNA due to the hydrogen bonding interaction of the coordinated NH with DNA. The residual charge on Cu(II), the coordination geometry and the ligand donor atom type play a key role in deciding the mode and extent of binding of complexes to DNA.

Structural Criteria for the Rational Design of Selective Ligands. 2. Effect of Alkyl Substitution on Metal Ion Complex Stability with Ligands Bearing Ethylene-Bridged Ether Donors.

A novel approach is presented for the application and interpretation of molecular mechanics calculations in ligand structural design. The methodology yields strain energies that (i) provide a yardstick for the measurement of ligand binding site organization for metal ion complexation and (ii) allow the comparison of any two ligands independent of either the number and type of donor atoms or the identity of the metal ion. Application of this methodology is demonstrated in a detailed examination of the influence of alkyl substitution on the structural organization of ethylene-bridged, bidentate, ether donor ligands for the alkali and alkaline earth cations. Nine cases are examined, including the unsubstituted ethylene bridge (dimethoxyethane), all possible arrangements of individual alkyl groups (monoalkylation, gem-dialkylation, meso-dialkylation,d,l-dialkylation, trialkylation, and tetraalkylation), and both cis and trans attachments of the cyclohexyl group. The calculated degree of binding site organization for metal ion complexation afforded by these connecting structures is shown to correlate with known changes in complex stability caused by alkyl substitution of crown ether macrocycles.

Energetics of the first and second coordination spheres of transition metal ions

For complexes of transition metals (manganese, iron, cobalt, nickel) with monodentate ligands, equilibrium metal-ligand distances and ligand bond energies in the first and second coordination spheres have been calculated by the CNDO method. Some effects of ligand bond energies in different coordination spheres are analyzed. These effects significantly differ between the first and second coordination spheres. In the first sphere, the ligand bond energy is mainly determined by the nature of the central ion and the type of donor atom of the ligand, but weakly depends on the structure of the ligand. Conversely, in the second coordination sphere, the ligand bond energy weakly depends on the nature of the central ion and the type of donor atom, but considerably depends on the structure of the ligands in the first coordination sphere. In the second coordination sphere, ligand binding is determined by ligand interactions with both the central ion and the ligands of the first sphere. In the general case, when strong specific interactions between ligands are absent, the energetics of the second sphere is determined by the size of the inner-spheric ligands, which may be considered to be a specific steric effect.

Liquid membrane transport of supramolecular complexes of some amines and amino acids with macrocyclic ligands

The transport of some amines in protonated form was studied (viz. methylamine, dimethylamine, diethylamine andn-propylamine) and α-amino acids (l-leucine,l-methionine,l-isoleucine,l-phenylalanine,l-valine,l-α-alanine andl-cysteine). The following macrocyclic ligands were used as carriers throughout the experiments: 15-crown-5 (15C5), 18-crown-6 (18C6), benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), diazacrown ether [2.2] (1,7,10,16-tetraoxa-4,13-diazacyclooctadecane) and cryptand [2.2.2] (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8.8.8] hexacosane). The active transport, assisted by pH gradient, of amino acids and amines in protonated form as ion pairs in the presence of picrate anion was performed. The experiments suggested the influence of the ligand size, the donor atom type, and the substituents on the transport phenomena.

Technetium(V) Complexes of Substituted Benzimidazolylalcohols and Thiols. Influence of the Donor Atom Type on the Configuration

Abstract Cationic square-pyramidal complexes of general formula (TcO(L)+ 2]Cl− were prepared by the reaction of (n-Bu4N)TcOCl4 with the potentially bidentate N,S-donor benzimidazolylthiols (HL) as ligands in the presence of triethylamine. The corresponding reaction with the N,O-donor benzimidazolylalcohols led to the formation and isolation of the neutral six-coordinate complexes TcOCl(L)2. These compounds were characterized by elementalanalysis, vibrational and optical spectroscopy. Mechanisms for the formation reactions and probable configurations of the complexes are proposed on the basis of qualitative studies and spectroscopic results.

The reaction of zinc sulphide with ionic copper—the biological implications. Part 1. Surface area studies of zinc sulphide and its reaction with copper complexes and copper containing proteins

The role of metal sulphides vis-à-vis the availability of dietary copper in ruminant animals has been investigated using zinc sulphide as a model metal sulphide and a selection of copper complexes and copper containing proteins as models for sources of dietary copper. The extent of reactivity of zinc sulphide towards the copper complexes is dependent upon the type of donor atom co-ordinated to copper: ▪ The order to reactivity is found to be CuO > CuN > CuS complexes and is in keeping with the reported values for the instability constant p K n of the complexes. In contrast, no reaction is observed between zinc sulphide and the copper containing proteins studied (azurin, superoxide dismutase and cerulophasmin) and is attributed to the protection of the copper centres by the protein backbone. The results facilitate an understanding of copper metabolism in ruminants and a mechanism is proposed for the removal of dietary copper sources in such species. Reactions between copper(II) sulphate solutions and samples of zinc sulphide having a range of specific surface areas (prepared by sintering at differing temperatures) have been studied. The fact that the reactivity is found to be highly dependent upon the specific surface area of the metal sulphide may well be of significance when considering the fate of copper in sulphur-rich biological systems.

113Cd NMR spectroscopy of coordination compounds and proteins

Despite the proven utility of 113Cd NMR chemical shifts for qualitatively or, in some cases, semiquantitatively assessing the nature of ligands at metal binding sites in metalloproteins and for studying enzyme function and mechanisms, it is likely that the full potential of 113Cd spectroscopy as a metallobioprobe has yet to be realized. The development of a more quantitative relationship between structural parameters and 113Cd chemical shift has been hampered in part by the lack of suitable model compounds and chemical dynamics [6]. As described throughout this review, these challenges are currently being met through applications of solid-state and low-temperature solution NMR methods and the design and study of less labile protein model compounds. However, even for cases where dynamics and solvent or anion effects can be ruled out, it appears that ligand atom type and number, bond lengths, coordination geometry, and neighboring atoms (including other metals) all influence 113Cd shifts. Studies of coordination compounds in solution have thus far provided useful information on the influence of donor-atom type and number on isotropic Cd chemical shifts. The influence on shift of coordination geometry and CdL bond lengths will probably only be adequately evaluated using the single crystal approach pioneered by Ellis and co-workers. Studies of the Cd ion bound exclusively by O-donor ligands will likely also be only adequately addressed through solid-state NMR methods due to lability problems with these ligands. In this area, model compounds with O-donor ligands which exhibit the unusual high-field shifts found for ICaBP and HRP are lacking. 113Cd NMR chemical shift information alone is at present inadequate for unambiguous determination of structural features or structural changes at the metal binding sites of metalloproteins. In this regard, HMQC NMR spectroscopy promises to be a particularly useful tool for identifying the ligands which are coordinated to Cd. For cases where ligand exchange or nuclear relaxation rates are high (relative to 1/ J), however, this method will not work and determination of structural features may rely exclusively on chemical shift analyses. It is therefore important that both techniques be developed and utilized.

Cation transport at 25°c from binary Na +M n+ , Cs +M n+ and Sr 2+M n+ nitrate mixtures in a H 2OCHCl 3H 2O liquid membrane system containing a series of macrocyclic carriers

Cation fluxes from binary mixtures of either Na +, Cs + or Sr 2+ with other alkali metal cations, alkaline earth metal cations, and Pb 2+ through a H 2OCHCl 3H 2O bulk liquid membrane system containing one of several macrocyclic carriers have been determined Nitrate salts were used in all cases. The most selective transport of Na + over all other cations studied was found with the carrier cryptand [2.2.1]. Selective transport of Na + relative to Li +, Cs + and the alkaline earth cations was found with cryptand [2.2.2B] and cryptand [2.2.2D]. The ligands 21-crown-7 and dibenzo-24-crown-8 showed selective transport of Cs + over the second cation in all cases. Several macrocycles showed selectivity for Sr 2+ over the second cation with the macrocycle 1,10-diaza-18-crown-6 showing the highest selectivity for this cation of all ligands studied. Relative fluxes from binary cation mixtures are rationalized in terms of macrocycle cavity size, donor atom type and ring substituents.

Diaza-crown ethers—V: Stability constants of N,N′-disubstituted diaza-crown ether complexes with various metal ions

Stability constants have been determined at 25°C in an aqueous 0.500 M Li(ClO 4) medium by potentiometric methods using the following ligands and metal ions 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane ( 2,2), N,N′-bis-(2-methoxyethyl)- 2,2(BME- 2,2), N,N′-bis-(2-hydroxyethyl)- 2,2(BHE- 2,2), N,N′-bis-(carbamoylmethyl)- 2,2(BCAM- 2,2) and Na +, K +, Mg 2+, Ca 2+, Cu 2+, Ag +, Cd 2+, Pb 2+, respectively. The magnitude of the stability constants are discussed in terms of metal ion hydration, metal ion/cavity size, hardness/softness of the various acceptors, the type of donor atom and the topology of the ligand. Comparisons are made with other cyclic and bicyclic diazapolyethers.

Chelate antidotes for acute Ni(II) intoxication

Twenty-seven chelating agents have been screened for efficacy as antidotes for acute Ni(II) acetate intoxication. Of these, thirteen have been found to be good to excellent antidotes when administered intraperitoneally at a level of chelate to Ni(II) of 10:1. An examination of the log of the ratio ( K NiX/ K CaX) for those compounds which are effective as antidotes showed values ranging from 3 up to 10. From the results obtained it would appear that there should be a very large number of chelating agents which should be capable of acting as antidotes in acute Ni(II) intoxication or in other circumstances where it was desired to enhance the urinary excretion of Ni(II). Sodium diethyldithiocarbamate, the antidote of choice for nickel carbonyl intoxication is not effective as an antidote in Ni(II) intoxication. The fact that N-Acetyl- d, l-penicillamine has none of the antidotal activity of d-penicillamine shows the essential role of nitrogen donor atom in this compound. Chelate molecules containing S, N, or O donors may be effective and no single type of donor atom can be described as essential to the functioning of these antidotes, though N and O seem somewhat better than S. Zinc diethylenetriaminepentaacetate is not an antidote, even though the stability constants favor its transformation into the nickel complex. In this case the slow rate of the transformation may be responsible for the absence of antidotal activity.

The Coextraction of Water into Nitrobenzene with Alkali and Alkaline Earth Metal Salts of 2,2′,4,4′,6,6′-Hexanitrodiphenylamine in the Presence of Several Crown Ethers and Cryptands

Abstract Alkali and alkaline earth metal salts of 2,2′,4,4′,6,6′-hexanitrodiphenylamine (HND) were extracted into nitrobenzene in the absence or presence of several crown ethers or cryptands which differ from one another in hole size. The quantity of water transferred to the nitrobenzene phase was determined by means of the Karl-Fischer method. The NMR and near-infrared spectra show that the coextraction of water is caused by the hydration of the cations. The number of water molecules attached to the cations increases upon going from Cs+ (0.6) to Li+ (5.6) and from Ba2+ (10.5) to Ca2+ (13.0). The complexation between these cations and the crown ethers causes a clear decrease in the hydration number; e.g., in the 1:1 metal–crown ether complexes, more than half of the water molecules are removed, and in the 1:1 metal–cryptand complexes, less than one water molecule remains unremoved. The complexes of a 1:2 stoichiometry are also found; some are stable and carry virtually no water molecules. The number and type of donor atoms have no significant effect on the coextraction of water. The presence of fused benzo rings on the crown ether also has no influence on the number of water molecules.

A systematic study of the effect of macrocycle ring size and donor atom type on the log K, .DELTA.H, and T.DELTA.S of reactions at 25.degree.C in methanol of mono- and divalent cations with crown ethers

A systematic study of the effect of macrocycle ring size and donor atom type on the log K, .DELTA.H, and T.DELTA.S of reactions at 25.degree.C in methanol of mono- and divalent cations with crown ethers