Substituent effects on the electron transfer reactivity of hydroquinones with laccase blue copper.

Stopped-flow kinetic studies of the anaerobic reduction of Rhus vernicifera laccase (monophenol, dihydroxyphenylalanine:oxygen oxidoreductase, EC 1.14.18.1) type 1 copper by 25 mono- and disubstituted hydroquinones (H2Q-X) have been performed at 25 degrees C and pH 7.0 in 0.5 M phosphate. All of the data are compatible with a mechanism involving rapid enzyme-substrate complex formation followed by rate-limiting intra-complex electron transfer. ES complex formation constants (Qp) for many substrates are strikingly insensitive to the electronic characteristics of the substituent X, falling within the range 5--50 M-1. It is shown that this result may be accounted for if only the singly ionized forms of the substituted hydroquinones are bound by the enzyme. All of the substrates exhibiting exceptionally high Qp values (greater than 50 M-1) have X groups capable of functioning as ligands; substituents with lone pairs of electrons may facilitate enzyme-substrate complex formation by enabling hydroquinone to function as a bidentate bridging ligand between the type 2 and type 3 copper sites. Intra-complex electron transfer rate constants for most substrates are remarkably insensitive to the thermodynamic driving force for the oxidation of H2Q-X to the corresponding semiquinone, the average value for ten substrates being 30 +/- 10 s-1. The electron transfer reactivity of polyphenols with laccase blue copper therefore appears to be controlled largely by protein-dependent activation requirements rather than by the oxidizability of the substrate.

Effect of structural variation within sym-(R)dibenzo-16-crown-5-oxyacetic acid resins upon the selectivity and efficiency of alkali-metal cation sorption.

Five new crown ether carboxyhc acid resins have been prepared by condensation polymerization of sym-(R)dibenzo-16-crown-5-oxyacetic acids with formaldehyde. Competitive alkali-metal cation sorption by these novel resins, which contain both ion-exchange and crown ether binding sites for metal ions, has been investigated. As the R-group was varied (methyl, ethyl, propyl, butyl, hexyl and decyl) both the alkali-metal cation sorption selectivity and efficiency were affected. The highest efficiency (loading) and Na(+) sorption selectivity were obtained when R = methyl, ethyl and propyl. The longer alkyl groups were found to be detrimental to both sorption efficiency and selectivity.

A comparative study of the effect of o-nitrophenyl octyl ether and o-nitrophenyl pentyl ether as plasticizers on the response and selectivity of carrier-based liquid membrane ion-selective electrodes.

Lithium, potassium and caesium-selective microelectrodes were prepared by coating the tips of preconditioned silver wires, incorporated in a flow-cell, with PVC membranes containing four different ionophores. A dicarboxamide, a 14-crown-4 carboxylic acid, benzo-18-crown-6 and di-(tert-butylbenzo)-21-crown-7 ionophores were used in the electrode matrix. The first two ionophores were used in lithium ion-selective electrodes, the third in a potassium ion electrode and the fourth in a caesium ion electrode. Two different plasticizers, o-nitrophenyl octyl ether (NPOE) and o-nitrophenyl pentyl ether (NPP'E) were used. Enhancement of the signal and the slope of the calibration curve and improvement of the curve linearity were observed in all cases when NPP'E was used as plasticizer. A general trend of enhanced selectivity of the electrodes incorporating crown ether ionophores was also observed when NPP'E was the plasticizer.

Influence of medium polarity upon selectivity and efficiency of alkali metal cation sorption by polyether carboxylic acid resins.

The influence of medium polarity upon competitive sorption of alkali-metal cations from aqueous and aqueous methanolic solutions by the polymethacrylic acid resin Amberlite CG-50, two acyclic polyether carboxylic acid resins, and six sym-(alkyl)dibenzo-16-crown-5-oxyacetic acid resins has been investigated. Addition of methanol was found to strongly depress the selective Li(+) sorption of CG-50 and one of the acyclic polyether carboxylic acid resins. Enhancement of metal ion-crown ether interactions as the percentage of methanol in the medium was increased accentuates the Na(+) sorption selectivity for the lariat ether carboxylic acid resins. The highest Na(+) sorption selectivity was obtained for sym-(alkyl)-dibenzo-16-crown-5-oxyacetic acid resins with ethyl and propyl groups in 80% methanol-20% water.

Influence of structural variation in room-temperature ionic liquids on the selectivity and efficiency of competitive alkali metal salt extraction by a crown ether.

An improved method for the preparation of 1-alkyl-3-methylimidazolium hexafluorophosphates provides a series of room-temperature ionic liquids (RTILs) in which the 1-alkyl group is varied systematically from butyl to nonyl. For competitive solvent extraction of aqueous solutions of alkali metal chlorides with solutions of dicyclohexano-18-crown-6 (DC18C6) in these RTILs, the extraction efficiency generally diminished as the length of the 1-alkyl group was increased. Under the same conditions, extraction of alkali metal chlorides into solutions of DC18C6 in chloroform, nitrobenzene, and 1-octanol was undetectable. The extraction selectivity order for DC18C6 in the RTILs was K+ > Rb+ > Cs+ > Na+ > Li+. As the alkyl group in the RTIL was elongated, the K+/ Rb+ and K+/Cs+ selectivities exhibited general increases with the larger enhancement for the latter. For DC18C6 in 1-octyl-3-methylimidazolium hexafluorophosphate, the alkali metal cation extraction selectivity and efficiency were unaffected by variation of the aqueous-phase anion from chloride to nitrate to sulfate.

A synthetic crown ether carboxylic acid ionophore displays synergistic transport of Pr3+ in conjunction with lasalocid.

Transport of Pr3+ across phosphatidyl choline vesicles, as monitored by 31P nmr, is second-order in the crown ether carboxylic acid 2, as it is with respect to lasalocid (X-537 A). When the synthetic (2) and the natural (lasalocid) ionophores are incorporated together in approximately 3:1 ratio into the lipidic phase, the transport velocity is markedly enhanced.

Effect of Structural Variation within Lipophilic Lariat Ether Phosphonic Acid Monoesters on the Selectivity and Efficiency of Competitive Alkali Metal Cation Extraction into Chloroform.

Lipophilic lariat ether phosphonic acid monoethyl esters with systematic crown ether ring size variation from 12-crown-4 to 24-crown-8 are utilized for competitive alkali metal cation extractions from aqueous solutions into chloroform. Effective alkali metal cation extraction from weakly acidic, neutral, and basic aqueous solutions is achieved. With 4, 5, and 6 oxygens in the crown ether rings, selectivities for Li(+), Na(+), and K(+), respectively, are observed. An 18-crown-6 phosphonic acid monoester exhibits excellent extraction selectivity for K(+) with K(+)/Li(+) and K(+)/Na(+) > 100. The lipophilic group attachment site, as well as the crown ether ring size, is shown to influence the extraction selectivity for the lariat ether phosphonic monoesters.

Design, synthesis and evaluation of diazadibenzocrown ethers as Pb(2+) extractants and carriers in plasticized cellulose triacetate membranes.

The ligands 4,7-diaza-2,3,8,9-dibenzo-15-crown-5 (L1), 4,10-diaza-2,3,11,12-dibenzo-18-crown-6 (L2), 4,10-diaza-2,3,11,12-di(4'-tert-butylbenzo)-18-crown-6 (L3) and N,N-di(methylenecarboxyethoxy) 4,10-diaza-2,3,11,12-dibenzo-18-crown-6 (L4) have been prepared. Partition coefficients and acid dissociation constants for these four diazadibenzocrown ether compounds were determined in water-chloroform. Their effectiveness was assessed in solvent extraction of Pb(2+) from aqueous solutions into toluene. Ligands L3 and L4 provide high selectivity for Pb(2+) over Cd(2+) and Zn(2+) in transport across plasticized cellulose triacetate membranes.

Optimization of the lariat ether carboxylic acid host structure for ferrioxamine B: demonstration of a second coordination shell chelate effect.

Host-guest supramolecular assembly formation constants involving the second-sphere complexation of the siderophore ferrioxamine B (FeHDFB(+)) by a lariat ether carboxylic acid host (L(n+2)COOH) in wet chloroform were obtained from liquid-liquid extractions at pH values above and below the host pK(a) (approximately 5.3). The host-guest formation constants, K(a), determined at pH = 3.2 for the assemblies [FeHDFB(+),L(n+2)COOH,ClO(4)(-)] (n = 4, 7, 10, 15) in wet chloroform are similar to those of the parent crown ether, benzo-18-crown-6. At pH = 9.3, the lariat ethers are ionized, and this results in a more stable assembly, [FeHDFB(+),L(n+2)COO(-)], as measured by the host-guest formation constant, K(app). This enhanced stability is shown to be a function of the lariat ether sidearm chain length (n = 4, 7, 10, 15) and is corroborated by molecular modeling calculations. Additionally, molecular modeling and extraction data demonstrate that there is an optimum lariat ether sidearm chain length with respect to host-guest assembly stability as measured by K(app). We attribute the enhanced stability effect of the ionized lariat ether in the host-guest assembly [FeHDFB(+),L(n+2)COO(-)], relative to [FeHDFB(+),L(n)+2)COOH,ClO(4)(-)] or to [FeHDFB(+),B18C6,ClO(4)(-)], to a second coordination shell chelate effect.

Chromogenic lariat ethers for selective alkali metal cation recognition.

Twelve new proton-ionizable, picrylamino-type chromogenic lariat ethers derived from dibenzo-14-crown-4, dibenzo-16-crown-5 and dibenzo-19-crown-6 demonstrate good selectivity and remarkable color response upon extraction of alkali metal cations from basic aqueous solutions into chloroform.

Effect of crown ethers on the ion-exchange behavior of alkaline Earth metals. Toward improved ion-exchange methods for the separation and preconcentration of radium.

The effect of selected crown ethers on the uptake of alkaline earth cations by sulfonic acid and diphosphonic acid-based cation-exchange resins from hydrochloric acid media is examined. The effect observed is shown to vary with the hydrophobicity of the crown ether. Water-soluble crown ethers enhance the sorption of certain cations, thereby improving the selectivity of the resin for other alkaline earths over calcium ion, an apparent result of a synergistic interaction between crown ether present in the resin phase and the ionic functional groups of the resin. In the presence of more hydrophobic crown ethers, a decrease in cation sorption is often observed, a result of the exclusion of the crown ether from the resin phase and the formation of cation-crown complexes in the solution phase. The result can be a reversal of the selectivity sequence ordinarily exhibited by the resin.

Immobilization of crown ether carboxylic acids on silica gel and their use in column concentration of alkali metal cations from dilute aqueous solutions.

Eight crown ethers with pendent carboxylic acid groups are immobilized on silica gel and utilized for column concentration of alkali metal cations from dilute aqueous solutions. The column concentration selectivity and efficiency are found to be strongly influenced by (1) the cavity size of the crown ether unit, (2) conformational positioning of the proton-ionizable side arm with respect to the crown ether cavity, and (3) capping of residual silanol surface groups with trimethylsilyl functions. By use of a chromatographic stripping technique, selective column concentration of Na(+), K(+), (Rb(+) and Cs(+)), and Cs(+) by different functionalized silica gels has been achieved.

Cloning of mRNA sequences from the human colon: preliminary characterisation of defined mRNAs in normal and neoplastic tissues.

RNA has been extracted from the normal human colon, converted into cDNA and cloned in the bacterial plasmid pBR322. About 4,000 sequences from this library were screened with probes derived from normal mucosa, familial polyposis mucosa, colonic adenocarcinomas and the colon tumour cell line HT29. Some mucosal sequences showed greatly reduced levels of transcription in neoplastic conditions, while a few showed elevated transcription. These have been further characterised by Northern and RNA dot-blot analysis.

Experimental and theoretical studies on the thermal decomposition of heterocyclic nitrosimines.

A series of substituted 2-nitrosiminobenzothiazolines (2) were synthesized by the nitrosation of the corresponding 2-iminobenzothiazolines (6). Thermal decomposition of 2a--f and of the seleno analogue 7 in methanol and of 3-methyl-2-nitrosobenzothiazoline (2a) in acetonitrile, 1,4-dioxane, and cyclohexane followed first-order kinetics. The activation parameters for thermal deazetization of 2a were measured in cyclohexane (Delta H(++) = 25.3 +/- 0.5 kcal/mol, Delta S(++) = 1.3 +/- 1.5 eu) and in methanol (Delta H(++) = 22.5 +/- 0.7 kcal/mol, Delta S(++) = -12.9 +/- 2.1 eu). These results indicate a unimolecular decomposition and are consistent with a proposed stepwise mechanism involving cyclization of the nitrosimine followed by loss of N(2). The ground-state conformations of the parent nitrosiminothiazoline (9a) and transition states for rotation around the exocyclic C==N bond, electrocyclic ring closure, and loss of N(2) were calculated using ab initio molecular orbital theory at the MP2/6-31G* level. The calculated gas-phase barrier height for the loss of N(2) from 9a (25.2 kcal/mol, MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE) compares favorably with the experimental barrier for 2a of 25.3 kcal/mol in cyclohexane. The potential energy surface is unusual; the rotational transition state 9a-rot-ts connects directly to the orthogonal transition state for ring-closure 9aTS. The decoupling of rotational and pseudopericyclic bond-forming transition states is contrasted with the single pericyclic transition state (15TS) for the electrocyclic ring-opening of oxetene (15) to acrolein (16). For comparison, the calculated homolytic strength of the N--NO bond is 40.0 kcal/mol (MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE).

A Calixarene-Based Fluorogenic Reagent for Selective Mercury(II) Recognition.

The first calixarene-based fluorogenic Hg(II)-selective extractant, 5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-bis(N-(5-dimethylaminonaphthalene-1-sulfonyl)carbamoylmethoxy)-26,28-dimethoxycalix[4]arene (2), is reported. In solvent extraction from aqueous acidic solutions (HNO(3)), 2 exhibits excellent selectivity for Hg(II) over a wide range of transition, alkali, and alkaline earth metal cations. Quenching of its fluorescence due to Hg(II) coordination is unaffected by the presence of 100-fold excesses of alkali metal cations, alkaline earth metal cations, Ag(I), Tl(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Pd(II), Zn(II), or Fe(III).

Selective Liquid Membrane Transport of Lead(II) by an Acyclic Polyether Dicarboxylic Acid Ionophore.

The effects of the chain structure and substitutents in six acyclic polyether dicarboxylic acids and one acyclic polyether carboxylic acid upon the efficiency and selectivity of pH-driven Pb(2+) transport in a bulk chloroform membrane system have been assessed. Among the carriers, 1,2-bis[2-(o-carboxyphenoxy)ethoxy]-4-tert-butylbenzene (1) is found to exhibit high selectivity for transport of Pb(2+) compared with alkali metal cations and a variety of other divalent metal ion species. Ionophore 1 also extracts Pb(2+) from aqueous solution into chloroform with the loss of two protons. A 1:1 complex of Pb(2+) with di-ionized 1 was isolated.

Screening metal binding selectivities of macrocycle mixtures by HPLC--ESI-MS and postcolumn reactions.

A rapid method for the screening of metal binding selectivities of host compounds in mixtures is presented. This method involves the separation of mixtures of hosts by HPLC, followed by postcolumn complexation with one or more metals, then analysis by mass spectrometry. The intensities of the host-guest complexes in the mass spectra correlate with the binding selectivities of the hosts. The method was applied to a series of lariat ethers that were synthesized as ion-selective reagents for ion-selective electrodes. The compounds most selective for Na+ vs Li+ and K+ were identified. Additionally, a mixture of substituted calixarenes was screened for alkali-metal-binding selectivity. These compounds were determined to be selective for Cs+ over Rb+, K+, and Na+.

Extraction Selectivities of Crown Ethers for Alkali Metal Cations: Differences between Single-Species and Competitive Solvent Extractions.

Separation factor values for pairs of alkali metal cations determined in competitive solvent extractions of alkali metal picrates from aqueous solutions into chloroform by a variety of benzo- and cyclohexano-group-containing crown ethers vary significantly from extrapolations based upon the results of single-species extraction experiments. For almost all of the crown ether-alkali metal cation combinations examined, the separation factor values are greater for competitive solvent extraction. In view of the unexpected results for sodium picrate extraction by dibenzo-24-crown-8, the solid-state structure of the isolated complex was determined.

Synthesis of a tetrahomodioxacalix[4]arene tetraamide and the crystal structure of its lead ion complex.

The 1,4-alternate tetrahomodioxacalix[4]arene tetraamide 4 with four p-phenyl groups on its upper rim was synthesized. In two-phase metal picrate extraction, 4 exhibited Pb2+ selectivity with formation of a 1:1 complex in chloroform. In the solid-state structure of the 4*Pb(Pic)2 complex, Pb2+ is bound by the carbonyl oxygens of two adjacent amide groups and an aryl-alkyl ether oxygen atom of one of these amide-containing substituents. The crystal structure and 1H NMR spectrum of the 4*Pb2+ complex reveals pi-metal ion complexation of one aromatic ring in the ligand with Pb2+.

Metal ion complexation by acyclic polyethers with lipophilic amide, thioamide, and amine end groups.

A series of acyclic polyethers with lipophilic amide, thioamide, and amine end groups was synthesized. Metal ion transport across bulk liquid membranes and measurement of thermodynamic parameters for ligand-metal ion complexation by titration calorimetry show strong selectivity for complexation of lead ion over other metal ion species for the diamide ligand. Lead ion complexation by the acyclic polyether diamide involves the amide oxygens and silver ion coordination by a dithioamide analog involves the thioamide sulfurs. With a proper length of the ethereal linkage, the ligand wraps around the metal ion in a pseudocyclic fashion.