Laser spectroscopic study on the conformations and the hydrated structures of benzo-18-crown-6-ether and dibenzo-18-crown-6-ether in supersonic jets

The laser-induced fluorescence spectra of jet-cooled benzo-18-crown-6 (B18C6) and dibenzo-18-crown-6 (DB18C6) exhibit a number of vibronic bands in the 35 000-37 000 cm(-1) region. We attribute these bands to monomers and hydrated clusters by fluorescence-detected IR-UV and UV-UV double resonance spectroscopy. We found four and two conformers for bare B18C6 and DB18C6, and the hydration of one water molecule reduces the number of isomers to three and one for B18C6-(H(2)O)(1) and DB18C6-(H(2)O)(1), respectively. The IR-UV spectra of B18C6-(H(2)O)(1) and DB18C6-(H(2)O)(1) suggest that all isomers of the monohydrated clusters have a double proton-donor type (bidentate) hydration. That is, the water molecule is bonded to B18C6 or DB18C6 via two O-H[dot dot dot]O hydrogen bonds. The blue shift of the electronic origin of the monohydrated clusters and the quantum chemical calculation suggest that the water molecule in B18C6-(H(2)O)(1) and DB18C6-(H(2)O)(1) prefers to be bonded to the ether oxygen atoms near the benzene ring.     

Conductance Study of the Thermodynamics of Tl+ ion Complexes with Different 18-Membered Crown Ethers in Binary Dimethylformamide-Acetonitrile Mixtures

A conductance study of the interactionbetween Tl⁺ ion and 18-crown-6 (18C6),dicyclohexano-18-crown-6 (DC18C6), benzo-18-crown-6(B18C6), diaza-18-crown-6 (DA18C6),dibenzyldiaza-18-crown-6 (DBzDA18C6) andhexaaza-18-crown-6 (HA18C6) indimethylformamide-acetonitrile mixtures was carriedout at various temperatures. The formation constantsof the resulting 1 : 1 complexes were determined fromthe molar conductance-mole ratio data and found tovary in the order HA18C6 > DA18C6 > DBzDA18C6 >18C6 > DC18C6 > B18C6. The enthalpy and entropy ofthe complexation reactions were determined from thetemperature dependence of the formation constants.     

Spectrophotometric Study of the Complexation of Some Lanthanide (III) Ions with a Series of 18-Crowns-6 in DMSO Solution Using Murexide as a Metallochromic Indicator

The complexation of La(III), Ce(III), Pr(III) and Er(III) with 18-crown-6(18C6), dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DCY18C6) anddibenzopyridino-18-crown-6 (DBPY18C6) has been studied in dimethylsulfoxide(DMSO) by means of a competitive spectrophotometric method using murexide asa metal ion indicator. The formation constants of the 1 : 1 complexeswere found tovary in the order La(III) > Ce(III) > Pr(III) > Er(III). It was foundthat the structure influences the formation and stability of the resultingcomplexes. The effects ofvarious parameters on complexation are discussed. The order of the stabilityconstants of each lanthanide ion with these macrocycles are18C6 > DC18C6 > DB18C6 > DBPY18C6.     

Structural and energetic effects in the molecular recognition of amino acids by 18-crown-6

Absolute 18-crown-6 (18C6) affinities of five amino acids (AAs) are determined using guided ion beam tandem mass spectrometry techniques. The AAs examined in this work include glycine (Gly), alanine (Ala), lysine (Lys), histidine (His), and arginine (Arg). Theoretical electronic structure calculations are performed to determine stable geometries and energetics for neutral and protonated 18C6 and the AAs as well as the proton bound complexes comprised of these species, (AA)H(+)(18C6). The proton affinities (PAs) of Gly and Ala are lower than the PA of 18C6, whereas the PAs of Lys, His, and Arg exceed that of 18C6. Therefore, the collision-induced dissociation (CID) behavior of the (AA)H(+)(18C6) complexes differs markedly across these systems. CID of the complexes to Gly and Ala produces H(+)(18C6) as the dominant and lowest energy pathway. At elevated energies, H(+)(AA) was produced in competition with H(+)(18C6) as a result of the relatively favorable entropy change in the formation of H(+)(AA). In contrast, CID of the complexes to the protonated basic AAs results in the formation of H(+)(AA) as the only direct CID product. H(+)(18C6) was not observed, even at elevated energies, as a result of unfavorable enthalpy and entropy change associated with its formation. Excellent agreement between the measured and calculated (AA)H(+)-18C6 bond dissociation energies (BDEs) is found with M06 theory for all complexes except (His)H(+)(18C6), where theory overestimates the strength of binding. In contrast, B3LYP theory significantly underestimates the (AA)H(+)-18C6 BDEs in all cases. Among the basic AAs, Lys exhibits the highest binding affinity for 18C6, suggesting that the side chains of Lys residues are the preferred binding site for 18C6 complexation in peptides and proteins. Gly and Ala exhibit greater 18C6 binding affinities than Lys, suggesting that the N-terminal amino group provides another favorable binding site for 18C6. Trends in the 18C6 binding affinities among the five AAs examined here exhibit an inverse correlation with the polarizability and proton affinity of the AA. Therefore, the ability of the N-terminal amino group to compete for 18C6 complexation is best for Gly and should become increasing less favorable as the size of the side chain substituent increases.     

Lipid-protein interactions in the membrane: studies with model peptides

We have used fluorescence quenching of tryptophan-containing trans-membrane peptides by bromine-containing phospholipids to study the specificity of peptide-lipid interactions. We have synthesized peptides Ac-K2GLm WLnK2A-amide where m = 7 and n = 9 (L16) and m = 10 and n = 12 (L22). Binding constants of L22 for dioleoylphosphatidylserine [di(C18 : 1)PS] or dioleoylphosphatidic acid [di(C18 : 1)PA] relative to dieoleoylphosphatidylcholine [di(C18 : 1)PC] were close to 1. However, for L16, whilst the bulk of the di(C18 : 1)PA molecules bound with a binding constant relative to di(C18 : 1)PC close to 1, a small number of di(C18 : 1)PA molecules bound much more strongly. Assuming just one high affinity binding site on L16 for anionic lipid, the affinity of the site for di(C18 : 1)PS was calculated to be ca. 8 times that for di (C18 : 1)PC. The relative binding constant was little affected by ionic strength and close contact between the anionic headgroup of di(C18 : 1)PS and a lysine residue on the peptide was suggested. The relative binding constant for di(C18 : 1)PS at this high affinity site was less than for di(C18 : 1)PA. Cholesterol interacts with L22 with an affinity about 0.7 of that of di(C18 : 1)PC. The structure of the peptide itself is important. The peptide Ac-KKGYL6WL8YKKA-amide (Y2L14) incorporated into bilayers of dinervonylphosphatidylcholine [di(C24 : 1)PC] whereas L16 did not incorporate into this lipid. It is suggested that thinning of a lipid bilayer around a peptide to give optimal hydrophobic matching is less energetically unfavourable when a Tyr residue is located in the lipid/water interfacial region.     

13C-NMR. Analysis of the Roxburghines

The ¹³C shifts of the alkaloids roxburghine B, C, D and E are determined. They confirm the following configurations for the last three bases: C(18α)-normal, C(18α)-pseudo and C(18β)-pseudo, respectively. Roxburghine B is shown to be a C(18β)-epi-allo isomer.     

Effect of the 18-crown-6 and benzo-18-crown-6 on the solvent extraction and separation of lanthanide(III) ions with 8-hydroxyquinoline

The synergistic solvent extraction of 13 lanthanides with mixtures of 8-hydroxyquinoline (HQ) and the crown ethers (S) 18-crown-6 (18C6) or benzo-18-crown-6 (B18C6) in 1,2-dichloroethane has been studied. The composition of the extracted species has been determined as LnQ₃ · S. The values of the equilibrium constant and separation factor have been calculated. Here, the effect of the synergistic agent (18C6 or B18C6) on the extraction process is discussed.     

Synergism of crown ethers in the thenoyl trifluoroacetone extraction of americium(III) in benzene medium

The synergism of the crown ethers (CE) dicyclohexano-18-crown-6 (DC18C6), dibenzo-18-crown-6 (DB18C6) and 18-crown-6 (18C6) has been investigated in the thenoyl trifluoroacetone (HTTA) extraction of americium(III) in benzene medium from an aqueous phase of ionic strength 0.5 and pH 3.50 at room temperature (23°C). The extracted synergistic species have the general formula Am(TTA)₃ · CE except for DC18C6 in which case the species Am(TTA)₃·2CE was also observed at high CE concentrations. The order of synergism was found to be DC18C6>DB18C6>18C6, which is the order of the basicity of CE as indicated by their ability to extract hydrogen ions from nitric acid solutions.     

Utilising retention correlation for the separation of oligostyrenes by coupled-column liquid chromatography

The separation of n = 2-5 n-butyloligostyrenes has been illustrated by reversed-phase reversed-phase (RP-RP) coupled-column liquid chromatography. The coupled-column separation has been achieved by use of a C18 column with methanol as the mobile phase followed by a DiamondBond C18 column with acetonitrile (ACN) mobile phase. The DiamondBond C18 is a hybrid carbon clad zirconia (CCZ)-C18 stationary phase. Unlike a C18-carbon clad zirconia two-dimensional liquid chromatographic system, which is orthogonal, the C18 and DiamondBond C18 columns combination exhibit correlations based upon the molecular weight of n-butyloligostyrenes. Using an alternative strategy to two-dimensional liquid chromatography, the molecular weight dependence displayed by both the C18 column and DiamondBond C18 has been used to increase throughput or decrease analysis time in the analysis of the n-butyloligostyrenes. However, this is at the expense of a portion of the two-dimensional peak capacity displayed by the C18-carbon clad zirconia system.     

Comparison of the acidity of residual silanol groups in several liquid chromatography columns

The silanol acidity of Waters Resolve C18, Waters Resolve silica, Waters Symmetry C18, Waters Symmetry silica, Waters XTerra MS C18 and underivatized XTerra columns has been measured from the retention of LiNO3 with a methanol/water (60:40) mobile phase buffered to different pH values. The Li+ cation is retained by cationic exchange with the background cation of the mobile phase (Na+) through the ionized silanols. The number of active silanols increases in the order: XTerra MS C18 < Symmetry C18 < underivatized XTerra < Resolve C18 < Resolve silica approximately equal to Symmetry silica. XTerra MS C18 does not present any residual silanol acidity up to s(s)pH 10.0 (pH in 60% methanol) as measured by LiNO3. The underivatized XTerra packing and Symmetry C18 present active silanols only at s(s)pH values higher than 7.0. For the other three columns, two different types of silanols with different acidity (s(s)pKa values about 3.5-4.6 and 6.2-6.8, respectively) have been observed. Symmetry C18 shows evidence of the presence of active basic sites that retain NO3- by anionic exchange.     

Progress in the synthesis of the lituarines: stereocontrol in sequential C-C bond formation on a spirobutenolide template

[structure: see text] We describe elaboration of a tricyclic spirobutenolide corresponding to the C(7-18) tricyclic substructure common to lituarines A-C. Conjugate addition, to install the C(16) methyl, is followed by construction of the crucial C(18-19) bond by silyl enol ether addition to the derived spiroacetal C(18)-O oxonium ion. Esterification with a C(1-6) acid, and selective ozonolysis to release the C(23) carbonyl, complete the assembly of all the carbons present in the lituarine macrocyclic core.     

Conductance and Thermodynamic Study of the Complexation of Ammonium Ion with Different Crown Ethers in Binary Nonaqueous Solvents

A conductance study of the interaction between ammonium ion and 18‐Crown‐6 (18C6), dicyclohexano‐18‐crown‐6 (DC18C6), ditertbutyl‐dicyclohexano‐18‐crown‐6 (t‐bu)₂DC18C6, diaza‐15‐crown‐5 (DA15C5), dibenzo‐21‐crown‐7 (DB21C7) and N‐Phenylaza‐15‐crown‐5 (NPA15C5) in acetonitril‐di‐methylsulfoxide mixture was carried out at various temperatures. The formation constants of the resultant 1:1 complexes were determined from the molar conductance‐mole ratio data and found to vary in the order of DA15C5 > DC18C6 > 18C6 > (t‐bu)₂DC18C6 > DB21C7 > NPA15C5. The enthalpy and entropy of the complexation reactions were determined from the temperature dependence of the formation constants.     

Intervalence charge transfer in mixed valence compound modified by the formation of a supramolecular complex

The electrochemical and spectroelectrochemical properties of N,N-diphenyl-1,4-phenylenediamine (PDA) were investigated in the absence and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether (DB24C8), in a solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile and in the presence of trifluoroacetic acid (TFA) only for 18C6. In neutral acetonitrile, PDA undergoes two reversible oxidation processes, which lead first to the formation of the cation-radical considered as mixed valence (MV) compound, and then to the dicationic species. When 18C6 is added in the medium and depending on 18C6 concentration, cyclic voltammetry shows a marked shift to more cathodic potentials of the current waves of the second redox process only. This is attributed to a strong interaction between the PDA(+2) dication and two 18C6 molecules, leading to the formation of a supramolecular complex with an association constant value K(a) = 7.0 × 10(7) M(-2). The interaction of 18C6 with PDA(+2) dication has a direct effect on the PDA(+.) cation-radical corresponding to a decrease in the lifetime of the MV compound and of the intramolecular electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the two amine centers in the MV compound (k(th) = 1.35 × 10(10) s(-1) in 18C6-free neutral solution containing 5.0 × 10(-4) M PDA, and k(th) = 3.6 × 10(9) s(-1) in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation constant K(co) falls from 6.0 × 10(6) in 18C6-free solution to 1.6 × 10(3) at [18C6]/[PDA] = 20/1. In acidified acetonitrile and when TFA concentration is increased, PDA still shows the two successive and reversible oxidation processes, but both are shifted to more anodic potentials. However, when 18C6 is added, the two oxidation waves shift to more cathodic potentials, indicating an interaction of all protonated PDA redox states with 18C6, resulting in the formation of supramolecular complexes. In the presence of TFA, the value of K(co) is decreased to 4.3 × 10(4), but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compound. In this medium, IV-CT in the MV compound is greater with 18C6 (k(th) = 2.3 × 10(10) s(-1) for [18C6]/[PDA] = 20/1) than without (k(th) = 1.4 × 10(9) s(-1)), which indicates a more important IV-CT rate when 18C6 is present. The results show for the first time that is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compound is the most important. This control is not obtained as usual by chemical modification of the structure of the starting molecule, but by varying either the acidity or the 18C6 concentration as external stimuli, which lead to reversible formation/dissociation of a supramolecular complex species. Moreover, we also studied the electrochemical properties of PDA in the presence of wider crown ether such as DB24C8. We showed that PDA undergoes the same electrochemical behavior with DB24C8 than with 18C6 in neutral organic medium (K(a) = 2.9 × 10(3) M(-1)). This result suggests that the complexation between the electrogenerated PDA(+2) dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which is supposed to form inclusion complexes.     

Characterization of the acidity of residual silanol groups in immobilized artificial membranes

Residual silanol acidity and activity of one immobilized artificial membrane (IAM) column have been measured from the retention of LiNO(3) in the column with a methanol/buffer (1mM in Na(+)) (60:40, v/v) mobile phase buffered to different pH values. Just one type of silanol with pssK(a)=7.61 (near the pH limit recommended by the manufacturer) was found, although these silanols show large activity. The results obtained have been compared with those obtained previously for Resolve C18, Resolve Silica, Symmetry C18, Symmetry Silica, XTerra MS C18, underivatized XTerra, Lichrospher 100 RP-18, Purospher RP-18e, Luna C18 (2) and Chromolith Perfomance RP-18e, showing that the IAM column is similar to Luna C18 and Symmetry C18 in terms of silica quality, as measured by Li(+) retention. A warning about the use of IAM columns for emulation of biological systems at physiological pH 7 is given because the ionized silanols may contribute to the retention of some drugs at this pH.     

The interaction between ketamine and some crown ethers in common organic solvents studied by NMR: the effect of donating atoms and ligand structure

1H NMR spectroscopy was used to investigate the stoichiometry and stability of the drug ketamine cation complexes with some crown ethers, such as 15-crown-5 (15C5), aza-15-crown-5 (A15C5), 18-crown-6 (18C6), aza-18-crown-6 (A18C6), diaza-18-crown-6 (DA18C6), dibenzyl-diaza-18-crown-6 (DBzDA18C6) and cryptant [2,2,2] (C222) in acetonitrile (AN), dimethylsulfoxide (DMSO) and methanol (MeOH) at 27 degrees C. In order to evaluate the formation constants of the ketamine cation complexes, the CH3 protons chemical shift (on the nitrogen atom of ketamine) was measured as function of ligand/ketamine mole ratio. The formation constant of resulting complexes were calculated by the computer fitting of chemical shift versus mole ratio data to appropriate equations. A significant chemical shift variation was not observed for 15C5 and 18C6. The stoichiometry of the mono aza and diaza ligands are 1:1 and 1:2 (ligand/ketamine), respectively. In all of the solvents studied, DA18C6 formed more stable complexes than other ligands. The solvent effect on the stability of these complexes is discussed.     

Conversion of the aggregation state of merocyanine dye, modification of the subcell packing of arachidic acid, and removal of the majority of n-octadecane by hydrothermal treatment in the liquid phase in a mixed Langmuir-Blodgett film of the ternary system

We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir-Blodgett (LB) film of merocyanine dye with an octadecyl group (MS(18))-arachidic acid (C(20))-n-octadecane (AL(18)) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS(18) hydrocarbon chain, the slight orientation change in the C(20) hydrocarbon chain, and the complete evaporation of AL(18). The dissociation of MS(18) is probably ascribed to the complete evaporation of AL(18) from the mixed LB film and the increase in thermal mobility of the long axis of the MS(18) hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS(18) aggregation state from H- to J-aggregates, the modification of the C(20) subcell packing from hexagonal to orthorhombic, and the removal of most of the AL(18) molecules. The conversion of the MS(18) aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS(18) aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL(18) molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS(18) permanent dipole interaction. Moreover, the modification of the C(20) subcell packing is possibly due to the hydrophobic effect, where the C(20) hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTL is quite effective to reorganize the chromophore alignment of MS(18), to modify the subcell packing of C(20) and to erase the majority of AL(18) molecules in the mixed LB film of the MS(18)-C(20)-AL(18) ternary system in a short time.     

有机阳离子[C18mim]＋在蒙脱土层间的物理化学吸附与聚集状态*

咪唑基离子液体(N鄄十八烷基鄄N忆鄄甲基溴化咪唑盐[C18mim]Br)中的[C18mim]+可以通过离子交换反应进入蒙脱土层间.本文用TGA、FTIR和XRD 研究了[C18mim]+在蒙脱土层间的物理和化学吸附及其聚集状态. 研究结果表明, 蒙脱土存在着饱和的化学吸附量, 其总吸附量随物理吸附量的增加而增加；随着[C18mim]+在蒙脱土层间吸附量的增加, 其聚集态结构的有序性增加, 并伴有分子链的构象变化,导致层间距的增加.     

Structure of hydrated clusters of dibenzo-18-crown-6-ether in a supersonic jet--encapsulation of water molecules in the crown cavity

The structure of dibenzo-18-crown-6-ether (DB18C6) and its hydrated clusters has been investigated in a supersonic jet. Two conformers of bare DB18C6 and six hydrated clusters (DB18C6-(H(2)O)(n)) were identified by laser-induced fluorescence, fluorescence-detected UV-UV hole-burning and IR-UV double-resonance spectroscopy. The IR-UV double resonance spectra were compared with the IR spectra obtained by quantum chemical calculations at the B3LYP/6-31+G* level. The two conformers of bare DB18C6 are assigned to "boat" and "chair I" forms, respectively, among which the boat form is dominant. All the six DB18C6-(H(2)O)(n) clusters with n = 1-4 have a boat conformation in the DB18C6 part. The water molecules form a variety of hydration networks in the boat-DB18C6 cavity. In DB18C6-(H(2)O)(1), a water molecule forms the bidentate hydrogen bond with the O atoms adjacent to the benzene rings. In this cluster, the water molecule is preferentially hydrogen bonded from the bottom of boat-DB18C6. In the larger clusters, the hydration networks are developed on the basis of the DB18C6-(H(2)O)(1) cluster.     

Spectroscopic studies of the interaction between crown ethers and organic nitriles

The interaction of 18-crown-6 (18C6), diaza-18-crown-6 (DA18C6), and dithia-18-crown-6 (DT18C6) with acetonitrile and malononitrile was investigated using infrared spectroscopy, 1H and 13C NMR spectroscopy, and molecular modeling. The interaction between 18C6 and the nitriles involves hydrogen bonding between the CH3/CH2 protons of the nitriles and the ether oxygens of the crown. In contrast, the interaction between the organic nitriles and DA18C6 involves the nitrogen of the nitrile group and the N-H groups of the crown. Due to the bulkiness of the sulfur atoms of DT18C6, no interaction was observed to occur between acetonitrile and the crown.     

Effect of Adding an Amphiphilic Solubilization Improver, Sucrose Distearate, on the Solubilization Capacity of Nonionic Microemulsions

We have studied the effect of adding sucrose distearate (2C(18)SE) on the solubilization capacity of microemulsions formed in the water/C(12)EO(6)/n-decane system. Upon addition of 2C(18)SE to the binary water/C(12)EO(6) system, a lamellar liquid crystal region developed. This suggests that the rigidity of the surfactant layer is strengthened. The solubilization of water and n-decane in the bicontinuous microemulsions increases about three times upon replacing 10% C(12)EO(6) with 2C(18)SE; besides, the HLB temperature is not greatly affected by 2C(18)SE. On the other hand, sucrose monostearate (C(18)SE) does not have such a function. The effect of added 2C(18)SE on the solubilization capacity of the discrete droplet-type o/w or w/o microemulsions was also studied. The efficiency of the solubilization-improving effect is reduced when the system is far from the HLB temperature. Copyright 2001 Academic Press.