Calorimetric measurement of the CH/pi interaction involved in the molecular recognition of saccharides by aromatic compounds

Can a benzene molecule differentiate between two isomeric carbohydrates? It is generally accepted that two factors govern molecular recognition: complementarity and preorganization. Preorganization requires the presence of cavities for positioning the host's groups of complementary nature to those of the guest. This study shows that, in fact, groups should be complementary to recognize each other (for the case presented here, it is controlled by the CH/pi interaction) but preorganization is not essential. Since weak interactions have their origin in dispersion forces, they also have impact on the enthalpic term of the free energy, so it was considered that their participation can be demonstrated by measuring the energy involved. For recognition to happen, two conditions must be satisfied: specificity and associated stabilizing energy. In this study we evaluated the heat of dissolution of different carbohydrates such as methyl 2,3,4,6-tetra-O-methyl-alpha-d-mannopyranoside and methyl 2,3,4,6-tetra-O-methyl-beta-d-galactopyranoside using different aromatic solvents. The solvation enthalpies in benzene were -78.8 +/- 3.9 and -88.7 +/- 5.5 kJ mol(-1) for each carbohydrate, respectively; and these values yielded a CH/pi energy of interaction of 9.9 kJ mol(-1). In addition, NMR studies of the effect of the addition of benzene to chloroform solutions of the two carbohydrates showed that benzene specifically interacts with the hydrogen atoms of the pyranose ring at positions 3, 4, and 5 located on the alpha face of the methyl-beta-galactoside, so it is, in fact, able to recognize it. Thus, the interactions between carbohydrates and the aromatic residues of proteins occur in the absence of the confinement generated by the protein structure. By experimentally measuring the energy associated with this interaction and comparing it to theoretical calculations, it was also possible to unequivocally determine the existence of CH/pi interactions between carbohydrates and proteins.     

Automatic on line preconcentration and determination of lead in water by ICP-AES using a TS-microcolumn

A simple, sensitive, low-cost and rapid, flow injection system for the on-line preconcentration of lead by sorption on a microcolumn packed with silica gel funtionalized with methylthiosalicylate (TS-gel) was developped. The metal is directly retained on the sorbent column and subsequently then eluted from it by EDTA. Five variables (sample flow rate, eluent flow rate, eluent concentration, pH and buffer concentration) were considered as factors in the optimization process. Interactions between analytical factors and their optimal levels were investigated using two level factorial and Box-Behnken designs. The optimum conditions established were applied to the determination of lead by flow injection inductively coupled plasma atomic emission spectrometry (FI-ICP-AES). The proposed method has a linear calibration range from 10 to at least 500 ng ml⁻¹ of lead. At a sample frequency of 24 h⁻¹ and a 120 s preconcentration time, the enrichment factor was 41, the detection limit was 15.3 ng ml⁻¹ (S/N=3) and the precision, expressed as relative standard deviation, was 0.9% (at 100 ng ml⁻¹). Validation of the developed method was carried out against electrothermal atomic absorption spectrometry analysis without statistically significant differences between the proposed method and the atomic absorption method.     

Quantum-chemical Study on the Structures and Properties of Uracil-BX3 （X = F, Cl） Complexes

1 INTRODUCTION The intermolecular interaction of bases in DNA or RNA is of immense interest and significance to che- mists and biologists alike. The interactions of these bases with metal cations, solvent molecules and other small molecules or ions would affect the struc- ture and biological properties or recognition process,which has been investigated widely[1～8]. Boron contained compounds are electron deficient com- pounds and have been extensively used as catalysts in chemical react…     

1H and 13C NMR spectra, structure and physicochemical features of phenyl acridine-9-carboxylates and 10-methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulphonates--alkyl substituted in the phenyl fragment

The 1H and 13C NMR spectra of twelve phenyl acridine-9-carboxylates--alkyl-substituted in the phenyl fragment--and their 10-methyl-9-(phenoxycarbonyl)acridinium salts dissolved in CD3CN, CD3OD, CDCl3 and DMSO-d6 were recorded in order to examine the influence of the structure of these compounds and the properties of the solvents on chemical shifts and 1H-(1)H coupling constants. Experimental data were compared with 1H and 13C chemical shifts predicted at the GIAO/DFT level of theory for DFT(B3LYP)/6-31G** optimised geometries of molecules, as well as with values of 1H chemical shifts and 1H-(1)H coupling constants, estimated using ACD/HNMR database software to ensure that the assignment was correct. To investigate the relations between chemical shifts and selected structural or physicochemical characteristics of the target compounds, the values of several of these parameters were determined at the DFT or HF levels of theory. The HOMO and LUMO energies obtained at the HF level yielded the ionisation potentials and electron affinities of molecules. The DFT method provided atomic partial charges, dipole moments, LCAO coefficients of pz LUMO of selected C atoms, and angles reflecting characteristic structural features of the compounds. It was found that the experimentally determined 1H and 13C chemical shifts of certain atoms relate to the predicted dipole moments, the angles between the acridine and phenyl moieties, and the LCAO coefficients of the pz LUMO of the C atoms believed to participate in the initial step of the oxidation of the target compounds. The spectral and physicochemical characteristics of the target compounds were investigated in the context of their chemiluminogenic ability.     

The hetero-Diels-Alder addition of sulfur dioxide to 1-fluorobuta-1,3-dienes: the sofa conformations preferred by 6-fluorosultines (6-fluoro-3,6-dihydro-1,2-oxathiin-2-oxides) enjoy enthalpic and conformational Anomeric effects

The reactivity of (E)- and (Z)-1-fluorobuta-1,3-diene ((E)- and (Z)-11), 2-fluorobutadiene (12), (E)- and (Z)-1-(fluoromethylidene)-2-methylidenecyclohexane ((E)- and (Z)-13) toward SO(2) has been explored and compared with that of (Z)- and (E)-1-(fluoromethylidene)-2-methylidene-3,4-dihydronaphthalene ((Z)-8 and (E)-8). In agreement with quantum calculations, 12 is unreactive toward SO(2) (no cycloaddition, only polymerization), whereas (E)-1-fluoro-1,3-dienes react more rapidly than their (Z)-isomers to give the corresponding 6-fluorosultines following the endo (Alder rule) mode of hetero-Diels-Alder addition. No sulfolene has been observed following the cheletropic mode of addition with the fluorodienes, in contrast to other substituted dienes. In agreement with the calculations, cis-2-fluoro-3,4-oxathiabenzobicyclo[4.4.0]dec-1(6),9-diene-4-oxide (cis-9, the sultine obtained by SO(2) addition to (Z)-8 under conditions of kinetic control) adopts a sofa conformation with the oxygen atom of the ring lying in the average plane of the four carbon atoms of its sultine moiety when it is in the crystalline state at -100 degrees C. A similar sofa conformation was found for its trans-isomer, trans-9, obtained by isomerization of cis-9 or by hetero-Diels-Alder addition of SO(2) to (E)-8. Experiments (equilibrium constant for hetero-Diels-Alder additions, bond lengths, and bond angles in crystalline fluorosultines cis-9 and trans-9) and high-level quantum calculations on cis- and trans-6-fluoro-3,6-dihydro-1,2-oxathiin-2-oxide (cis- and trans-20) confirm the existence of a stabilizing, enthalpic, anomeric (gem-disubstitution by sulfinyloxy and fluoro groups) effect, which is interpreted in terms of (lone pair) n(O1)-->sigma*(C-F) hyperconjugative interactions. This effect is strongest in the sofa conformers with a gauche arrangement of the sigma(O1,S2) and sigma(C6,F) bonds. The calculations suggest also that n(O1)-->sigma*(S2,O2'), pi*(S=O), and n(S2)-->sigma*(O1,C6) interactions intervene and affect the relative stability of the conformers (sofa, boat, pseudo-chair) found for 6-fluorosultines cis- and trans-20.     

The structure of ammonium pyrazolates in the solid state

The crystals obtained by mixing equimolar amounts of diethyl 1H-pyrazole-3,5-dicarboxylate and the primary amines phenethylamine and homoveratrylamine are ammonium pyrazolate salts as determined by 13C and 15N CPMAS NMR.     

Second-derivative constant-wavelength synchronous scan spectrofluorimetry for determination of benzo[b]fluoranthene, benzo[a]pyrene and indeno[1,2,3-cd]pyrene in drinking water

The use of derivative constant-wavelength synchronous scan fluorimetry is reported for the determination of three polycyclic aromatic hydrocarbon pollutants in drinking water (linearity range 0.4-4 mug 1(-1)). The limits of detection (LD) and quantification (LQ) (mug 1(-1)) are 0.01 and 0.07 for benzo[b]fluoranthene, 0.03 and 0.12 for benzo[a]pyrene and 0.19 and 0.57 for indeno[1,2,3-cd]pyrene in the presence of three other pollutants, benzo[k]fluoranthene, benzo[ghi]perylene and fluoranthene. The precision (RSD /= 85%) were satisfactory.     

The key role of the intermolecular pi-pi interactions in the presence of spin crossover in neutral [Fe(abpt)2A2] complexes (A = terminal monoanion N ligand)

New iron(II) complexes of formulas [Fe(abpt) 2(tcm) 2] ( 1), [Fe(abpt) 2(tcnome) 2] ( 2), and [Fe(abpt) 2(tcnoet) 2] ( 3) (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, tcm (-) = [C(CN) 3] (-) = tricyanomethanide anion; tcnome (-) = [(NC) 2CC(OCH 3)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-methoxypropenide anion; tcnoet (-) = [(NC) 2CC(OC 2H 5)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-ethoxypropenide anion) have been synthesized and characterized by infrared spectroscopy, magnetic properties and by variable-temperature single-crystal X-ray diffraction. The crystal structure determinations of 1 and 2 reveal in both cases centrosymmetric discrete iron(II) monomeric structures in which two abpt chelating ligands stand in the equatorial plane and two terminal polynitrile ligands complete the distorted octahedral environment in trans positions. For 3, the crystallographic studies revealed two polymorphs, 3- A and 3- B, exhibiting similar discrete molecular structures to those found for 1 and 2 but with different molecular arrangements. In agreement with the variable-temperature single-crystal X-ray diffraction, the magnetic susceptibility measurements, performed in the temperature range 2-400 K, showed a spin-crossover phenomenon above room temperature for complexes 1, 3- A, and 3- B with a T 1/2 of 336, 377, and 383 K, respectively, while complex 2 remains in the high-spin ground state ( S = 2) in the whole temperature range. To understand further the magnetic behaviors of 1, 3-A, and 3-B, single-crystal X-ray diffraction measurements were performed at high temperatures. The crystal structures of both polymorphs could not be obtained above 400 K because the crystals decomposed. However, single-crystal X-ray data have been collected for compound 1, which reaches the full high-spin state at lower temperatures. Its crystal structure, solved at 400 K, showed a strong modification of the iron coordination sphere (average Fe-N = 2.157(3) A vs 1.986(3) A at 293 K). In agreement with the magnetic properties. Such structural behavior is a signature of the spin-state transition from low-spin (LS) to high-spin (HS). On the basis of the intermolecular pi stacking observed for the series described in this paper and for related complexes involving similar discrete structures, we have shown that complexes displaying frontal pi stacking present spin transition such as 1, 3-A, and 3-B and those involving sideways pi stacking such as complex 2 remain in the HS state.     

Direct analysis of inorganic anions in samples with high salt content by capillary zone electrophoresis

Trace-level inorganic anions in seawater are separated efficiently by capillary zone electrophoresis using direct UV detection. The carrier electrolyte is 50 mM borate at pH 9.3 and contained 1.5M NaCl. This buffer solution is adopted to prevent interference from high concentrations of the chloride ion in seawater. No electro-osmotic flow reverser is used to shorten the analysis time. The experimental conditions such as the concentration of NaCl in the carrier electrolyte, capillary inner diameter, applied current, and temperature are optimized. Linear plots are obtained in the concentration range of 0.1 to 20 microg/mL. The quantitation limits of the anions are in the order of 0.02 to 0.1 microg/mL. The proposed method may be applicable to the determination of inorganic anions in other environmental samples and effluents of a power plant.