Positional effect of solute functional group among positional isomers in hydroxyl group-solvent and carbonyl group-solvent specific interactions in menthanol--water mixed solvents monitored by high-performance liquid chromatography

We have evaluated the hydroxyl group–solvent and carbonyl group–solvent specific interactions by using mostly an Alltima C₁₈ stationary phase and subsidiarily squalane-adsorbed C₁₈ phase, and by measuring the retention data of carefully selected solutes in 60:40, 70:30, 80:20 and 90:10 (%, v/v) methanol–water eluents at 25, 30, 35, 40, 45 and 50°C. The selected solutes are four positional isomers of phenylbutanol, 5-phenyl-1-pentanol, three positional isomers of alkylarylketone derived from butylbenzene, and 1-phenyl-2-hexanone. The magnitudes of carbonyl group–solvent specific interaction enthalpies are larger than those of hydroxyl group–solvent specific interaction enthalpies in general. We observed clear discrepancies in functional group–solvent specific interactions among positional isomers. The spatial accessibility of the functional group by the solvent molecules seems to govern the strength of interaction. The relationships between molecular structures and functional group accessibilities have been discussed. The specific functional group–mobile phase interactions obtained by the Alltima C₁₈ stationary phase were systematically different from those obtained by the squalane-impregnated C₁₈ stationary phase, which may be due to structural differences between the two phases.     

Binding properties of 2,4,5-trichlorophenoxyacetic acid-imprinted polymers prepared with different molar ratios between template and functional monomer

Several molecularly-imprinted polymers binding the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were prepared with a molar ratio between the functional monomer and the template molecule in the pre-polymerisation mixture set between 1+2 and 20+1. The functional monomer used was 4-vinylpyridine (4-VP), the cross-linker was ethylene dimethacrylate, and the porogenic solvent was a mixture of methanol–water 3+1 (v/v). The polymers obtained were grinded, sieved and packed in 100 mm×3.9 mm HPLC columns. The effects of the mobile phase composition were evaluated by eluting the columns with acetonitrile–water mixtures. The results obtained indicate that column capacity, selectivity factor and the imprinting effect are controlled by ion-pair and hydrophobic interactions between the analyte and the stationary phase. In the full range of ratios considered, column capacity, selectivity factor and imprinting effect are inversely proportional to the molar ratio between the template molecule and the functional monomer.     

Separation of polar and non-polar analytes using dimethyl sulfoxide-modified subcritical water

Separations using methanol–water or acetonitrile–water mixtures at different temperatures have been well investigated in reversed-phase liquid chromatography. However, reversed-phase separation with dimethyl sulfoxide (DMSO)–water mixtures is much less studied. In this work, separations of polyhydroxybenzenes, phenol derivatives, benzene, toluene, ethylbenzene, and xylenes (BTEX), and polycyclic aromatic hydrocarbons (PAHs) with DMSO-modified subcritical water were performed at several temperatures to evaluate the effect of temperature on the elution strength of DMSO–water mixtures. The column efficiency obtained by using DMSO-modified subcritical water was also studied. Finally, the resolution of ethylbenzene and p-xylene was investigated.     

Structure of small TiCn clusters: A theoretical study

A theoretical study of the TiC_n (n = 1–8) clusters has been carried out at the B3LYP/6-311+G(d) level. Molecular properties for three different isomers, namely linear, cyclic, and fan species, have been determined. The fan isomers, where the titanium atom is essentially side-bonded to the entire C_n unit, are predicted to be more stable than both linear and cyclic isomers. Only for the largest studied species, TiC₈, the cyclic isomer is located lower in energy. An even–odd parity effect in the incremental binding energies is observed for the three isomers, n-even species being in general more stable for linear and fan isomers, whereas for the cyclic species n-odd clusters are favoured. A topological analysis of the electronic charge density shows that all cyclic isomers correspond to true monocyclic rings, whereas for the fan species a variety of different connectivities has been observed.     

Structure and stability of thiourea with water, DFT and MP2 calculations

The relative stabilities of thiourea in water are investigated computationally by considering thiourea–water complexes containing up to 1–6 water molecules (CS(NH₂)₂(H₂O)_n=1–6) using density functional theory and MP2 ab initio molecular orbital theory. The results show that the thiourea complex is stable and has an unusually high affinity for incoming water molecules. The clusters are progressively stabilized by the addition of water molecules, as indicated by the increasing of the binding energy. The binding energy of the cluster to each H₂O molecule is about 33 kJ mol⁻¹ for n=1–5.The C–S bond, N–C bond distance, Mulliken populations and binding energy keep approximately constant as the clusters increase in size with an increasing number of H₂O molecules. As the solvation progresses, the C–S distance increases monotonically while the Mulliken populations on the C–S bond reduces monotonically with the addition of each H₂O molecule, indicating that the C–S bond of the thiourea unit in the clusters is de-stabilized with an increasing number of H₂O molecules. Charge transfers for the clusters are mainly found at N, S atoms of the thiourea.     

Double H-bridged and single H-bridged diboryl radicals

The structures of B₂H₅·, B₂H₅CO·, and B₂H₅N₂· radicals are investigated using the 6–31G^* basis set. Both double H-bridged and single H-bridged isomers are found to be local minima on the potential energy surface. The effects of electron correlation are taken into account using single point MP4/6–31G^* calculations and, for the diboryl radicals, complete MP3/6–31G^* optimizations. In all cases the single H-bridged isomers are found to be more stable than the corresponding double H-bridged isomers.The transition state for the double H-bridged to single H-bridged B₂H₅· isomerization reaction is calculated to be 2.54 kcal mol^–1 above the double H-bridged radical at the MP4SDTQ/6-31G^*//UHF/ 6–31 G^* level when corrected for zero point energy. Barrier tunneling increased the reaction rate by a factor of 2.5–3.0, strongly suggesting the system is fluxional at this temperature.The addition of CO and N₂ to the diboryl radicals leads to relocation of the unpaired electron and rehybridization of the C and N atoms adjacent to the boron atoms. The isomers of B₂H₅CO· and B₂H₅N₂· are different and should be distinguishable experimentally. While the CO moiety is bound to the diboryl radicals isomers by over 19 kcal mol^–1, no binding energy is evident for N₂.     

The spectrophysics of warfarin: implications for protein binding

The photophysical behavior of the isomers of the anticoagulant drug warfarin in various solvents and solvent mixtures was investigated using absorption, 1H NMR, and steady-state and time-resolved fluorescence spectroscopies in conjunction with B3LYP-based theoretical treatments. Complex absorption patterns were observed, indicative of the presence of different isomers of warfarin in the various solvents studied. In alkaline aqueous solution, the deprotonated open side form of warfarin is highly dominant and only one S0-->S1 singlet transition could be observed in the absorption spectrum centered at 320 nm. These observations were supported by theoretical density functional calculations (B3LYP) in which the geometries of nine isomers of warfarin were optimized and their respective eight lowest singlet and three lowest triplet excitation energy levels were predicted. Examination of the fluorescence excitation and emission spectra of the isomers in nonpolar and polar organic solvents showed the presence of the deprotonated open side chain form of warfarin in 2-propanol, ethanol, and acetonitrile. Time-resolved fluorescence experiments revealed a short decay time constant, tau1, in all solvents studied while in more polar environments a second longer one, tau2, was evident varying between 0.5 and 1.6 ns depending on solvent polarity. The variation of number and length of fluorescence lifetimes as a function of solvent environment has provided a tool for examining warfarin protein binding. Studies on the binding of warfarin to human serum albumin (HSA) have been undertaken, and different modes of binding were observed which are indicative of binding to the anion-selective Sudlow I and, second, a lower affinity mode of interaction.     

Interactions of Zn(II) with single and multiple amino acids. Insights from density functional and ab initio calculations

Calculations were performed to study the interactions of metal ions (M) with (multiple) amino acids (AA) and fill the gap between single AA and proteins. A complete conformational search results in nine and eleven ZnGly isomers at B3P86 and MP2 levels, respectively, and four populated conformers of glycine are responsible for production of these isomers. For all M, the isomers via the OO and NO binding modes are the main constituents, and the OO mode is favored by stronger electrostatic interactions. Binding with more glycines causes larger structural distortions, improves relative stabilities of monodentate binding isomers and generates new binding modes (e.g. ZnB_III via only the hydroxyl group). The scaling factor of Zn(Gly)_n structures, the ratio of its binding affinity versus the sum of comprising ZnGly isomers, is linear with glycine number (n), and the linear relationship may not be altered by mutations of glycines and M. It thus allows to estimate M(AA)_n binding affinities (n ≥ 2) from the comprising MAA structures and analyze their structures with kinetic methods. The DFT and MP2 results become comparable by increasing metal coordination, e.g. the ZnB_III versus ZnA_I (zwitterionic) relative energy differs by 41.9 kcal mol^?1 at B3P86 and MP2 levels and is close by addition of three water molecules (4.1 kcal mol^?1). The presence of water solvent improves the relative stabilities of monodentate binding isomers and results in a broader conformational distribution. Copyright © 2012 John Wiley & Sons, Ltd.     

Determination of theophylline and its metabolites in biological samples by liquid chromatography-mass spectrometry

Liquid chromatography–mass spectrometry (LC–MS) is a powerful tool for analysis of drugs and their metabolites. We used a column-switching system in combination with atmospheric pressure chemical ionization LC–MS (LC–APCI–MS) for the determination of theophylline and its metabolites in biological samples. The separation was carried out on a reversed-phase column using methanol–20 mM ammonium acetate as a mobile phase at a flow-rate of 1 ml/min in 30 min. In the mass spectrum, the molecular ions of these drugs and metabolites were clearly observed as base peaks. This method is sufficiently sensitive and accurate for the pharmacokinetic studies of these drugs.     

The dynamics of water exchange in gadolinium DOTA complexes studied by transition path sampling and potential of mean force methods

The mechanism of water exchange at the Gd centre of the two isomers of [Gd(iii)DOTA](-) (gadolinate(1-), [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetato(4-)-N1,N4,N7,N10,O1,O4,O7,O10]) has been explored using transition path sampling and potential of mean force methods to sample those regions of phase space inaccessible to standard molecular dynamics simulation. We find that there are definite differences in the details of the solvent rearrangement accompanying the exchange of the capping water molecule for the two isomers. We conclude that these solvent effects, rather than any differences in the binding energy of the capping water, are central in determining the exchange rate. We find that the potential of mean force studies yield absolute and relative rates of water exchange for the two isomers that are in good agreement with experiment.     

Influence of structure on electron correlation effects and electron-water dispersion interactions in anionic water clusters

Electronic structure calculations at the level of second-order M?ller-Plesset perturbation theory have been performed on anionic water clusters, (H2O)n(-), in the n = 14-33 size regime. The contribution to the electron binding energy that arises from electron correlation is found to be significantly larger for cavity-bound electrons than it is for surface-bound electrons, even for surface states with electron binding energies well above 1 eV. A decomposition of the correlation energy into interactions between pairs of Boys-localized molecular orbitals is used to demonstrate that the larger correlation energy found in the cavity isomers arises from electron-water dispersion interactions, and that the dispersion interaction is larger in cavity-bound isomers because the unpaired electron penetrates well beyond the first solvation shell. In contrast, a surface-bound electron exhibits virtually no penetration into the interior of the cavity. To obtain a qualitatively accurate picture of this phenomenon, one must plot molecular orbitals using isoprobability surfaces rather than arbitrarily-selected isocontours.     

Exploring the correlation between network structure and electron binding energy in the (H(2)O)(7)(-) cluster through isomer-photoselected vibrational predissociation spectroscopy and ab initio calculations: addressing complexity beyond types I-III

We report a combined photoelectron and vibrational spectroscopy study of the (H(2)O)(7)(-) cluster anions in order to correlate structural changes with the observed differences in electron binding energies of the various isomers. Photoelectron spectra of the (H(2)O)(7)(-) . Ar(m) clusters are obtained over the range of m=0-10. These spectra reveal the formation of a new isomer (I') for m>5, the electron binding energy of which is about 0.15 eV higher than that of the type I form previously reported to be the highest binding energy species [Coe et al., J. Chem. Phys. 92, 3980 (1990)]. Isomer-selective vibrational predissociation spectra are obtained using both the Ar dependence of the isomer distribution and photochemical depopulation of the more weakly (electron) binding isomers. The likely structures of the isomers at play are identified with the aid of electronic structure calculations, and the electron binding energies, as well as harmonic vibrational spectra, are calculated for 28 low-lying forms for comparison with the experimental results. The HOH bending spectrum of the low binding type II form is dominated by a band that is moderately redshifted relative to the bending origin of the bare water molecule. Calculations trace this feature primarily to the bending vibration localized on a water molecule in which a dangling H atom points toward the electron cloud. Both higher binding forms (I and I') display the characteristic patterns in the bending and OH stretching regions signaling electron attachment primarily to a water molecule in an AA binding site, a persistent motif found in non-isomer-selective spectra of the clusters up to (H(2)O)(50)(-).     

Hydration of small peptides

The results for the sequential hydration of small peptides (<15 residues) obtained in our group are reviewed and put in perspective with other work published in the literature where appropriate. Our findings are based on hydration equilibrium measurements in a high-pressure drift cell inserted into an electrospray mass spectrometer and on calculations employing molecular mechanics and density functional theory methods. It is found that the ionic functional groups typically present in peptides, the ammonium, guanidinium, and carboxylate groups, are the primary target of water molecules binding to peptides. Whereas the water–guanidinium binding energy is fairly constant at 9 ± 1 kcal/mol, the water binding energy of an ammonium group ranges from 7 to 15 kcal/mol depending on how exposed the ammonium group is. A five-residue peptide containing an ammonium group is in favorable cases large enough to fully self-solvate the charge, but a pentapeptide containing a guanidinium group is too small to efficiently shield the charge of this much larger ionic group. The water–carboxylate interaction amounts to 13 kcal/mol with smaller values for a shielded carboxylate group. Both water bound to water in a second solvation shell and charge remote water molecules on the surface of the peptide are bound by 7–8 kcal/mol. The presence of several ionic groups in multiply charged peptides increases the number of favorable hydration sites, but does not enhance the water–peptide binding energy significantly. Water binding energies measured for the first four water molecules bound to protonated bradykinin do not show the declining trend typically observed for other peptides but are constant at 10 kcal/mol, a result consistent with a molecule containing a salt bridge with several good hydration sites. Questions regarding peptide structural changes as a function of number of solvating water molecules are discussed. Not much is known at present about the effect of individual water molecules on the conformation of peptides and on the stability of peptide zwitterions.     

On the contribution of vibrational anharmonicity to the binding energies of water clusters

The second-order vibrational perturbation theory method has been used together with the B3LYP and MP2 electronic structure methods to investigate the effects of anharmonicity on the vibrational zero-point energy (ZPE) contributions to the binding energies of (H2O)n, n = 2-6, clusters. For the low-lying isomers of (H2O)6, the anharmonicity correction to the binding energy is calculated to range from -248 to -355 cm(-1). It is also demonstrated that although high-order electron correlation effects are important for the individual vibrational frequencies, they are relatively unimportant for the net ZPE contributions to the binding energies of water clusters.     

High-performance liquid chromatographic determination of ochratoxin A in artificially contaminated cocoa beans using automated sample clean-up

A HPLC method is described for the analysis of ochratoxin A at low-ppb levels in samples of artificially contaminated cocoa beans. The samples are extracted in a mixture of methanol–water containing ascorbic acid, adjusted to pH and evaporated to dryness. Samples in this state are then placed onto a Benchmate sample preparation workstation where C₁₈ solid-phase extraction operations are performed. The resulting materials are evaporated to dryness and analyzed by reversed-phase HPLC with fluorescence detection. The method was evaluated for accuracy and precision with R.S.D.s for multiple injections of sample and standard calculated to be 1.1% and 2.5% for sample and standard, respectively. Recoveries of ochratoxin A added to cocoa beans ranged from 87–106% over the range of the assay.     

Relaxation of excited Franck–Condon states of molecular electron donor–acceptor complexes in liquid solution: the role of orientational isomers

The direct consequences of the presence of ground state orientational isomers of molecular complexes are discussed in terms of the adiabatic potential energy surfaces calculated for the ground and excited states of electron donor–acceptor complexes of tetracyanobenzene with toluene and with mesitylene. Some earlier experimental results that confirm the presence of orientational isomers are also recalled and reviewed, together with the recent results for molecular exciplexes under supersonic molecular beam conditions. Exploration of potential energy surfaces shows that the relaxation pathways of excited Franck–Condon states of the ground state isomers may differ considerably and in liquid solution may be sensitive to physical conditions, which in fact is observed in time-resolved fluorescence spectra of the electron donor–acceptor systems under consideration, upon excitation of high-energy Van der Waals orientational isomers. It is concluded that, in weak electron donor–acceptor complexes in liquid solutions, the role of such isomers may be limited, but it may become crucial for the kinetics and dynamics of excited states if the system is simultaneously capable of forming an exciplex.     

Separation of phenylthiohydantoin-amino acids by highpressure liquid chromatography

A method for the separation of all phenylthiohydantoin (PTH)-amino acids except PTH-arginine and PTH-histidine by high-pressure liquid chromatography on a silica column is described. Elution is performed with a concave solvent gradient from hexane—methanol—propanol (3980:9:11) to methanol—propanol (9:11). A complete run is achieved in 40 rain with a pressure drop of 1000 p.s.i. over the 250 mm × 2.1 mm column. Eluted peaks of 2–5 nmole are easily detected by their ultraviolet absorption at 254 nm. This method is superior to existing gas—liquid and thin-layer chromatographic techniques since all PTH-amino acids except PTH-arginine and PTH-histidine may be both separated and quantitated in a single run of 40 min.

The use of the technique in conjunction with an automated peptide sequence analyser is illustrated.     

Theoretical investigation of gas phase ethanol–(water)n (n = 1–5) clusters and comparison with gas phase pure water clusters (water)n (n = 2–6)

Various properties (such as optimal structures, structural parameters, hydrogen bonds, natural bond orbital charge distributions, binding energies, electron densities at hydrogen bond critical points, cooperative effects, and so on) of gas phase ethanol–(water)_n (n = 1–5) clusters with the change in the number of water molecules have been systematically explored at the MP2/aug‐cc‐pVTZ//MP2/6‐311++G(d,p) computational level. The study of optimal structures shows that the most stable ethanol‐water heterodimer is the one where exists one primary hydrogen bond (O? H…O) and one secondary hydrogen bond (C? H …O) simultaneously. The cyclic geometric pattern formed by the primary hydrogen bonds, where all the molecules are proton acceptor and proton donor simultaneously, is the most stable configuration for ethanol–(water)_n (n = 2–4) clusters, and a transition from two‐dimensional cyclic to three‐dimensional structures occurs at n = 5. At the same time, the cluster stability seems to correlate with the number of primary hydrogen bonds, because the secondary hydrogen bond was extremely weaker than the primary hydrogen bond. Furthermore, the comparison of cooperative effects between ethanol–water clusters and gas phase pure water clusters has been analyzed from two aspects. First of all, for the cyclic structure, the cooperative effect in the former is slightly stronger than that of the latter with the increasing of water molecules. Second, for the ethanol–(water)₅ and (water)₆ structure, the cooperative effect in the former is also correspondingly stronger than that of the latter except for the ethanol–(water)₅ book structure. © 2012 Wiley Periodicals, Inc.     

quantitation of p- and m-methylstyrenes by liquid chromatography and 13C nuclear magnetic resonance spectrometry

The molar ratio of p- and m-methylstyrene in mixtures is determined by liquid chromatography and ¹³C NMR spectrometry. In the former instance, separation and quantitations are achieved using an adamantyl-modified column with a mobile phase of ethanol/water (30 + 70, v/v) containing 0.01 M β-cyclodextrin. In the latter instance, a ¹H-decoupled ¹³C NMR technique is described for the determination of the positional isomers. The results obtained by both methods are self-consistent, with a standard deviation for the molar ratio of the para and meta isomers of less than ±0.01.