Low-temperature structural phase transition in deuterated and protonated lithium acetate dihydrate

Heat capacity measurements of protonated lithium acetate dihydrate show a structural phase transition at T = 12 K. This finding is in contrast to earlier work, where it was thought that only the deuterated compound undergoes a low temperature structural phase transition. This finding is confirmed by low temperature ultrasound spectroscopy, where the structural phase transition is associated with a velocity decrease of the ultrasonic waves, i.e. with an elastic softening. We compare the thermodynamic properties of the protonated and deuterated compounds and discuss two alternatives for the mechanism of the phase transition based on the thermal expansion measurements.     

Giant deuteron migration during the isosymmetric phase transition in deuterated 3,5-pyridinedicarboxylic acid

Deuterated 3,5‐pyridinedicarboxylic acid exhibits reversible temperature‐induced deuteron migration of a magnitude unprecedented in this class of compounds. We used a combination of variable‐temperature powder and single‐crystal neutron diffraction and density functional theory (DFT)‐based computational methods to elucidate the origin of this remarkable behaviour. Single‐crystal neutron diffraction shows that between 15 and 300 K, the deuteron moves by 0.32(1) Å and the structure changes from a low‐temperature N? D???O form to a high‐temperature N???D? O form. Variable‐temperature powder neutron‐diffraction data, which was fitted by using parametric Rietveld refinement, show that this deuteron migration is due to an isosymmetric, first‐order phase transition that occurs by growth of the daughter phase in the parent‐phase matrix. Similar phase transitions are observed in two selectively deuterated forms of the material. DFT calculations demonstrate the role of phonons and show that vibrational free‐energy stabilisation, which plays a key role in the observed structural phase transitions, is more pronounced in the fully deuterated material and proportional to the mass of the molecule, that is, the level of deuteration. This is consistent with our experimental work, for which distinct crystallographic phase transitions were clearly observed for the three deuterated systems, but not for the fully protonated material.     

Real time scanning force microscopy observation of a structural phase transition in self-assembled alkanethiols

We present here a new approach based on high resolution scanning force microscopy that permits molecular determination and frictional discrimination between coexisting molecular configurations of alkanethiols on Au(111). The method has allowed visualizing in real time a phase transition between two differently ordered self-assembled configurations, which spontaneously takes place with time at constant coverage and under ambient conditions. Frictional force measurements indicate that the molecular rearrangement observed occurs via an order-disorder-order process.     

Two-component chiral phase transfer catalysts: enantioselective esterification of an N-acylated amino acid

[see reaction]. The first example of a two-component chiral phase transfer catalyst is described which, operating in a biphasic solvent system, preferentially esterifies one enantiomer of a racemic N-acylated amino acid. The two-component catalyst is comprised of an achiral quaternary ammonium ion and a proline-derived chiral selector initially developed for the liquid chromatographic separation of enantiomers.     

Dansyl amino acid enantiomer separation on a teicoplanin chiral stationary phase: effect of eluent pH

A sensitive method for the simultaneous determination of loratadine and its major active metabolite descarboethoxyloratadine (DCL) in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The samples were extracted from plasma with toluene followed by back-extraction into formic acid (2%) for DCL after which the toluene containing the loratadine was evaporated, the analyte reconstituted and combined with the DCL back-extract. Chromatography was performed on a Phenomenex Luna C18 (2) 5-microm, 150x2.1-mm column with a mobile phase consisting of acetonitrile-0.1% formic acid using gradient elution (10 to 90% acetonitrile in 2 min) at a flow-rate of 0.3 ml/min. Detection was achieved by a Perkin-Elmer API 2000 mass spectrometer (LC-MS-MS) set at unit resolution in the multiple reaction monitoring mode. TurbolonSpray ionisation was used for ion production. The mean recovery for loratadine and descarboethoxyloratadine was 61 and 100%, respectively, with a lower limit of quantification at 0.10 ng/ml for both the analyte and its metabolite. This is the first assay method described for the simultaneous determination of loratadine and descarboethoxyloratadine in plasma using one chromatographic run. The method is sensitive and reproducible enough to be used in pharmacokinetic studies.     

Synthesis and structural characterization of trivalent amino acid derived chiral phosphorus compounds

Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13.     

On the helix-coil transition in alanine based polypeptides in gas phase

Using multicanonical simulations, the authors study the effect of charged end groups on helix formation in alanine based polypeptides. They confirm earlier reports that neutral polyalanine exhibits a pronounced helix-coil transition in gas phase simulations. Introducing a charged Lys+ at the C terminal stabilizes the alpha helix and leads to a higher transition temperature. On the other hand, adding the Lys+ at the N terminal inhibits helix formation. Instead, a more globular structure was found. These results are in agreement with recent experiments on alanine based polypeptides in gas phase. They indicate that present force fields describe accurately the intramolecular interactions in proteins.     

Determination of amino acid enantiomers in orange juices by chiral phase capillary gas chromatography

Summary The configurations of free amino acids (AAs) in orange juice beverages (commercial products of satisfactory and unsatisfactory quality), an orange juice concentrate (bulk product suspected of being adulterated), and in an orange juice that has been contaminated by addition ofLactobacillus plantarum as a model for microbial spoilage, were determined, after derivatization, by means of gas-liquid chromatography (GC) using fused-silica capillary columns coated with Chirasil-L-Val or Chirasil-D-Val as stationary phases. AAs were isolated from juices by treatment with Dowex WX8 ion-exchanger and were investigated, by GC, as theirN(O)-pentafluoropropionylorN(O)-trifluoroacetyl 1-propyl esters. It was found that the high quality orange juice beverage contained L-AAs exclusively whereas this juice, after fermentation withLactobacillus, contained free D-Ala (32.7%), D-Val (62.3%), D-Phe (20.0%), D-Glu (24.3%), D-Ser (2.6%), D-Asp (0.8%), and significant amounts of D-Pro [% D=100 D/(D+L)]. D-Ala (8.8%) and D-Ser (4.2%) were found in a sensory and analytically unsatisfactory orange juice beverage, whilst D-Ala (27.5%) and D-Ser (14.3%) were detected in the orange juice concentrate suspected of being adulterated.Although capillary GC on chiral stationary phases is regarded as being highly suitable for the determination of AA enantiomers in fruit juice beverages, detection of D-AAs is currently not considered as conclusive proof of fruit juice adulteration caused by addition of AA racemates since a non-microbial origin of D-AAs in the respective juice, or an original occurrence of D-AAs, in either the free, substituted, or peptide-bonded form in the fruits, cannot be excluded with certainty.Presented in part at the Deutscher Lebensmittelchemikertag, Sept. 18–21, 1990, Frankfurt and at the 14. Jahrestagung Deutscher Lebensmitteltechnologen, Nov. 15–17, 1990, Düsseldorf.     

Thermal and kinetic study of the ferroelectric phase transition in deuterated triglycine selenate

The specific heat and the enthalpy variation of a highly deuterated crystal of ferroelectric triglycine selenate have been measured around its first-order phase transition using the technique square modulated differential thermal analysis (SMDTA). The low temperature variation rate has allowed analyzing the kinetics of the phase transition. Due to an internal crack in the sample, the transition is carried out in two steps and an intermediate region where the transition is blocked and both phases coexist without transformation has been found. The latent heat on cooling (L _c=1.32±0.02 J g^–1) is higher than on heating (L _h=1.08±0.02 J g^–1) due to the thermal hysteresis and the great difference between the specific heat in both phases. Nevertheless, the enthalpy balance is fulfilled on heating and on cooling.     

Enantioseparation of amino acids on a polysaccharide-based chiral stationary phase

Sulfonic acids have been shown to be more effective than the commonly used trifluoroacetic acid (TFA) in the chiral resolution of underivatized aromatic amino acids on an amylosic column. Sulfonic acid additives give a more UV transparent mobile phase, possibly allowing the detection of non-aromatic analytes. Work presented demonstrates that through the combination of sulfonic acid mobile phase additives, amine mobile phase additives and solvent modifier variations, the enantiomers of 20 of 25 probe amino acids are fully resolved, four are partially resolved with only one failing to be separated on a common amylosic column.     

2D-NMR strategy dedicated to the analysis of weakly ordered,fully deuterated enantiomers in chiral liquid crystals

Methods for the assignment of the quadrupolar doublets in the deuterium NMR spectra of weakly ordered, perdeuterated or partially deuterated enantiomers dissolved in chiral liquid crystals are described which use robust 2D correlation NMR experiments. To overcome a lack of resolution in deuterium tilted Q-COSY 2D spectra in such materials, we propose and explore a correlation 2D sequence which is based on deuterium-carbon 2D correlation spectroscopy. The technique results in a (13)C-(2)H contour plot and allows the full resonance assignment of overcrowded deuterium 1D spectra using carbon-deuterium correlations. The (2)H autocorrelation and (13)C-(2)H correlation experiments are applied in the case of a racemic mixture of 2-ethylhexanoic acid-d(15) dissolved in a polypeptidic chiral oriented solvent. The performance and the limits of both techniques are presented and discussed. For the last step of the assignment procedure, we propose a simple method for obtaining two coherent sets of quadrupolar splittings, one for each enantiomer.     

The effects of a second branched alkyl chain and double chiral centres on the phase transition and spontaneous polarization of a chiral smectic liquid crystal

The effects of a second branched alkyl chain, lateral substitution, and double chiral centres on the phase transition and spontaneous polarization of the ferroelectric liquid crystal having a 2-methylalkanoyl group have been investigated. The introduction of another branched alkyl chain away from the 2-methylalkanoyl group causes a sharp S_C*-S_A transition peak and also enhances the ferroelectric properties in the S_C* phase. Since the order within the layers is liquid-like in the S_C* phase, the alkyl chain branching away from both the chiral centre and a polar group affects the overall motion of the molecule in the S_C* phase. In the system of a compound with double chiral centres, the existence of the chiral centre in the 2-methylalkanoyl group affects the phase transition temperatures and the magnitude of the spontaneous polarization in the S_C* phase. On the other hand, the existence of the chiral centre in the 2-methylbutyl group only affects the stability of a more highly ordered smectic phase appearing below the S_C* phase.     

Cumulative reaction probabilities and transition state properties: a study of the F + H2 reaction and its deuterated isotopic variants

A comparative quantum mechanical (QM) and quasiclassical trajectory (QCT) study of the cumulative reaction probabilities (CRPs) is presented in this work for the F + H(2) reaction and its isotopic variants for low values of the total angular momentum J. The agreement between the two sets of calculations is very good with the exception of some features whose origin is genuinely QM. The agreement also extends to the CRP resolved in the helicity quantum number k. The most remarkable feature is the steplike structure, which becomes clearly distinct when the CRPs are resolved in odd and even rotational states j. The analysis of these steps shows that each successive increment is due to the opening of the consecutive rovibrational states of the H(2) or D(2) molecule, which, in this case, nearly coincide with those of the transition state. Moreover, the height of each step reflects the number of helicity states compatible with a given J and j values, thus indicating that the various helicity states for a specific j have basically the same contribution to the CRPs at a given total energy. As a consequence, the dependence with k of the reactivity is practically negligible, suggesting very small steric restrictions for any possible orientation of the reactants. This behavior is in marked contrast to that found in the D + H(2) reaction, wherein a strong k dependence was found in the threshold and magnitude of the CRP. The advantages of a combined QCT and QM approaches to the study of CRPs are emphasized in this work.     

Chiral separation of enantiomers of amino acid derivatives by high-performance liquid chromatography on a norvancomycin-bonded chiral stationary phase

A novel norvancomycin-bonded chiral stationary phase (NVC-CSP) was synthesized by using the chiral selector of norvancomycin. The chiral separation of enantiomers of several dansyl-amino acids by high-performance liquid chromatography (HPLC) in the reversed-phase mode is described. The effects of some parameters, such as organic modifier concentration, column temperature, pH and flow rate of the mobile phase, on the retention and enantioselectivity were investigated. The study showed that ionic, as well as hydrophobic interactions were engaged between the analyte and macrocycle in this chromatographic system. Increasing pH of buffers usually improved the chiral resolution for dansyl--amino-n-butyric acid (Dns-But), dansyl-methionine (Dns-Met) and dansyl-threonine (Dns-Thr), but not for dansyl-glutamic acid (Dns-Glu) which contains two carboxylic groups in its molecular structure. The natural logarithms of selectivity factors (ln ) of all the investigated compounds depended linearly on the reciprocal of temperature (1/T), most processes of enantioseparation were controlled enthalpically. Interestingly, the process of enantioseparaton for dansyl-threonine was enthalpy-controlled at pH of 3.5, while at pH of 7.0, it was entropy-controlled according to thermodynamic parameters Δ_R,SΔH° and Δ_R,SΔS° afforded by Van’t Hoff plots. In order to get baseline separation for all the solutes researched, norvancomycin was also used as a chiral mobile phase additive. In combination with the NVC-CSP, remarkable increases in enanselectivity were observed for all the compounds, as the result of a “synergistic” effect.     

Pressure-induced structural phase transition in the half-Heusler compound CaAuBi

We report a structural phase transition of the ternary compound CaAuBi under pressure, from the known cubic half-Heusler phase to a hexagonal LiGaGe type phase, based on synchrotron X-ray diffraction patterns taken under pressures up to 18 GPa. We report lattice parameters and atomic coordinates, and perform total energy calculations for both the cubic and hexagonal phases under different pressures. Finally, we present a structure map that places CaAuBi in the context of related 18 electron XYZ ternary systems.     

Sodium maleopimaric acid as pseudostationary phase for chiral separations of amino acid derivatives by capillary micellar electrokinetic chromatography

A new type of chiral surfactant, sodium maleopimaric acid (SMA), was synthesized, and employed for the enantioselective micellar electrokinetic chromatographic (MEKC) separation of amino acid enantiomers derivatized with naphthalene-2,3-dicarboxaldehyde (NDA-D/L-AAs). The effect of the surfactant concentration, type and concentration of the BGE, and buffer pH on the resolution was studied, and optimized conditions were used to evaluate the ability of this new surfactant to perform chiral separations toward NDA-D/L-AAs by MEKC. Enantiomeric separations of NDA-D/L-AAs were achieved with a running buffer consisting of 100 mM borate (pH 9.5) and 20 mM SMA in a 58.5 cm length x 50 microm id capillary. Under the conditions selected, two pairs of tested amino acid enantiomers including NDA-D/L-trptophan (Trp) and NDA-D/L-kynurenine (Kyn) were resolved.     

Effect of amino acid surfactants on phase transition of poly(N-isopropylacrylamide) gel

The volume phase transition behavior of a poly(N-isopropylacrylamide) gel (NIPA gel) in solutions of N-acyl amino acid surfactants were studied as a function of surfactant concentration. The addition of a surfactant beyond the critical micelle concentration (cmc) produced elevation in the transition temperature of the NIPA gel and its swelling. The changes in the volume phase transition temperature and in the swelling of the NIPA gel became more significant with the decreasing size of the amino acid side chain. This result could almost be explained only by the binding amount of surfactant onto the NIPA gel regardless of molecular structure of the amino acid. The binding amount increased in the order of sodium N-lauroyl-glycinate>-alaninate>-valinate>-leucinate>or=-phenylalaninate. For an N-acyl amino acid surfactant to bind onto the NIPA gel, to increase the transition temperature, and to facilitate swelling of the gel, the steric hindrance of the amino acid side chain was more effective than its hydrophobicity.     

Catalytic asymmetric cyanosilylation of ketones by a chiral amino acid salt

In this Communication, we describe the first highly enantioselective cyanosilylation of ketones catalyzed by simple chiral amino acid salt. A broad range of aromatic, alpha,beta-unsaturated, heterocyclic ketones catalyzed by l-phenylglycine sodium salt gave the corresponding trimethylsilyl ethers of cyanohydrins in 75-96% yields with 86-97% enantioselectivities. A catalytic cycle based on experimental phenomena and studies has been proposed to explain the origin of this activation. The reaction employed commercially available and fully recyclable catalysts and involves a simple experimental procedure.     

Direct separation of amino acid enantiomers using a chiral crown ether stationary phase. Application to 2-amino-omega-phosphonoalkanoic acids

The resolution of a series of 2-amino-omega-phosphonoalkanoic acid enantiomers using a crown ether chiral stationary phase is described. The method is applicable to other primary amino acid and shows some advantages over chiral derivatization with fluorometric detection. Optical isomers of under 0.5% may be quantified.     

Separation of amino acid enentiomers by adding a chiral complex to the eluent

Amino acid enantiomers are separated by reverse-phase chromatography with an optically active polyamine/Cu(II) complex in the mobile phase. The polyamine was obtained by reaction of ( — )-trans-1,2-diaminocyclohexane with ethylene glycol diglycidyl ether; the complex with copper(II) ions was used in the eluent. Separation factors are reported for ten amino acids and four barbiturates.