Study of the mechanism of enantioseparation part VI: thermodynamic study of HPLC separation of some enantiomers of phenylcarbamic acid derivatives on a (S,S) Whelk-O 1 column

The enantiomers of thirteen 2-, 3-, and 4-alkoxyphenylcarbamic acid 2-methoxy-1-[(4-methylpiperazino)methyl]ethyl ester were separated on a (S,S) Whelk-O 1 CSP column isothermally in the range of 0-50 degrees C at 10 degrees C increments, using methanol/ water (90/10, v/v) containing 17.5 mmol L(-1) acetic acid and 14.36 mmol L(-1) triethylamine as a mobile phase. The dependence of the natural logarithms of retention and selectivity factors (In k, In alpha, respectively) on the inverse of temperature, 1/T, was used to determine thermodynamic data of enantiomers of alkoxysubstituted phenylcarbamic acid 2-methoxy-1-[(4-methylpiperazino) methyl]ethyl esters. Enthalpyentropy compensation plots showed that all of the compounds in this study separate via the same enthalpy-driven chiral recognition mechanism.     

Aptamer-oligonucleotide binding studied by capillary electrophoresis: cation effect and separation efficiency

Novel features of DNA structure, recognition and discrimination have been recently elucidated through the solution structural characterization of DNA aptamers that bind cofactors, amino acids and peptides with high affinity and specificity. Multidimensional nuclear magnetic resonance methodologies have been successfully applied to solve the solution structures. In this work, it was demonstrated that capillary electrophoresis was a powerful tool allowing the fundamental study of the binding mechanism between a DNA aptamer and three ligands, adenosine and adenylate compounds, i.e., adenosine diphosphate (ADP) and adenosine triphosphate (ATP). In order to gain further insight into this binding, thermodynamic measurements under different values of parameters (such as salt nature and its concentration (x) in the run buffer) were carried out. The results showed that dehydration at the binding interface, van der Waals interactions, H-bonding and adjustment of the aptamer recognition surface were implied in the aptamer-ligand association. As well, it was demonstrated that the addition in the medium of the sodium monovalent cation Na(+) or the nickel divalent cation Ni(2+) decreased the complex formation. Separation efficiency and peak shape can also be improved by Mg(2+) divalent cation, which increased the mass transfer kinetics during the ligand-aptamer binding process. A significant separation for the worst separated pair of peaks on the electropherogram ((ADP, ATP) peak pair) was thus achieved.     

Thermochemistry of Aqueous Solutions of Alkylated Nucleic Acid Bases. X. Enthalpies of Hydration of Cytosine and some Methylated, Hydroxy and Methoxy Derivatives of Cytosine

Enthalpies of solution in water, vapor pressures, and enthalpies of sublimation were determined for cytosine and a number of crystalline derivatives of cytosine: 1-methylcytosine, 1-methyl-N ⁴-hydroxycytosine, 1,5-methyl-N ⁴-methoxycytosine, 1,N ⁴,N ⁴-trimethylcytosine, 1,5,N ⁴-trimethylcytosine, 1,5-dimethyl-N ⁴-methoxycytosine, and l,N ⁴-dimethyl-5-ethylcytosine. Enthalpies of hydration were calculated. The latter were analyzed on the basis of the group additivity method. The relation between enthalpy of hydration and polarity of compounds is discussed.     

Enthalpies of formation of some phases present in the Y−Ba−Cu−O system by solution calorimetry

Enthalpies of some of the phases in the Y–Ba–Cu–O system were determined by solution calorimetry using a Calvet microcalorimeter. The standard enthalpies of formation for the phases were found to be YBa₂Cu₃O_6.60, –2627.9; YBa₂Cu₃O_6.77, –2641.8; YBa₂Cu₃O_6.90, –2652.0; YBa₂Cu₃O_6.99, –2659.3; Y₂Cu₂O₅, –2198.6; Y₂BaCuO₅, –2656.4; BaCuO_2.33, –788.6; and BaCuO_2.42, –796.2 kJ-mol^–1.     

Enthalpic Interaction of Amino Acids with 2-Chloroethanol in Aqueous Solutions at 298.15 K

The enthalpies of mixing have been determined for five kinds of aqueous amino acids solutions (glycine, L-alanine, L-valine, L-serine, and L-proline) with 2-chloroethanol by an LKB-2277 Bio Activity Monitor at 298.15 K. In addition, the enthalpies of dilution at 298.15 K of aqueous solutions containing the five kinds of amino acids and 2-chloroethanol have been obtained. The heterotactic enthalpic pairwise interaction coefficients of the virial expansion of excess enthalpy were evaluated and interpreted from the point of view of solute–solute interactions. In comparison with ethanol, 2-chloroethanol shows a stronger exothermic interaction with amino acids because of its hydrophilic Cl atom and a more acidic –OH group. Using the additivity groups concept by Savage and Wood (SWAG), contributions of each of functional group of the amino acids and ethanol and 2-chloroethanol have been estimated.     

Enthalpies of mixing of liquid alloys in the In-Pd-Sn system and the limiting binary systems

The partial and integral enthalpies of mixing of molten binary In-Pd (up to about 29 at.% Pd), In-Sn (entire compositional range) and Pd-Sn (up to about 53 at.% Pd) alloys were determined at 900 °C. A Calvet-type microcalorimeter was used for the measurements employing a drop calorimetric technique. Additionally, five sections in the ternary In-Pd-Sn system (compositions up to about 40 at.% Pd) were investigated at 900 °C. The ternary interaction parameters were fitted using the Redlich-Kister-Muggiano model for substitutional solutions. The isoenthalpy curves for In-Pd-Sn at 900 °C were constructed for the integral molar enthalpy of mixing. Furthermore, the experimental results in the ternary system were compared with calculated values obtained by employing different binary extrapolation models.     

Enthalpies of solvation of alkylated uracils in water,nonaqueous and mixed solvents

The enthalpies of solution of uracil and its alkylated derivatives in water, methanol, N,N-dimethylformamide (DMF) and water+DMF mixtures were measured at 25°C. The enthalpies of solvation were determined. The enthalpies of cavity formation, corresponding to the enthalpies of solvent-solvent interactions were calculated and the enthalpies of solute-solvent interactions were obtained. The presence of the alkyl groups was found to have different effects on the enthalpy of interaction depending on the position and size of the substitution. The effect of alkylation at the nonpolar side of the uracil ring was found to arise mostly from the enhancement of the van der Waals interactions. The alkyl substitutions at the polar side resulted also in the removal of the solvent molecules interacting specifically with the polar groups of uracil. The enthalpy of those specific interactions was determined and found to be stronger in methanol and DMF than in water. Enthalpies of solvation in the binary water+DMF solvent were found to depend in a nonlinear way on the solvent composition. The nonlinearities in the water-rich region were found to arise from the decay of the hydrophobic hydration of the solutes with the increasing DMF content. The substitution of two methyl groups caused the uracil molecule to bahave as a predominantly hydrophobic solute. The nonlinearities in the DMF-rich region were found only for those solutes which can form hydrogen bonds with DMF.     

Determination of the enthalpy change for anabolism by four methods

The enthalpy change for anabolism is needed to model the growth/respiration relation in plants. If all CO₂ production is assigned to catabolism, the anabolic reaction becomes C_substrate→C_products+xO₂ with an enthalpy change, ΔH_b. Four methods are proposed for determining ΔH_b: (a) From the difference in the heats of combustion of substrate and anabolic products (i.e. newly grown tissue). (b) From the composition of newly grown tissue and application of Thornton’s rule. (c) From independently measured values of the specific growth rate, R_SG, and of the product (R_SG ΔH_b). The product (R_SG ΔH_b) equals (−ΔH_CO2R_CO2−R_q) where R_CO2 is the specific rate of CO₂ production by respiration, ΔH_CO2 is the heat of combustion of respiratory substrate per mole of CO₂ and R_q is the specific metabolic heat rate. ΔH_b is then calculated as the ratio (R_SG ΔH_b)/R_SG. (d) From (ΔH_b=−(R_q/R_CO2+ΔH_CO2) [(1−)/] where is the substrate carbon conversion efficiency obtained from a total carbon balance. The first three methods have been tested and compared on oat seedlings and the last on corn seedlings. ΔH_b values from all four methods are in reasonable agreement despite the different assumptions involved.     

Thermochemistry of Bu<Subscript>4</Subscript>NBr solutions in binary solvents containing formamide

The heats of solution of tetrabutylammonium bromide have been measured in mixtures of formamide (FA) with methanol (MeOH) and ethylene glycol (EG) at 313.15 K by calorimetric method. The standard enthalpies of solution in binary mixtures have been extrapolated to infinite dilution by Redlich–Rosenfeld–Meyer type equation using the literary data at 298.15 K and the present paper data at 313.15 K. The Debye–Hückel limiting law slope A _H required for calculation of the ∆_sol H ⁰ value has been obtained with application the new additive scheme of determination of the physic-chemical characteristics of binaries. The scheme is tested on the example of Bu₄NBr solutions in FA–MeOH mixture at 298.15 K. Its application yields the ∆_sol H ⁰ value very closed on the ones determined with the real (non-additive) characteristics of binaries. The standard enthalpies of solution extrapolated by Redlich–Rosenfeld–Meyer type equation are in a good agreement with the ones computed in terms of the Debye–Hückel theory in the second approximation. The heat capacities characteristics of Bu₄NBr have been calculated in H₂O–FA, MeOH–FA and EG–FA mixtures using the literary and present data. The sequence of solvents H₂O > FA > EG > MeOH located on their ability to solvophobic solvation found by us earlier for enthalpic characteristics is confirmed by the ∆C _p ⁰ values. The comparison of thermochemical characteristics of Bu₄NBr solutions in aqueous and non-aqueous mixtures containing FA has been carried out. The own structure of water remains in the region of small additions of formamide to co-solvents. It considerably differs the H₂O–FA mixture from the investigated non-aqueous systems.     

Extending bond orbital-connection matrix method to the QSPR study of alkylbenzenes: Some thermochemical properties

The properties of alkylbenzenes were estimated, using the contributions of the σ bonds, the conjugated π bond and the steric effect in alkylbenzene molecule. And a novel bond orbital-connection matrix, conjugated π bond orbital-connection matrix in aromatic molecules, was proposed. The eigenvalues of the conjugated π bond orbital-connection matrix can well express the contribution of the conjugated π bond to the properties of aromatic molecules. Using this eigenvalue together with the parameters proposed in our early works, the bond orbital-connection matrix method was extended successfully to the QSPR studies of alkylbenzenes and a general model was obtained to evaluate the thermochemical properties of alkylbenzenes, that is,

p_{(alkylbenzene)}=aN_C–C+b∑X_1CC+cN_C–H+d∑X_1CH+eS_Z/E+k∑X_1π,