Immobilization of catalase on membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors

The immobilization of catalase on grafted membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors is described. The introduction of carboxylic acid groups onto a hydrophobic support provides a good environment for subsequent enzyme immobilization. This single membrane, hydrogen peroxide sensor showed significant improvement with respect to the double membrane versions. The response is very rapid, the linear range being from 10 μM up to 6 mM, with a detection limit of 4.7 μM, and a lifetime of more than 4 months.     

Molecular dynamics and quantum chemical approaches in the study of the hydration of protonated cyclohexyldiamines

Explicit hydration of the neutral and charged cyclohexylamine and of the cyclohexyldiamine isomers in their mono- or diprotonated forms is investigated through classical molecular dynamics (MD) simulations in aqueous solutions combined with DFT calculations in amine–water complexes. The MD studies performed in the monoamines reveal that the structure of the hydration shell around the neutral amino group (NH₂) is quite distinct from that around the charged one (NH₃⁺). On average, the number of water molecules surrounding the two groups is calculated to be ～2 and 3–4, respectively. The variation of the hydration structure prompted by the groups’ proximity is discussed based on the data found for the mono- and diprotonated diamines. To have a more detailed picture of the water molecules’ arrangement around the amino groups and of the amine–water hydrogen bonds, geometry optimisations in hydrates with up to six water molecules are carried out at the B3LYP/aug-cc-pVDZ level. Complexation energies are also computed. The main findings emerging from these calculations are found to be very helpful to rationalise the mutual influence of the amino groups and therefore to better elucidate the MD findings. The complementary nature of the two research methods is emphasised as an excellent tool in order to closely examine the hydration of polyamines, as exemplified for the cyclohexyldiamines.     

Thermal analysis and crystallization from melts

The growth of atenolol, pindolol and betaxolol hydrochloride from melt was investigated by differential scanning calorimetry (DSC) and polarized light thermal microscopy (PLTM). Phase transitions occurring on cooling and subsequent reheating runs performed between −160 °C and a temperature above the respective melting points were studied by DSC. The thermal cycles were also followed by PLTM. Details about the dynamic of the crystallization front taken from microscopic observations are given. An explanation of the results on the basis of molecular supramolecular recognition is advanced.     

Infrared study of the acidic and basic forms of betaxolol

Betaxolol and its respective hydrochloride salt were studied in solution by computational calculations and infrared spectroscopy. The solution molecular conformations were taken to be the same as those exhibited by the compounds in the solid state given by X-ray diffraction and calculated after full geometry optimization by ab initio Hartree-Fock methods using the 6-31G(d) basis set. Infrared spectra of carbon tetrachloride solutions provide valuable information on the structure of the compounds in non-polar solvents at different concentrations.     

Structure of Isolated 1,4-Butanediol: Combination of MP2 Calculations, NBO Analysis, and Matrix-Isolation Infrared Spectroscopy

Theoretical calculations at the MP2 level, NBO and AIM analysis, and matrix-isolation infrared spectroscopy have been used to investigate the structure of the isolated molecule of 1,4-butanediol (1,4-BDO). Sixty-five structures were found to be minima on the potential energy surface, and the three most stable forms are characterized by a folded backbone conformation leading to the formation of an intramolecular H-bond. To better characterize the intramolecular interactions and particularly the hydrogen bonds, natural bond orbital analysis (NBO) was performed for the four most stable conformers, and was further complemented with an atoms-in-molecules (AIM) topological analysis. Infrared spectra of 1,4-BDO isolated in low-temperature argon and xenon matrixes show a good agreement with a population-weighted mean theoretical spectrum, and the spectral features of the conformers expected to be trapped in the matrixes were observed experimentally. Annealing the xenon matrix from 20 to 60 K resulted in significant spectral changes, which were interpreted based on the barriers to intramolecular rotation. An estimation of the intramolecular hydrogen bond energy was carried out following three different methodologies.     

Infrared spectroscopy of racemic and enantiomeric forms of atenolol

The molecular structure of conformational isomorphs given by X-ray diffraction for racemic and enantiomeric atenolol were optimized at the HF/6-31G* level of theory and the infrared spectra of the structure were calculated. These spectra are used to characterize the differences between the various atenolol conformers. The spectra of the (R,S)- and S-atenolol solid forms were recorded and the bands corresponding to the functional groups identified with the aid of the calculated spectra, fitting analysis, temperature effect and H/D isotopic exchange. Particular attention was paid to the stretch vibration modes of the functional groups present in the atenolol.     

Phase Transition in Trans and Cis-1,2-Cyclohexanediol Studied by Infrared Spectroscopy

The study of trans- and cis-1,2-cyclohexanediol by infrared spectroscopy was performed. The variation of the maximum frequency and of the bandwidth of the OH stretching vibration give evidence of the role played by hydrogen bonding in the solid and liquid phases of both isomers and allows to follow the phase transitions. A solid rotator phase is shown for the cis compound.This revised version was published online in November 2005 with corrections to the Cover Date.     

Infrared spectroscopy and the characterization of terfenadine crystallized from solvents

In this paper the structural characterization of terfenadine crystallized from ethanol-water, ethanol and methanol is performed by infrared spectroscopy. The OH stretching vibration, composed of three markedly overlapped bands, is analyzed by peak fitting. The assignment of the hydrogen bonds was conducted making use of band parameters, spectroscopic data for CCl₄ solutions, and molecular dynamics calculations from dimeric systems. Terfenadine just precipitated from solvents is never in the highest crystalline state. This state is reached when the samples are heated at a temperature above 100°C. Some amorphous solid is coprecipitated with the crystalline phase, particularly in methanol. This revised version was published online in August 2006 with corrections to the Cover Date.     

Enthalpy of sublimation in the study of the solid state of organic compounds. Application to erythritol and threitol

The enthalpies of sublimation of erythritol and L-threitol have been determined at 298.15 K by calorimetry. The values obtained for the two diastereomers differ from one another by 17 kJ mol(-1). An interpretation of these results is based on the decomposition of this thermodynamic property in a term coming from the intermolecular interactions of the molecules in the crystal (delta(int)H degrees) and another one related with the conformational change of the molecules on going from the crystal lattice to the most stable forms in the gas phase (delta(conf)H degrees). This last term was calculated from the values of the enthalpy of the molecules in the gas state and of the enthalpy of the isolated molecules with the crystal conformation. Both quantities were obtained by density functional theory (DFT) calculations at the B3LYP/6-311G++(d,p) level of theory. The results obtained in this study show that the most important contribution to the differences observed in the enthalpy of sublimation are the differences in the enthalpy of conformational change (13 kJ mol(-1)) rather than different intermolecular forces exhibited in the solid phase. This is explained by the lower enthalpy of threitol in the gas phase relative to erythritol, which is attributed to the higher strength of the intramolecular hydrogen bonds in the former. The comparison of the calculated infrared spectra obtained for the two compounds in the gas phase supports this interpretation.     

Conformational study of monomeric 2,3-butanediols by matrix-isolation infrared spectroscopy and DFT calculations

The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argon and xenon matrixes were studied, allowing the identification of two different conformers for each compound. These conformers were characterized by a +/-gauche arrangement around the O-C-C-O dihedral angle, thus enabling the establishment of a very weak intramolecular hydrogen bond of the O...H-O type. No other forms of these compounds were identified in matrixes, despite the fact that these four conformers had calculated relative energies from 0 to 5.1 kJ mol(-1) and were expected to be thermally populated from 50 to 6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained in terms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compounds during deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to be less than 4 kJ mol(-1) and are easily overcome in matrixes within the family of conformers with the same heavy atom backbone. The barriers for intramolecular rearrangement of the O-C-C-O dihedral angle in both diastereomers were calculated to range from 20 to 30 kJ mol(-1). Interconversions between the latter conformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers, and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G theory. Additional MP2/6-311++G calculations of energies and vibrational frequencies were performed on the most relevant conformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules were also obtained based on theoretical structural data and from vibrational frequencies (using well-established empirical correlations). The obtained values for -DeltaH for both diastereomers of 2,3-butanediol amount to ca. 6-8 kJ mol(-1).