Hydrogen Bonding and Vaporization Thermodynamics in Hexafluoroisopropanol-Acetone and -Methanol Mixtures. A Joined Cluster Analysis and Molecular Dynamic Study

Binary mixtures of hexafluoroisopropanol with either methanol or acetone are analyzed via classical molecular dynamics simulations and quantum cluster equilibrium calculations. In particular, their populations and thermodynamic properties are investigated with the binary quantum cluster equilibrium method, using our in-house code Peacemaker 2.8, upgraded with temperature-dependent parameters. A novel approach, where the final density from classical molecular dynamics, has been used to generate the necessary reference isobars. The hydrogen bond network in both type of mixtures at molar fraction of hexafluoroisopropanol of 0.2, 0.5, and 0.8 respectively is investigated via the molecular dynamics trajectories and the cluster results. In particular, the populations show that mixed clusters are preferred in both systems even at 0.2 molar fractions of hexafluoroisopropanol. Enthalpies and entropies of vaporization are calculated for the neat and mixed systems and found to be in good agreement with experimental values.     

Reaction of aldehydes and ketones with t-butyl bromide-dimethyl sulphoxide

Reacting aldehydes and ketones with the “Bu^tBr-Me₂SO” system produces the corresponding α-bromoderivatives $\text{2}$. In the case of ketones, is possible, bromination is obtained exclusively at the more highly substituted α-position. With slight modifications of the reaction conditions (add to obtain “in situ” formation of either dimethyl(2-oxo-2-phenylalkyl)sulphonium salts $\text{3}$ or of α-methylthioderivatives $\text{4}$. Dimethyl(l-m bromide ($\text{3h}$) during crystallization undergoes spontaneous resolution of the two enantiomers, as demonstrated by single crystal X-ray analysis a     

Chiral aminoacid containing acyclic ligands - II. Complexation of alkaline earth cations

The abilities of the title ligands for alkali and alkaline earth cations are estimated by ¹H and ¹³C NMR and by picrate extraction from water to methylene chloride. These ligands are shown to be excellent complexing agents for alkaline earth cations. Complexation occurs preferably at the ether and amide carbonyl groups. The stoichiometry of complexation is evaluated by ¹H NMR.     

Recognition and strand displacement of DNA oligonucleotides by peptide nucleic acids (PNAs). High-performance ion-exchange chromatographic analysis

Peptide nucleic acids (PNAs) are oligonucleotide mimics containing a pseudopeptide chain, which are able to bind complementary DNA tracts with high affinity and selectivity. Two mixed-sequence PNA undecamers (1 and 2) were synthesized and their double-stranded adducts with the complementary oligonucleotides (3 and 4) were revealed by the appearance of the corresponding peak in anion-exchange HPLC. A DEAE column was used and elution was performed with aqueous Tris buffer (pH 8) and an ionic strength gradient (0-0.5 M NaCl). The same effect was not observed with non-complementary oligonucleotides. The stability of the PNA-DNA adducts under the conditions used in the chromatographic system was studied as a function of temperature. Furthermore, in competition experiments double-stranded oligonucleotides were challenged by a PNA complementary to one strand: the formation of the PNA-DNA hybrid and the displacement of the non-complementary strand were observed with high specificity. The results suggest a possible use of ion-exchange HPLC for studying PNA-DNA interactions, and indicate the efficiency of PNA probes in the chromatographic analysis of DNA.     

Histamine-modified cationic beta-cyclodextrins as chiral selectors for the enantiomeric separation of hydroxy acids and carboxylic acids by capillary electrophoresis.

The enantiomeric separation of alpha-hydroxy acids and carboxylic acids was successfully performed by using 6-deoxy-6-N-histamino-beta-cyclodextrin (CD-hm), a monosubstituted positively charged beta-cyclodextrin (beta-CD) bearing a histamine moiety linked to the C6 of a glucose unit in the upper CD rim via the amino group. Good results were obtained at a low selector concentration (1 mM). The number of positive charges on the upper rim may be modulated as a function of pH, because of the different pKa of the amino and the imidazolyl groups, and was found to affect both the enantioselectivity and resolution factors. With the analogous 6-deoxy-[4-(2-aminoethyl)imidazolyl]-beta-cyclodextrin (CD-mh) bearing the histamine moiety linked to the C6 via the imidazolyl group, very poor results were obtained, showing that the proximity of the positive charge to the cavity plays an important role in the enantiomeric recognition. The complexation mode was studied by electrospray ionization-mass spectrometry (ESI-MS) and two-dimensional nuclear magnetic resonance (2-D NMR) ROESY experiments: the recognition model is consistent with an inclusion complexation of the aromatic ring of the analyte within the CD cavity coupled to electrostatic interactions between the carboxylate and the protonated amino group of the cyclodextrin.     

Study of the oligopeptide fraction in Grana Padano and Parmigiano-Reggiano cheeses by liquid chromatography electrospray ionization mass spectrometry

The oligopeptide fractions of Grana Padano and Parmigiano Reggiano cheese samples, at various stages of ageing (6, 12, 18 and 24 months), were analysed by means of high-performance liquid chromatography coupled to electrospray ionisation mass spectrometry (ESI-MS). The main oligopeptides (< 5000 Da) present in the samples were extracted by using an originally developed method which allowed good enrichment of the oligopeptide fractions and identified according to their molecular weights. The casein sequences compatible with the found molecular weights were determined and the exact oligopeptide sequences were identified by using the fragmentation peaks originated by in-source collisionally-induced dissociation (CID). The ESI-MS method reported here allowed the sequence identification of oligopeptides very quickly with a single liquid chromatography-mass spectrometry run. Oligopeptides were also semi-quantified by comparison with a suitable internal standard (the dipeptide phenylalanyl-phenylalanine, Phe-Phe). This methodology demonstrated that, as far as the main oligopeptides are concerned, Grana Padano and Parmigiano Reggiano presented very similar composition. Anyway, the evolution of the fractions during ageing time was characteristically distinct among the two cheeses: in Grana Padano cheeses most of the oligopeptides reached a maximum at 12 months of ageing and then decreased, whereas in Parmigiano Reggiano cheeses the oligopeptide amounts were usually lower and had a less regular trend. The reason for this different behaviour may be ascribed to the different production techniques.     

The local and electronic structures of Al90FexCe10−x alloys (x=3, 5, 7): XANES analysis

The x-ray absorption near-edge structure (XANES) of Al₉₀Fe_xCe_10?x (x=3, 5, 7) is measured above the Fe K edge and analyzed theoretically by the multiple scattering method. By comparing the experimental data with the theoretical findings, the most adequate model for describing the neighbor environment of Fe atoms is chosen. Since the dipole transition matrix element is a smoothly varying function of energy near the Fe K edge, the x-ray absorption fine structure in this energy range characterizes the density of free p states of Fe in the conduction band of Al₉₀Fe_xCe_10?x alloys.