On the biological importance of the hydrogen bond breaking potency of fluorocarbons

Fluorocarbons containing higher halogens have the potency of breaking certain hydrogen bonds in solutions. The relative strength of this potency varies in the series F < Cl < Br < I and the presence of hydrogen atoms in the fluorocarbon increases it. There is a striking parallelism between the hydrogen bond breaking and the anaesthetic potency of these molecules. It is suggested that the breaking of hydrogen bonds is an important step in the mechanism of anaesthesia.     

Stereospecific uncatalyzed alpha-O-glycosylation and alpha-C-glycosidation by means of a new D-Gulal-derived alpha vinyl oxirane

The reaction of alpha vinyl oxirane 5, prepared through a new route to the d-gulal system, with O-nucleophiles (alcohols and di-O-isopropylidene-alpha-d-monosaccharides) and C-nucleophiles (lithium alkyls) affords, in a completely stereoselective way, the corresponding 2-unsaturated alpha O- and C-glycosides having the same configuration as the starting epoxide.     

Enantioselective total synthesis of (−)-microcarpalide

The enantioselective total synthesis of the actin-targeting metabolite (−)-microcarpalide is described. Key steps include ring-closing metathesis (RCM) for the final construction of the 10-membered lactone framework and stereoselective homologation of boronic esters for the insertion of all stereocentres with the desired absolute configuration. In particular, the acidic fragment was prepared in seven steps from a suitable chiral bromomethane boronate by means of two sequential stereoselective homologations to install the two stereocentres with the correct final R stereochemistry, employing (−)-pinanediol as the chiral director. Subsequent elaboration to the required C₇ backbone entailed nucleophilic displacement with a vinyl Grignard reagent, oxidative cleavage of the boronic scaffold and protection-deprotection manipulations. Interestingly, when the tribenzyloxy diene ester resulting from DCC-mediated coupling of the two key synthons was subjected to RCM in the presence of Grubbs' catalyst, the reaction proceeded stereoselectively to yield the desired trans oxecin-2-one, albeit with poor conversion.     

On the use of symmetry in first-order perturbed HF theory. II

A method is described whereby molecular symmetry is employed to reduce the number of two-electron integrals in perturbed Hartree–Fock calculations of second-order properties. The method is a generalization of the Dacre–Elder procedure. First- and second-rank perturbing tensor operators are examined in the coupled HF approach to electric dipole polarizabilities, magnetic susceptibilities, quadrupole polarizabilities, and spin-dopolar contributions to spin–spin coupling constants. The procedure sketched here permits a large saving of computer efforts, which is shown by some illustrative examples.     

DNA codes for nanoscience

The nanometer scale is a special place where all sciences meet and develop a particularly strong interdisciplinarity. While biology is a source of inspiration for nanoscientists, chemistry has a central role in turning inspirations and methods from biological systems to nanotechnological use. DNA is the biological molecule by which nanoscience and nanotechnology is mostly fascinated. Nature uses DNA not only as a repository of the genetic information, but also as a controller of the expression of the genes it contains. Thus, there are codes embedded in the DNA sequence that serve to control recognition processes on the atomic scale, such as the base pairing, and others that control processes taking place on the nanoscale. From the chemical point of view, DNA is the supramolecular building block with the highest informational content. Nanoscience has therefore the opportunity of using DNA molecules to increase the level of complexity and efficiency in self-assembling and self-directing processes.     

Subcanonical curves and complete intersections in projective 3-space

Summary Un classico teorema di G. Gherardelli afferma che una curvaC P ³ è intersezionè completa se e soltanto se è proiettivamente normale e sottocanonica. Qui si prova che, seC e a- sottocanonica ed inoltre le superficie di grado 1 + (a/2) (a pari) ovvero (a + l)/2 o (a + 3)/2 o (a + 5)/2 (a dispari) tagliano suC serie complete, alloraC è intersezione completa. Si determina inoltre un bound d funzione di a tale che, seC è a-sottocanonica e di grado d d, alloraC è intersezione completa se e soltanto se le superficie di grado a tagliano suC una serie completa. Si discutono poi numerosi esempi di curve sottocanoniche non intersezioni complete.Paper written while P. Valabrega was member of C.N.R. (G.N.A.S.G.A.) and both authors were supported by M.P.I. funds.     

Fourier transform-ion cyclotron resonance study of the gas-phase acidities of germane and methylgermane; Bond dissociation energy of germane

An accurate gas-phase acidity for germane (enthalpy scale, equivalent to the proton affinity of GeH₃ ^?), ΔH _acid ^o(GeH₄) = 1502.0 ± 5.1 kJ mol^?1, is obtained by constructing a consistent acidity ladder between GeH₄, and H₂S by using Fourier transform-ion cyclotron resonance spectrometry, and 0 and 298.15 K values for the first bond dissociation energy of GeH₄ are proposed: D₀ ^o(H₃Ge-H) = 352 ± 9 kJ mol^?1; D ^o(H₃Ge-H) = 358 ± 9 kJ mol^?1, respectively. These results are compared with experimental and theoretical data reported in the literature. Methylgermane was found to be a weaker acid than germane by approximately 35 kJ mol^?1: ΔH _acid ^o = 1536.6 kJ mol^?1.     

ANVAS: Artificial Neural Variables Adaptation System for descriptor selection

A new algorithm model-oriented for variable selection is presented in this study. It is based on the combination of genetic algorithms (GA) for hyperspace exploration, and counterpropagation artificial neural network (CP ANN) for deriving the fitness score. The proposed method performed very well on both well defined synthetic data sets and real academic data sets.     

Separation and characterisation of sphingoceramides by high-performance liquid chromatography-electrospray ionisation mass spectrometry

We developed a simple and reliable analytical method for the quantification and the characterization of ceramides extracted from biological samples by high-performance liquid chromatography (HPLC) coupled to electrospray ionisation tandem mass spectrometry (ESI/MS/MS). The chromatographic separation of analytes was carried out in a RP8 column, eluting with a methanol-water mixture in gradient elution mode. The separated lipids were detected by total ion monitoring and characterised by MS/MS spectra; quantitative analysis was performed by integrating the extracted ion peaks obtained in the negative ion mode. Good repeatability was obtained for retention time (0.3-2%), peak area ratio (A(S)/A(IS), 2-8%), as well as limit of detection (LOD, 5-26 pg) and quantification (LOQ, 13-53 pg). The method was validated for the analysis of N-palmitoyl-D-erythro-sphingosine (Cer16), N-stearoyl-D-erythro-sphingosine (Cer18), N-tetracosanoyl-D-erythro-sphingosine (N24:0, lignoceric ceramide, Cer24:0), and N-tetracos-15'-enoyl-D-erythro-sphingosine (N24:1, nervonic ceramide, Cer24:1), giving good results. Lipid mixtures, extracted from skin and epidermal cells, were analysed for their content of the studied ceramides.