S=N versus S+-N-: an experimental and theoretical charge density study

To elucidate the bonding situation in the widely discussed hypervalent sulfur nitrogen species, the charge density distributions rho(r) and related properties of four representative compounds, methyl(diimido)sulfinic acid H(NtBu)(2)SMe (1), methylene-bis(triimido)sulfonic acid H(2)C[S(NtBu)(2) (NHtBu)](2) (2), sulfurdiimide S(NtBu)(2) (3), and sulfurtriimide S(NtBu)(3) (4), were determined experimentally by high-resolution low-temperature X-ray diffraction experiments (T = 100 K). This set of molecules represents an ideal frame of reference for the comparison of SN bonding modes, because they contain short formal S=N double bonds as well as long S-N single bonds, some of them influenced by inter- or intramolecular hydrogen bonds. For comparison, the gas-phase ab initio calculations of the four model compounds, H(NMe)(2)SMe, H(2)C[S(NMe)(2)(NHMe)](2), S(NMe)(2), and S(NMe)(3), were performed. The topological features were found to be not particularly sensitive with respect to different substituents R (R = H, Me, tBu). In this paper, it is documented that theory and experiment differ in the eigenvalues of the Hessian matrix because of systematically differing positions of the bond critical points but agree very well concerning the spatial Laplacian distribution and the distinct polarization of all investigated sulfur-nitrogen bonds. Both recommend the S(+)-N(-) formulation of sulfur nitrogen bonds in 1 and 2 since all nitrogen atoms are found to be sp(3) hybridized. The planar SNx (x = 2, 3) units in the diimide 3 and the triimide 4 reveal characteristics of m-center-n-electron systems. For none of the investigated S-N bonds, a classical double bond formulation can be supported. This is further substantiated by the NBO/NRT approach. Valence expansion to more than eight electrons at the sulfur atom can definitely be excluded to explain the bonding.     

Constituents of holothuroidea.9. Isolation and structure of a new ganglioside molecular species from the sea cucumber Holothuria pervicax

A new ganglioside molecular species, HPG-7 (1) was obtained from the polar fraction of the chloroform/methanol extract of the sea cucumber, Holothuria pervicax. On the basis of chemical and spectroscopic evidence, the structure of 1 was determined, and the major component was 1-O-[alpha-L-fucopyranosyl-(1-->4)-(N-acetyl-alpha-D-neuraminosyl) - (2-->11)-(N-glycolyl-alpha-D-neuraminosyl)-(2-->4)-(N-acetyl -alpha-D- neuraminosyl)-(2-->6)-beta-D-glucopyranosyl]-(2S,3S,4R)-2-[(2R)-2- hydroxytetracosanoylamino]-14-methyl-hexadecane-1,3,4-triol. This is the first report on the isolation and structure elucidation of trisialo-ganglioside from sea cucumber. 1 showed neuritogenic activity toward the rat pheochromocytoma cell line, PC-12 cell.     

Tetradecyltrimethylammonium bromide water-in-oil microemulsions: dependence of the minimum amount of alkanol required to produce a microemulsion with the alkanol and organic solvent topology

The amount of alcohol required to produce a microemulsion in a quaternary water-in-oil system was evaluated for a series of alcohols and hydrocarbon solvents of different size or topology. It was observed that the amount of n-hexanol and n-decanol required was similar in all the solvents considered. On the other hand, considerably higher concentrations of the branched alcohols (2,4-dimethyl-3-pentanol and 3-ethyl-3-pentanol) were required to produce the microemulsion, irrespective of the solvent topology (n-hexane or 2,2,4-trimethylpentane). From an analysis of the change in the analytical alcohol concentration with the surfactant concentration the amounts of alcohol present at the microaggregates' surface at the point of microemulsion formation were obtained. It is concluded that the high amounts of branched alcohols required are due to both less efficient incorporation at the interface and the larger number of alcohol molecules per surfactant required to stabilize the microemulsion.     

Indirect laser photoacoustic detection for trace samples on membranes

An indirect laser photoacoustic technique is described. The basic principle, the experimental set-up and the influencing factors are discussed in detail. This method was used to quantify phenylamine, arginine and methylene blue on membranes. The linear ranges of the calibration curve are up to two orders of magnitude. With 1 microliter of sample solution, the detection limits are 0.25, 1.52 and 0.14 pmol for phenylamine, arginine and methylene blue, respectively.