Characterization of Phenolic Plant Exudates by Nuclear Magnetic Resonance Spectroscopy.

The class of plant exudates that contain the phenol functionality, termed phenolics, is defined, surveyed, and characterized by solid-state 13C NMR spectroscopy and by solution-state 1H NMR spectroscopy. Materials in this group are identified by the phenolic 13C resonance (from the ipso carbon of ArOH) at δ 145-160 (δ 160-167 for ArOR). The resonance patterns define several subclasses based on the collective similarity of their 13C spectra, specifically, aloetics from the genus Aloe, guaiacs from the genus Guaiacum and other eurosid and conifer genera, xanthics from the genus Garcinia, and kinos from the genus Eucalyptus and many other genera. Phenolic exudates often are mixed with terpenoid materials (the building block of exudates known as resins) and carbohydrates (the building block of exudates known as gums) to form hybrid subgroups such as guaiac gums, guaiac resins, and kino resins. There are numerous phenolic exudates not affiliated with any of these groups, both as pure phenolics and as hybrids (phenolic resins, phenolic gum resins, and phenolic waxes).

Structural changes from heating amber and copal as observed by nuclear magnetic resonance spectroscopy.

Structural changes caused by heating of fossilized (amber) and semifossilized (copal) resins have been examined by nuclear magnetic resonance spectroscopy. A set of 28 samples was constituted to include different geographical sources, degrees of maturation, colors, and structural groupings. The onset of structural alterations was determined by observation of the lowest temperature at which spectral changes occurred. Both proton spectra in solution and carbon-13? spectra in the solid state then were recorded of cooled samples after heating for 12 hr at temperature increments, until liquification of the sample began. The spectra of both nuclides exhibit loss of a few peaks, broadening of most peaks, and enhancement of the unsaturated or aromatic region at the expense of saturated resonances. Such changes are irreversible and lead to a harder and less soluble material on cooling. The changes parallel those that occur with maturation of fossil resins or materials that lead to coal.

Examination of amber and related materials by NMR spectroscopy.

Examination of the solid-state (13)C and solution (1)H NMR spectra of fossilized resins (ambers) has generated five groupings of materials based on spectral characteristics. The worldwide Group A is associated with the botanical family of the Araucariaceae. The worldwide Group B is associated with the Dipterocarpaceae. Baltic amber or succinite (Group C) is related to Group A but with a disputed conifer source. Amber from Latin America, the Caribbean, and Africa is associated with the Fabaceae, the genus Hymenaea in particular. The minor Group E contains the rare fossil polystyrene. The spectra of jet indicate that it is a coal-like material with a rank between lignite and sub-bituminous coal.

Molecular classification of the natural exudates of the rosids.

Exudates of the rosid clade of the eudicots have been surveyed and characterized by carbon-13 and proton nuclear magnetic resonance spectroscopy. Of 554 samples divided roughly equally between the subclades fabids and malvids, about two-fifths are resins, a third gums, one-ninth gum resins, one-twelfth kinos, and the remaining not affiliated with these four main molecular classes. Two small new molecular classes, respectively from the Clusiaceae (xanthics) and the Zygophyllaceae (guaiacs), are identified and described.

Characterization of plant exudates by principal-component and cluster analyses with nuclear magnetic resonance variables.

Principal-component and cluster analyses have been applied to nuclear magnetic resonance data for exudates derived from both conifers and angiosperms in order to classify these materials on the basis of molecular structure. The method succeeds in distinguishing resins produced by the conifer families Araucariaceae, Cupressaceae, and Pinaceae from each other and from resins produced by the angiosperm family Fabaceae. Other exudate types, including gums, gum resins, and kinos, also are distinguished from each other and from the resins.

The silicate-mediated formose reaction: bottom-up synthesis of sugar silicates.

Understanding the mechanism of sugar formation and stabilization is important for constraining theories on the abiotic origin of complex biomolecules. Although previous studies have produced sugars from small molecules through the formose and related reactions, the product mixtures are complex and unstable. We have demonstrated that simple two- and three-carbon molecules (glycolaldehyde and glyceraldehyde), in the presence of aqueous sodium silicate, spontaneously form silicate complexes of four- and six-carbon sugars, respectively. Silicate selects for sugars with a specific stereochemistry and sequesters them from rapid decomposition. Given the abundance of silicate minerals, these observations suggest that formose-like reactions may provide a feasible pathway for the abiotic formation of biologically important sugars, such as ribose.

Matisse to Picasso: a compositional study of modern bronze sculptures.

Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used to determine the bulk metal elemental composition of 62 modern bronze sculptures cast in Paris in the first half of the twentieth century from the collections of The Art Institute of Chicago and the Philadelphia Museum of Art. As a result, a comprehensive survey of the alloy composition of the sculptures of many prominent European artists of the early twentieth century is presented here for the first time. The sculptures in this study consist of predominantly copper with two main alloying elements (zinc and tin). By plotting the concentrations of these two elements (zinc and tin) against each other for all the sculptures studied, three clusters of data become apparent: (A) high-zinc brass; (B) low-zinc brass; (C) tin bronze. These clusters correlate to specific foundries, which used specific casting methods (sand or lost wax) that were influenced by individual preferences and technical skills of the foundry masters. For instance, the high-zinc brass alloys (with the highest levels of tin and zinc and the lowest melting temperature) correspond to most of the Picasso sculptures, correlate with the Valsuani foundry, and are associated with the most recent sculptures (post-WWII) and with the lost-wax casting method. By expanding the ICP-OES database of objects studied, these material correlations may become useful for identifying, dating, or possibly even authenticating other bronzes that do not bear foundry marks. Figure.

Nuclear magnetic resonance spectroscopic characterization of legume exudates.

Exudates from the plant family Fabaceae have been characterized by proton and carbon-13 nuclear magnetic resonance spectroscopy. The 79 identified species from 38 genera represent all three subfamilies of this widespread and economically important angiospermous (flowering) family. The observed exudates include resins, gums, kinos, gum resins, and a few materials as yet unclassified molecularly. Exudates from the subfamily Caesalpinoideae are primarily resins, whereas those from the Mimoisoideae and Faboideae are primarily gums. Three species of the Mimoisoideae produce both gums and kinos.

Strong conductance variation in conformationally constrained oligosilane tunnel junctions.

The effects of molecular conformation on conductance in oligosilane-bridged metal-molecule-metal junctions are studied theoretically using density functional theory combined with a nonequilibrium Green's function approach. Varying the internal SiSiSiSi dihedral angles in hexasilane diamine chains changes the conductance by up to 3 orders of magnitude. This conformational dependence is due to the effects of sigma-delocalization on the positions of the highest occupied molecular orbital (HOMO) energies. The conductance values for the different conformations are related to electron transfer rates in donor-bridge-acceptor systems, and the effect of shifting the injection energy is examined. The transport properties are found to be extremely sensitive to the alignment between the HOMO energies and Fermi level of the gold electrodes.

Synthesis of calixarene-capped carbosilane dendrimers.

A new class of polycalix[4]arene hosts has been constructed based on a carbosilane dendrimer architecture, in which each dendritic branch terminates with a calix[4]arene entity. This study reports the synthesis and characterization of the zeroth generation example with four calix[4]arenes and of the first generation example with 12 calix[4]arenes.

Chemical signatures of fossilized resins and recent plant exudates.

Amber is one of the few gemstones based on an organic structure. Found over most of the world, it is the fossil form of sticky plant exudates called resins. Investigation of amber by modern analytical techniques provides structural information and insight into the identity of the ancient plants that produced the source resin. Mass spectrometric analysis of materials separated by gas chromatography has identified specific compounds that are the basis of a reliable classification of the different types of amber. NMR spectroscopy of bulk, solid amber provides a complementary classification. NMR spectroscopy also can be used to characterize modern resins as well as other types of plant exudates such as gums, gum resins, and kinos, which strongly resemble resins in appearance but have very different molecular constitutions.

Distinctions among conifer exudates by proton magnetic resonance spectroscopy.

Proton nuclear magnetic resonance spectra have been recorded of exudates harvested from 12 species from the family Araucariaceae, 40 from the Cupressaceae, and one from the Podocarpaceae. These spectra were compared with the spectra previously recorded of 82 species from the Pinaceae. These four families together represent all major groups of extant, resin-bearing conifers. A common set of 10 COSY two-dimensional cross-peaks generally define samples from the Pinaceae, a different set of six peaks define the Araucariaceae, and yet a third set of 10 peaks define the Cupressaceae, with a few exceptions. It is important that proton spectra can distinguish the Araucariaceae and the Cupressaceae, since carbon-13 spectra do not. The one-dimensional peaks not only confirm these familial distinctions but also often characterize genus and species uniquely.

Proton nuclear magnetic resonance characterization of resins from the family pinaceae.

Proton magnetic resonance spectra were recorded for solutions of resinous materials harvested from 82 species in seven genera of the gymnospermous plant family Pinaceae. Data were recorded in both one and two (COSY) dimensions. Approximately 11 peaks in the 1D spectra and 10 cross-peaks in the 2D spectra were present in almost all pinacean spectra, providing a familial diagnostic. Some 40 1D peaks or peak clusters and 60 2D cross-peaks or clusters were considered significant and are reported, when present, for all species. Whereas previous solid-state 13C data were diagnostic primarily at the family level, the patterns of 1D and 2D peaks may provide diagnostic information at the genus and species levels. These spectra constitute the first broad use of 1H NMR to study plant exudates in general and to provide taxonomic characterization in particular.

General but discriminating fluorescent chemosensor for aliphatic amines.

Aliphatic amines are sensitively and discriminatively detected through binding with demethylated naphthol AS-BI (7-bromo-3-hydroxy-2-naphth-o-hydroxyanilide, 2) and fluorescence of the resulting complex. Recognition of the amine by the chemosensor 2 occurs via proton transfer of the naphtholic proton to the amine and is facilitated by the presence of the phenol group. Amine basicity is the primary controller of detection. Poorly basic aromatic and conjugated amines such as pyridine and aniline are not detected. Hydrogen bonding within the complex allows further differentiation of aliphatic amines. Doubly primary, conformationally flexible diamines are the most sensitive to detection, followed by secondary amines.

Taxonomic and chemical relationships revealed by nuclear magnetic resonance spectra of plant exudates.

Exudates collected from 65 species of gymnosperms and angiosperms were examined by solid-state carbon-13 (13C) nuclear magnetic resonance (NMR) spectroscopy. Diagnostic criteria were developed to distinguish resins, gums, and gum resins. The typology generated from the exudate spectra generally follows current taxonomic classifications, suggesting that 13C NMR spectroscopy may have applications in exudate identification, at least at the familial level, and in some cases at the generic or specific levels.

Silicate complexes of sugars in aqueous solution.

Certain sugars react readily with basic silicic acid to form soluble 2/1 (sugar/silicic acid) silicate complexes. Failure of monohydroxy compounds to give soluble products under these conditions indicates that the sugar silicates are chelates: five-membered diolato rings. Only furanose forms react. Pyranose sugars are stable under these conditions. Because all glycosides fail to react with silicic acid under these conditions, reaction appears to involve the anomeric position (C1 in aldoses, C2 in ketoses), which has a more acidic hydroxy group. Reaction is completed only when the anomeric hydroxy group is cis to an adjacent hydroxy group. The appropriate furanose form must have sufficient natural abundance and solubility to provide an observable product, as measured by (29)Si and (13)C NMR spectroscopy. These structural and practical constraints rationalize the successful reaction of the monosaccharides ribose, xylose, lyxose, talose, psicose, fructose, sorbose, and tagatose and the disaccharides lactulose, maltulose, and palatinose. Glucose, mannose, galactose, and sucrose, among others, failed to form complexes. This high selectivity for formation of sugar silicates may have ramifications in prebiotic chemistry.

Simple surfactant-free route to mesoporous organic-inorganic hybrid silicas containing covalently bound cyclodextrins.

Novel mesoporous organic-inorganic hybrid silicas containing covalently bound cyclodextrins have been synthesized by a simple, surfactant-free sol-gel route and are shown to have high BET surface areas and narrow pore-size distributions.

The unexpected motional isotropy of the tin atom in a tricoordinate stannylium cation.

The dynamics of the metal atom in the recently isolated tricoordinate tin complex tris(2,4,6-triisopropylphenyl)stannylium tetrakis(pentafluorophenyl)borate was examined by temperature-dependent (119)Sn Mössbauer spectroscopy over the temperature range 90 K < T < 170 K. Contrary to expectation, the metal atom motion in this temperature range is isotropic within experimental error of the Mössbauer data, and is only moderately anisotropic, even at 293 K, as evidenced by single crystal X-ray diffraction data. The hyperfine parameters at 90 K are completely consistent with trigonal coordination involving sp(2) hybridization of the 5s5p bonding orbitals of tin.

The C5SiMe7+ cation: pyramidal, bicyclic, or cyclohexadienyl?

The monosila analogue of the C6Me7+ cation can be accessed by hydride abstraction from (pentamethylcyclopentadienyl)dimethylsilane, Cp*SiMe2H. Treatment of this material with triphenylmethylium tetrakis(pentafluorophenyl)borate in dichloromethane at -50 degrees C produced a single cationic species stable for a day or longer. The presence of a single resonance for the ring methyls and a single resonance for the silyl methyls in both the 1H and the 13C spectra indicated either a static structure with fivefold symmetry or a dynamic structure in which the dimethylsilyl group equilibrates rapidly among positions. Density function theory calculations found a minimum whose structure had the unsymmetrical silabicyclo[3.1.0]hexenyl structure. Calculated 1H, 13C, and 29Si chemical shifts for this structure were in good agreement with the observed values for this structure. Another calculated minimum was the silacyclohexadienyl structure, but its calculated chemical shifts were very different from those observed. We conclude that the cation has the bicyclic structure but rapidly interconverts the ring positions to produce the very simple NMR spectra.