The organic content of the Tagish Lake meteorite.

The Tagish Lake meteorite fell last year on a frozen lake in Canada and may provide the most pristine material of its kind. Analyses have now shown this carbonaceous chondrite to contain a suite of soluble organic compounds (approximately 100 parts per million) that includes mono- and dicarboxylic acids, dicarboximides, pyridine carboxylic acids, a sulfonic acid, and both aliphatic and aromatic hydrocarbons. The insoluble carbon exhibits exclusive aromatic character, deuterium enrichment, and fullerenes containing "planetary" helium and argon. The findings provide insight into an outcome of early solar chemical evolution that differs from any seen so far in meteorites.

Assembly of the inorganic double helix Mg6(Et2NCO2)12 by fixation of carbon dioxide: crystal and solution structure.

The homoleptic magnesium carbamato complex Mg6(Et2NCO2)12, 1 (Et2NCO2- = diethylcarbamato anion), was prepared by the reaction of dibutylmagnesium with diethylamine, followed by carboxylation using gaseous carbon dioxide. Crystallographic characterization demonstrated that 1 has the standard M6(R2NCO2)12 structure and is a double helix of MgO(x)(x = 5, 6) coordination polyhedra with Delta or Lambda stereochemistry arising from the configuration around the six-coordinate Mg2+ cations. It crystallized in the orthorhombic space group Ccca with two molecules of Delta1 and two of Lambda1 per unit cell (a = 21.548 A, b = 25.094 A, c = 15.4485(11) A, alpha = beta = gamma = 90 degrees ). Extensive solution characterization of 1 by 1-dimensional proton and 13C NMR spectroscopy and by two-dimensional 1H-[13C] NMR correlation techniques verified that the helical structure is maintained in solution. Moreover, these measurements indicated that the intramolecular dynamics of 1 relating to motions of the ethyl groups was substantially hindered in solution. Correlation of the crystallographic and NMR structural studies indicated that this arises from a combination of hindered rotation about the carbamato C-N bond and efficient packing of the ethyl groups around the Mg6O24 core. The result is an inverted-micelle-like structure for 1 in which the hydrophobic ethyl groups form a sheath largely restricting access to the hydrophilic Mg6O24 core.

Evidence for isomerization in myotoxin a from the prairie rattlesnake (Crotalus viridis viridis).

Myotoxin a, from the venom of the prairie rattlesnake, Crotalus viridis viridis, exists as a temperature-dependent equilibrium of two interconverting forms. Reverse-phase high-performance liquid chromatography (RP-HPLC) shows that the two forms interconvert slowly enough at 25 degrees C to be seen as two separate peaks with a molar ratio of c. 1:4. Each peak can be isolated and individually injected to give the same two peaks in the same ratio of areas. The two peaks merge during chromatography at elevated temperatures, indicating an increase in the rate of interconversion. At low temperature, c. 5 degrees C, the individual peaks can be isolated and maintained for several days without reaching equilibrium. Mass analysis by matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry shows that myotoxin a is present in both RP-HPLC peaks, suggesting that the two resolved forms are conformational isomers. Capillary zone electrophoresis (CZE) also shows two resolved, but interconvertible peaks over a range of pH values. Furthermore, RP-HPLC chromatograms of myotoxin a at concentrations from 0.013 mM to 0.41 mM maintain a consistent ratio of peak areas, without evidence of dimerization. Two-dimensional 1H-NMR nuclear Overhauser enhancement spectroscopy indicates the presence of a cis-proline peptide bond, consistent with an equilibrium mixture of cis-trans isomers; however, addition of peptidyl-prolyl cis-trans isomerase (PPI) does not enhance the rate of equilibration of the RP-HPLC peaks isolated at c. 5 degrees C.

Evidence for DNA phosphate backbone alkylation and cleavage by pyrrolo[1,2-a]benzimidazoles: small molecules capable of causing base-pair-specific phosphodiester bond hydrolysis.

This report presents evidence that a reduced pyrrolo[1,2-a]benzimidazole (PBI) cleaves DNA as a result of phosphate alkylation followed by hydrolysis of the resulting phosphate triester. The base-pair specificity of the phosphate alkylation results from Hoogsteen-type hydrogen bonding of the reduced PBI in the major groove at only A.T and G.C base pairs. Alkylated phosphates were detected by 31P NMR and the cleavage products were detected by 1H NMR and HPLC. Evidence is also presented that a reduced PBI interacts with DNA in the major groove rather than in the minor groove or by intercalation.

Antineoplastic agents, 177. Isolation and structure of phyllanthostatin 6.

The isolation and structural elucidation of a new Phyllanthus glycoside, phyllanthostatin 6 [7], was summarized. Phyllanthostatin 6 [7] was isolated from the roots of Phyllanthus acuminatus (Euphorbiaceae) and was found to inhibit (ED50 = 0.35 micrograms/ml) growth of the murine P-388 lymphocytic leukemia cell line. Two other new constituents were shown to be didesacetylphyllanthostatin 3 [9] and descinnamoylphyllanthocindiol [10]. Structure determinations were achieved employing hrfabms and 2D-nmr spectroscopy. Application of an hplc separation technique to the Phyllanthus glycosides and development of a new isolation procedure for the major antineoplastic constituent, phyllanthoside [1], are also described.

Stishovite at the cretaceous-tertiary boundary, raton, new Mexico.

Stishovite, a dense phase of silica, has become widely accepted as an indicator of terrestrial impact events. Stishovite occurs at several impact structures but has not been found at volcanic sites. Solid-state silicon-29 magic-angle spinning nuclear magnetic resonance (silicon-29 MAS NMR) and X-ray diffraction of samples from the Cretaceous-Tertiary boundary layer at Raton, New Mexico, indicate that stishovite occurs in crystalline mineral grains. Stishovite was indicated by a single, sharp resonance with a chemical shift value of -191.3 ppm, characteristic of silicon in octahedral coordination, that disappeared after heating the sample at 850 degrees Celsius for 30 minutes. An X-ray diffraction pattern of HF residuals from the unheated sample displayed more than 120 peaks, most of which correspond to quartz, zircon, rutile, and anatase. Eight unambiguous weak to moderate reflections could be ascribed to d-spacings characteristic of stishovite.

Intracellular pH of Cotton Embryos and Seed Coats during Fruit Development Determined by P Nuclear Magnetic Resonance Spectroscopy.

The pH of the phosphate-containing compartments of developing cotton seed coat and embryo tissues was determined by means of (31)P nuclear magnetic resonance spectroscopy. The pH values of these tissues varied as a function of developmental age. From 27 to approximately 38 days postanthesis, a strong pH differential existed between the two tissues; the seed coat was up to 1.4 pH units more acid than developing cotton embryos. The pattern of pH values found with this technique agrees with pH values of tissue homogenates in distilled water. The results confirm an earlier suggestion that seed coat cells are more acidic than embryo cells during key developmental stages of the seed. The pH differential between these two tissues causes abscisic acid to diffuse from seed coats to embryos against its apparent concentration gradient to prevent viviparous germination, despite a higher abscisic acid concentration in the embryo.

Assignment of the aromatic 1H-NMR resonances of myotoxin a isolated from the venom of Crotalus viridis viridis.

The 400 MHz 1H-NMR spectrum of myotoxin a from the venom of Crotalus viridis viridis is described. The identification of spin systems in the aromatic region corresponding to the six aromatic residues of myotoxin a was completed using both one- and two-dimensional NMR spectroscopy and the pH dependence of chemical shifts. Assignments of these spin systems to specific residues was possible for the singly occurring amino acids Tyr-1 and Phe-12. Resonances from Tyr-1, His-5 and His-10 were shifted significantly from their random coil values in a pH-dependent manner. These shift perturbations were deemed evidence of a helical arrangement of the amino terminal region which placed these residues in close proximity to each other.

Ionization of tyrosine residues in horse-heart ferricytochrome c and its guanidinated and acetylated-guanidinated derivatives.

Spectrophotometric titration curves were obtained at 242 nm for native and fully guanidinated horse-heart ferricytochrome c. The cytochrome c data were fit over the pH range 9-12 (I = 0.35) by a theoretical curve with pK' values of 10.35 and 11.70. The slope of the experimental data increases sharply above pH 12.5 suggesting that two tyrosine residues with pK' values greater than 12.5 are exposed by conformation change. The guanidinated cytochrome c data after correction for the alkaline spin-state transition were fit over the entire pH range 9-13.6 (I = 0.35) by a theoretical curve with pK' values 10.37, 10.78, 11.50, and 13.60. These results along with viscosity measurements indicate that the unfolding transition occurs at higher pH in the guanidinated derivative. N-Acetylimidazole was used to acetylate specific tyrosyl groups of guanidinated cytochrome c. Assignments of acetylated tyrosine residues were confirmed by peptide mapping of 14C-labelled derivatives. Spectrophotometric titrations with rapid data acquisition of two monoacetylated derivatives allowed assignments of pK'1 (10.37) to Tyr-67 and pK'4 (13.60) to Tyr-97. The basis for the large differences in acidity and chemical reactivity of these two residues is not obvious from the crystallographic structure and may arise from differences in solvent access due to motions of the polypeptide chain.

Assignment of fluorine nuclear magnetic resonance signals from rabbit cyanomethemoglobin.

A fluorine NMR study of cyanomethemoglobin prepared from hemoglobin isolated from rabbits maintained on a diet containing DL-p-fluorophenylalanine is described. The results indicate that substitution of fluorophenylalanine occurs essentially randomly at all phenylalanine positions of the alpha- and beta-globin chains; a set of hybrid hemoglobins in which only the alpha- or only the beta-chains contain the fluorinated amino acid was prepared and used to ascertain the fluorine NMR signals arising from each chain. The temperature and pH dependences of chemical shifts, spin-lattice relaxation times, 19F(1H) nuclear Overhauser effects, and the effect of chemical modification of the beta-93 sulfhydryl groups were examined. When considered in light of presently available X-ray structures of human and horse hemoglobins, the available data permit a tentative assignment of most signals to particular fluorophenylalanine/phenylalanine positions in the globin sequences.

13C-NMR spectroscopy of acetyltyrosyl-guanidinated horse heart cytochrome c.

The tyrosine residues of guanidinated horse heart cytochrome c have been specifically acetylated by reaction with N-[1-13C]acetylimidazole (90 atom%). Acetylation was monitored by 13C-NMR spectroscopy. The tyrosine residues were found to show widely varying reactivities ranging from one that is completely and exclusively acetylated at low reagent concentration (residue 67) to one that is acetylated only when the protein is unfolded (residue 97). Homogeneous derivatives were prepared containing one (either residue 67 or 97), three 48, 67 and 74), or four (residues 48, 67, 74 and 97) O-[1-13C]acetyl groups. 13C-NMR spectra of selected derivatives were obtained at pH 5.8, in the presence of cyanide ion, in the ferrous and ferric oxidation states, and after denaturation with 6M guanidine hydrochloride. The O-[1-13C]acetyltyrosyl resonances gave chemical shift values ranging from 171.8 to 176.0 ppm. These resonances were assigned to specific groups based on the known order of reactivity of the tyrosyl side chains toward N-acetylimidazole. The chemical shift of O-[1-13C]acetyltyrosyl 67 was found to be particularly sensitive to changes in protein structure. The proximity of this group to the heme makes it subject to distance-dependent paramagnetic and ring current effects. Acetylation of tyrosyl 74 gives rise to a pH-dependent equilibrium between conformers in the ferric state and a conformation change in the ferrous state. Acetylation of this residue also leads to an absorbance decrease at 695 nm that can be related to the 13C-NMR-detected conformational equilibrium. Addition of cyanide ion abolished this equilibrium.