Structure and conformation of the monohydrate of N-t-boctyrosyl-proline (Boc-Tyr-Pro·H2O)

The structure and conformation of the monohydrate of N-t-boc-tyrosyl-proline (Boc-Tyr-Pro·H₂O) (C₁₉H₂₆O₆N₂·H₂O) has been investigated with X-ray crystallographic and spectroscopic methods. Boc-Tyr-Pro crystallized in an extendedtrans conformation in the space group P2₁2₁2₁ with cell dimensionsa=8.566(1),b=9.996(1),c=24.734(1). The conformation of Boc-Tyr-Pro reflex -helix type prolines. Three intermolecular hydrogen bonds are observed. Crystal water is involved in two hydrogen bonds (to the hydroxyl group of the C-terminal of the proline residue; to the carbonyl group of the t-Boc functionality) while the hydroxyl group of the tyrosyl residue (to the carbonyl group of the amide bond) is involved in one hydrogen bond. The puckering mode of the pyrrolidine ring of the proline residue is similar to what has been previously observed for other proline-containing peptides.Cis-trans isomerism is observed in the NMR spectra of Boc-Tyr-Pro with a predominance for the extended side chain for the tyrosyl residue.     

Electron-impact fragmentation of some secondary aliphatic nitramines. Migration of the nitro group in heterocyclic nitramines

The mass spectra of a series of nitramines, including the explosives RDX and HMX, are reported. The nature of the major fragment ions is established by labeling with ¹⁵N or¹³C and by high resolution mass spectrometry. Electron-impact-induced fragmentation pathways including the migration of the nitro group in the heterocyclic nitramines, RDX and HMX, are postulated.     

Diol—Based Sol–Gel as a Novel Binder for Barium Titanate Ceramics

The relative merits of using a diol-based sol–gel precursor as a binder in the fabrication of barium titanate ceramics made by die-pressing a mixed oxide starting powder have been investigated. The characteristics of powder compacts were compared to samples prepared using a conventional PVB organic binder. The green strength of pellets with the PVB binder was higher than for samples containing the gel, but strength after binder burnout from gel samples was double that of pellets made with PVB. The gel led to improvements in densification during sintering, but these were most noticeable at low sintering temperatures and at intermediate stages of densification. After sintering for 2 h at 1150C, the gel-containing samples reached 75% of theoretical density, some 5% more than for the organic polymer binder. Sintering at 1300C for 2 h, gave a sintered density of ~96% of theoretical density using the gel additive, which was only 2% higher than for the conventional PVB binder system. The decomposition characteristics of the gel were investigated by TGA, FTIR and GC-MS techniques.     

Mass spectrometry of nucleoside derivatives. Seleno analogs of purine and pyrimidine bases and nucleosides

Mass spectra of certain selenobases and selenonucleosides, and some of their trimethylsilyl and O,N-permethyl derivatives have been studied from the standpoint of structural characterization, and in order to ascertain the influence of selenium on normal fragmentation patterns. Molecular ion abundances of the selenouracils are intermediate between those of the corresponding oxygen and sulfur analogs. Fragmentation processes are similar to those of the corresponding normal bases and nucleosides but with additional ions resulting from expulsion of Se or SeH in most cases. Trimethylsilylation occurs at approximately the same rate as for normal bases and nucleosides but the products show decreasing stability with prolonged heating. A least squares procedure is demonstrated which generates monoisotopic mass patterns and assists in interpretation of the mass spectra.     

The synthesis and chemical reactivity of 6-selenoguanosine and certain related derivatives

The synthesis of 6-selenoguanosine ( 2 ) has been accomplished by a nucleophilic displacement of the chloro group from 2-amino-6-chloro-9-(β- D -ribofuranosyl)purine ( 1 ) with either selenourea or sodium hydrogen selenide. Treatment of 2 with Raney nickel has revealed that the seleno group can be removed much easier under these conditions than the corresponding mercapto group. Alkylation of 2 with several alkylating agents occurred at the exocyclic 6-seleno group to furnish several 6-alkylseleno-2-amino-9-(β- D -ribofuranosyl)purines. Nucleophilic displacement of the 6-benzylseleno group from 2-amino-6-benzylseleno-9-(β- D -ribofuranosyl)purine ( 3c ) with sodium methoxide has been observed to occur at a faster rate than that observed for the corresponding 6-benzylmercapto derivative. A study on the relative stability between 2 and 6-seleno-9-(β- D -ribofuranosyl)purine toward basic conditions has revealed that the amino group at position two imparts an increase in stability.     

The NCI Drug Information System. 4. Inventory and Shipping Modules

The Inventory/Shipping package of the NCI Drug Information System (DIS) is designed to support all inventory and shipping operations associated with the testing by the NCI of large numbers of chemicals for anticancer activity. Two major databases, an Inventory database and a Shipping History database, contain all of the data associated with these operations. Software that supports the operations in an online interactive manner also provides for the accessing and updating of these databases as necessary. Special hardware in the form of barcode reader/printers and digital balances is also interfaced to the system to improve the efficiency of the operations.     

TGA–FTIR study of a lead zirconate titanate gel made from a triol-based sol–gel system

A Pb(Zr,Ti)O₃ precursor gel made from a sol prepared using 1,1,1,-tris(hydroxymethyl)ethane, lead acetate and zirconium and titanium propoxides, stabilised with acetylacetone, was analysed using TGA–FTIR analysis. Decomposition under nitrogen (N₂) gave rise to evolved gas absorbance peaks at 215 °C, 279 °C, 300 °C and 386 °C, but organic vapours continued to be evolved, along with CO₂ and CO until 950 °C. The final TGA step in N₂ is thought to relate to decomposition of an intermediate carbonate phase and the final elimination of residues of triol or acetylacetonate species which form part of the polymeric gel structure. By contrast, heating in air promoted oxidative pyrolysis of the final organic groups at ≤450 °C. In air, an intermediate carbonate phase was decomposed by heating at 550 °C, allowing Pb(Zr,Ti)O₃ to be produced some 400 °C below the equivalent N₂ decomposition temperature.     

Determination of pesticide residues (> 0.5 microg/L) in soft drinks and sports drinks by gas chromatography with mass spectrometry: collaborative study

A collaborative study was conducted on a method for the measurement of 19 low-level pesticide residues in soft drinks and sports drinks by gas chromatography with mass spectrometry (GC/MS). The pesticide residues determined were 2,4'-dichlorodiphenyldichloroethylene (2,4'-DDE); 2,4'-dichlorodiphenyldichloroethane (2,4'-DDD); 4,4'-dichlorodiphenyldichloroethylene (4,4'-DDE); 2,4'-dichlorodiphenyltrichloroethane (2,4'-DDT); 4,4'-dichlorodiphenyltrichloroethane (4,4'-DDT); 4,4'-dichlorodiphenyldichloroethane (4,4'-DDD); alpha-endosulfan; endosulfan-sulfate; dieldrin; aldrin; ethion; chlorpyrifos; beta-endosulfan; malathion; methyl-parathion; alpha-hexachlorocyclohexane (alpha-HCH); beta-HCH; delta-HCH; and gamma-HCH. Blind fortification solutions containing 4 different levels of pesticide residues (0, 0.1, 0.5, and 1.0 microg/L) were provided to 8 collaborating laboratories who used them to create test samples in 6 matrixes (also provided): 2 colas, a diet cola, a clear lemon-lime soft drink, an orange soft drink, and a sports drink. Reproducibility (RSDR) for all 19 pesticide residues in all matrixes ranged from 7 to 151% at the 0.1 microg/L level, 11 to 121% at 0.5 microg/L, and 14 to 67% at 1.0 microg/L. Repeatability (RSDr), applicable to the diet cola and the sports drink, ranged from 1 to 76% for the 19 pesticide residues at the 0.1 microg/L level, 9 to 38% at 0.5 microg/L, and 9 to 38% at 1.0 microg/L. Recoveries for the 19 pesticide residues in all matrixes ranged from 77 to 645% at the 0.1 microg/L level, 60 to 231% at 0.5 microg/L, and 61 to 146% at 1.0 microg/L. It is recommended that the method be accepted by AOAC as Official First Action with a limit of quantification (LOQ) equal to 0.5 microg/L for 4,4'-DDT; 2,4'-DDT; 2,4'-DDD; 4,4'-DDE; 4,4'-DDD; 2,4'-DDE; aldrin; dieldrin; alpha-endosulfan; endosulfan-sulfate; chlorpyrifos; and ethion, and an LOQ equal to 1.0 microg/L for beta-endosulfan; alpha-HCH; beta-HCH; delta-HCH; gamma-HCH; methyl-parathion; and malathion.