Stereochemistry of cytidine: conformational analysis and hydrogen bonding

The energy calculations using the INDO molecular orbital method and classical potential function show that the syn conformation of pyrimidine nucleosides having the C_(2')-endo or C_(2')-endo-C_(3')-endo sugar ring puckering is as stable as the corresponding anti form.The O_(2')-O_(2') base-ribose hydrogen bonding significantly alters the Φ_CN conformation by confining the allowed Φ_CN ranges to about 150–210°.     

Comparative studies of electron-impact (EI) and field ionization (FI) mass spectra of pentaerythritol derivatives

The EI Mass spectra of the polyfunctional pentaerythritol derivatives show that the molecular ions [M]⁺· exhibit extensive ion fragmentation. No Information about [M]⁺·. can be obtained. In Contrast to this, the FI Mass spectra of these compounds show intense [M]⁺· and/or [M + 1]⁺, and a characteristic ion at m/e 31, which is assumed to be the oxonium ion \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_2 = \mathop {\rm O}\limits^ + {\rm H} $\end{document}. Because of surface adsorption and field attraction, FI mass spectrometry presents a serious problem in quantitative analysis of a mixture containing compounds with quite different degrees of polarization.     

Amide-water interactions in cosolvent systems. I. Solubilities and apparent molar volumes of methyl paraben

Solubility and apparent molar volume data are used to demonstrate effects of amide alkylation on amide-water interactions at 25° C. Precise measurements were made of the apparent molar volumes of the amides in binary amide-water mixtures using a dilatometric technique. The results show that the apparent molar volumes of alkyl-substituted amides in water pass through a minimum at an amide concentration which varies inversely with the degree of alkylation. Further studies showed that the solubilities of methyl paraben (methyl-p-hydroxybenzoate) and naphthalene in various amide-water solvent systems increased in characteristic fashion with amide alkylation.     

Photon-induced dissociation with a four-sector tandem mass spectrometer

The feasibility of using photodissociation of protonated peptide molecules to sequence specific fragment ions with a 193-nm pulsed laser beam in a magnetic deflection tandem mass spectrometer of EBEB configuration was demonstrated. Although the short pulse (15 ns) and low repetition rate (100 Hz) of the excimer laser permitted the irradiation of only ～ 0.02% of the (M + H)⁺ ions exiting MS-1, a photon-induced decomposition spectrum of the heptapeptide angiotensio III (M _r 930.5) was produced that was practically the same (but with better signal-to-noise ratio) as that generated by collision-activated dissociation at the same low duty cycle. Because of the low and pulsed fragment ion currents, an array detector was used to record the spectra. A dependence between laser power and abundance of fragment ions was observed (increased power increases the relative abundance of ions of low mass). Laser power was varied from 6 to 80 mJ. Formation of fragment ions from a large peptide (melittin, M, 2844.75) was also observed. The results permit the design of modifications that may increase the fragment ion yield to 10% or higher, which would make photon-induced decomposition a useful method for magnetic deflection mass spectrometers.     

Tuning the band gap in hybrid tin iodide perovskite semiconductors using structural templating

Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.     

Molecular characteristics of the inhibition of human neutrophil elastase by nonsteroidal antiinflammatory drugs

Nonsteroidal antiinflammatory drugs(NSAIDs) are known as clinically effective agents for treatment of inflammatory diseases. Inhibition of cyclooxygenase has been thought to be a major facet of the pharmacological mechanism of NSAIDs. However, it is difficult to ascribe the antiinflammatory effects of NSAIDs solely to the inhibition of prostaglandin synthesis. Human neutrophil elastase (HNElastase; HNE, EC 3.4.21.37) has been known as a causative factor in inflammatory diseases. To investigate the specific relationship between HNElastase inhibition and specificity of molecular structure of several NSAIDs, HNElastase was purified by Ultrogel AcA54 gel filtration, CM-Sephadex ion exchange, and HPLC (with TSK 250 column) chromatography. HNElastase was inhibited by aspirin and salicylate in a competitive manner and by naproxen, ketoprofen, phenylbutazone, and oxyphenbutazone in a partial competative manner, but not by ibuprofen and tolmetin. HNElastase-phenylbutazone-complex showed strong Raman shifts at 200, 440, 1124, 1194, 1384, 1506, and 1768 cm(-1). The Raman bands 1194, 1384, and 1768 cm(-1) may represent evidences of the conformational change at -N=N-phi radical, pyrazol ring, and -C=O radical of the elastase-drug complex, respectively. Phenylbutazone might be bound to HNElastase by ionic and hydrophobic interaction, and masked the active site. Inhibition of HNElastase could be another mechanism of action of NSAIDs besides cyclooxygenase inhibition in the treatment of inflammatory diseases. Different inhibition characteristics of HNE-lastase by NSAIDs such as aspirin, phenylbutazone-like drugs and ineffective drugs could be important points for drawing the criteria for appropriate drugs in clinical application.     

Total synthesis of (+)-belactosin A

A concise first total synthesis of the antitumour antibiotic belactosin A is reported, involving coupling of beta-lactone carboxylic acid 3 with N-Ala-aminocyclopropyl alanine 11.     

Oxorhenium(V) dithiolates catalyze the oxidation by tert-butyl hydroperoxide of sulfoxides and sulfides, including 4,6-dimethyldibenzothiophene

tert-Butyl hydroperoxide (TBHP) efficiently converts a wide variety of sulfides to sulfoxides and sulfones. The method offers the advantage that one product or the other can be obtained in high purity by a modest variation of conditions. The reactions occur smoothly at 25minus sign50 C in chloroform and, to the extent studied, in toluene and methylene chloride. A catalyst is required; the most extensively studied was MeReO(mtp)PPh(3), 1, where mtpH(2) is 2-(mercaptomethyl)thiophenol. Other chelating dithiolate ligands can be used with comparable results. These oxidations were tested for dialkyl, alkylminus signaryl, and diaryl sulfides; thiophenes; and thianthrene. Even the "hard" sulfide, 4,6-dimethyldibenzothiophene (DMDBT) was quantitatively oxidized to the dioxide with TBHP:DMDBT 3.0-3.5 and 0.05-3.8 mol % 1. The mechanism was explored in kinetics studies carried out only for methyl tolyl sulfide. The product buildup curve was complex, with an induction period followed by a rapid growth phase. The kinetic data could be modeled adequately but not perfectly by allowing five rate constants to refine. Their values are consistent with the chemical sense of the mechanism.     

Synthesis,spectroscopic studies,and x-ray crystallographic analysis of the organotin carbohydrate: 1,2∶3,4-di-O-isopropylidene-6-O-triphenylstannylmethyl-α-D-galactopyranose

The title organotin carbohydrate, C₃₁H₃₆O₆Sn, has been synthesized and its molecular structure has been determined in solution and in the solid state. NMR, infrared, mass and X-ray crystallographic techniques were used. The chiral molecules crystallize in the monoclinic space group P2₁ withZ=2. The triphenyltin and carbohydrate moieties are linked by a trans methylene-oxygen-methylene arrangement. The pyranosyl ring adopts a twist-boat conformation and the isopropylidene rings adopt different (half-chair and envelope) forms. Solution and solid-state conformations are similar as only three ¹³C shift values are greater than 2ppm; the ¹¹⁹Sn value is 12 ppm.