Enhanced analysis of purine and pyrimidine bases by the use of double-strand polyaniline coatings in micellar electrokinetic capillary chromatography

A double-strand polymeric complex, which suppresses electroosmotic flow relative to fused-silica, is described. The polymeric complex contains a strand polyaniline (PAN) with the second strand containing polyacrylic acid (PAA) and methacrylate (MA) groups. The complex is referred to as PAN:P(AAMA). This polymeric complex has pH-controlled electroactive and hydrophobic characteristics and can be easily coated onto fused-silica. Enhanced separations of theophylline, theobromine, caffeine and adenine, thymine, uracil and cytosine were obtained by the use of the coated capillary in the micellar electrokinetic capillary electrophoresis (MEKC) system. The purine and pyrimdine bases were separated on the coated capillary with a 20 mM, pH 7 phosphate buffer which contains 0.05 M sodium dodecyl sulfate (SDS) as an additive.     

Mass spectral characterization of tetracyclines by electrospray ionization,H/D exchange,and multiple stage mass spectrometry

Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N2 and with ND3 as the nebulizer gas. CID mass spectra were obtained for [M + H]+ and [M - H]- ions and the exchanged analogs, [M(Dx) + D]+ and [M(Dx) - D]- , and produced by ESI using a Sciex API-III(plus) and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MS(N) spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H2O (D2O) and NH3 (ND3) if there is a tertiary OH at C-6. The loss of H2O (D2O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH3 (ND3) initially. MSN experiments showed easily understandable losses of HDO, HN(CH3)2, CH3 - N=CH2, and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH3 (ND3) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol.     

Crystal structures of 4,11-pregnadiene-3,20-dione and 21-methyl-20-oxa-4-pregnene-3,20-dione

The structures of the title compounds were solved by direct methods and refined by anisotropic full-matrix least-squares methods. 4,11-Pregnadiene-3,20-dione, C₂₁H₂₈O₂(1) crystallizes in the monoclinic space groupP2₁ (Z=2). The unit cell parametersa, b, c (Å), and (°) were: 12.319(2), 7.700(2), 9.717(2), 109.41(2). TheA- andC-rings exhibit intermediate sofa-halfchair conformations. TheB-ring has a chair conformation and theD-ring assumes an intermediate envelope-half-chair conformation. The progesterone side chain has a typical conformation; the C16–C17–C20–O20 torsion angle is –15.1(4)°. 21-Methyl-20-oxa-4-pregnene-3,20-dione, C₂₁H₃₀O₃ (2) crystallizes in the orthorhombic space groupP2₁2₁2₁ (Z=4). The unit cell parametersa, b, c (Å) were: 12.926(2), 19.447(4), 7.313(1). The progesterone side chain has an unusual conformation; the C16–C17–C20–O20 torsion angle is 174.6(4)°. TheA-ring has a 1, 2-half-chair conformation, ringsB andC exhibit chair conformations and ringD is in a 13,14-half-chair conformation.     

Reversed-Phase Retention of Nucleic Acid Components

Abstract

Hydrogels in solutions can reversibly swell and collapse with small changes in the process conditions. In this paper we focus on using this characteristic as a purification device. The gels prove to be effective and gentle substitutes for ultrafiltration membranes for the purification of sensitive products such as biomaterials.     

Simultaneous Electroanalysis of Guaiacol and its Analogs Based on their Differential Complexation with α‐Cyclodextrin on Nafion Modified Boron‐doped Diamond Electrode

Guaiacol and its analogs, a distinct flavor in whiskey, foods, and beverages, display different inclusion complexes with α‐Cyclodextrin (α‐CD) and can be detected on a Nafion modified boron‐doped diamond (BDD) electrode. The stable Nafion layer preconcentrates the analyte‐α‐CD complexes prior to electroanalysis by square wave voltammetry (SWV) to enhance detection sensitivities. This sensing mode together with peak deconvolution successfully distinguishes between guaiacol, 4‐ethylguaiacol (4‐EG), three cresol isomers, and phenols in a popular whiskey brand. The results obtained are corroborated by high‐performance liquid chromatography (HPLC). Anodic oxidation at + 2 V in phosphate buffer, pH 7 is proven as an effective method to renew the electrode surface after its exposure to the guaiacols. This robust approach circumvents several drawbacks associated with the use of enzymes and nanomaterials for the analysis of such analytes as reported in the literature.     

Thermal chemistry of bicyclo[4.2.0]oct-2-enes

At 300 degrees C, bicyclo[4.2.0]oct-2-ene (1) isomerizes to bicyclo[2.2.2]oct-2-ene (2) via a formal [1,3] sigmatropic carbon migration. Deuterium labels at C7 and C8 were employed to probe for two-centered stereomutation resulting from C1-C6 cleavage and for one-centered stereomutation resulting from C1-C8 cleavage, respectively. In addition, deuterium labeling allowed for the elucidation of the stereochemical preference of the [1,3] migration of 1 to 2. The two possible [1,3] carbon shift outcomes reflect a slight preference for migration with inversion rather than retention of stereochemistry; the si/sr product ratio is approximately 1.4. One-centered stereomutation is the dominant process in the thermal manifold of 1, with lesser amounts of fragmentation and [1,3] carbon migration processes being observed. All of these observations are consistent with a long-lived, conformationally promiscuous diradical intermediate.     

Effect of a methoxy substituent on the vinylcyclobutane carbon migration

Over the temperature range 250-300 degrees C, 8-exo-methoxybicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-exo- and 5-endo-methoxybicyclo[2.2.2]oct-2-enes, 2a and 2b, respectively, with a clear preference for the si product: si/sr = 3.2. Both 1a and its 8-endo epimer 1b experience appreciable epimerization and fragmentation. A long-lived intermediate with weakly interacting diradical centers, one of which is stabilized by a methoxy substituent, can account for all such observations.