Mass-spectrometric studies on alkylated 6-thiopurines

6-Thiopurine and its N- or C-alkyl derivatives all form an [M – 1]-ion upon fragmentation. In the 7-alkyl derivatives, this ion represents the major component of the spectrum. This is ascribed to formation of a five-membered thiazoline-like ring. Similar ring formation stabilises the [M – 1]-ion in the 7-methyl derivatives of hypoxanthine, adenine and 6-selenopurine.     

Routine determination of major anions in atmospheric aerosols by capillary electrophoresis

Capillary electrophoresis (CE) with indirect UV detection utilizing a pyromellitate-based electrolyte was used for the routine analysis of major anions in atmospheric aerosols collected on filters with high-volume (Hi-Vol) samplers. The long-term reliability of the CE system was checked over an 8-month period during which over 2900 samples were analyzed. In addition, approximately 1100 samples were analyzed in parallel by ion chromatography (IC). It has been shown that acceptable analytical performance can be routinely obtained. The agreement between the CE and IC results is good, generally better than 20% at concentrations larger than 1 mg l⁻¹.     

Stripping voltammetry of thorium based on adsorptive accumulation

A sensitive stripping voltammetric procedure for quantifying thorium is described. The chelate of thorium with the azo dye Mordant Blue 9 is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated chelate is measured during a negative-going potential scan. Cyclic voltammetry is used to characterize the interfacial and redox behaviors. The effects of pH, dye concentration and accumulation potential are discussed. The detection limit is 4 × 10^?10 M (4-min accumulation), a linear current-concentration relationship is observed up to 1.3 × 10^?7 M, and the relative standard deviation (at the 6 × 10^?8 M level) is 3.1%. Possible interferences by trace metals and organic surfactants are investigated. Simultaneous quantitation of thorium and nickel is illustrated.     

Rheological properties of multisticker associative polyelectrolytes in semidilute aqueous solutions

Multisticker associative polyelectrolytes of acrylamide (≈86 mol %) and sodium 2‐acrylamido‐2‐methylpropanesulfonate (≈12 mol %), hydrophobically modified with N,N‐dihexylacrylamide groups (≈2 mol %), were prepared with a micellar radical polymerization technique. This process led to multiblock polymers in which the length of the hydrophobic blocks could be controlled through variations in the surfactant‐to‐hydrophobe molar ratio, that is, the number of hydrophobes per micelle (N_H). The rheological behavior of aqueous solutions of polymers with the same molecular weight and the same composition but with two different hydrophobic block lengths (N_H = 7 or 3 monomer units per block) was investigated as a function of the polymer concentration with steady‐flow, creep, and oscillatory experiments. The critical concentration at the onset of the viscosity enhancement decreased as the length of the hydrophobic segments in the polymers increased. Also, an increase in the N_H value significantly enhanced the thickening ability of the polymers and affected the structure of the transient network. In the semidilute unentangled regime, the behavior of the polymer with long hydrophobic segments (N_H = 7) was studied in detail. The results were well explained by the sticky Rouse theory of associative polymer dynamics. Finally, the viscosity decreased with an increase in the temperature, mainly because of a lowering of the sample relaxation time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1640–1655, 2004     

Two-, three-, and four-coordinate Ag(I) coordination polymers formed by the novel phosphinite PPh2(3-OCH2C5H4N)

The novel phosphinite PPh(2)(3-OCH(2)C(5)H(4)N) (1) has been synthesized, and its coordination properties to Ag(I) have been studied. When reacted in a 1:1 ratio with Ag(I), coordination polymers with different coordination numbers about the Ag are found depending on the anion. For PPh(2)(3-OCH(2)C(5)H(4)N)AgBF(4) (2), a two-coordinate Ag is observed with a P-Ag-N angle of 167 degrees. Mixed three and four coordination about Ag is observed for PPh(2)(3-OCH(2)C(5)H(4)N)AgOTf (3), and for the trifluoroacetate derivative, PPh(2)(3-OCH(2)C(5)H(4)N)Agtfa (4), only a four-coordinate Ag is produced. X-ray crystal-structure determinations for compounds 2-4 have been carried out. The X-ray structures show a wide range of Ag-Ag distances in the polymers, which are dependent on the conformation of the bridging ligand.     

Homolysis of weak Ti-O bonds: experimental and theoretical studies of titanium oxygen bonds derived from stable nitroxyl radicals

Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium.     

Rapid quantitative capillary zone electrophoresis method for monitoring the micro-heterogeneity of an intact recombinant glycoprotein

A simple high-resolution capillary zone electrophoresis (CZE) method capable of rapidly assessing the micro-heterogeneity of a 24 kDa molecular weight glycoprotein, has been developed. Separation is carried out using a bare silica capillary at a pH of 2.5 in a commercially available electrophoresis buffer system composed of triethanolamine and phosphoric acid. Over 30 peaks were detected within a run time of 15 min using a 27 cm capillary and approximately 60 peaks were detected using a 77 cm capillary. Although most of the peaks arise from differences in the oligosaccharide structures present on the one glycosylation site on this molecule, other forms of micro-heterogeneity due to the presence of the nonglycosylated form of this glycoprotein and various types of chemical degradation, e.g., deamidation, are also responsible for the multitude of peaks observed. Although the exact chemical identity of each peak in the resulting electropherogram of this glycoprotein is not known, useful information can be obtained for assessing comparability, stability, and batch consistency. Factors impacting the resolution, precision, accuracy, and robustness of the assay are also discussed along with inherent advantages and limitations associated with measuring the micro-heterogeneity of intact glycoproteins.     

An Asymmetric Synthesis of L-694,458, a Human Leukocyte Elastase Inhibitor, via Novel Enzyme Resolution of beta-Lactam Esters

A convergent synthesis of [S-(R,S)]-2-[4-[(4-methylpiperazin-1-yl)carbonyl]phenoxy]-3,3-diethyl-N-[1-[3,4-(methylenedioxy)phenyl]butyl]-4-oxo-1-azetidinecarboxamide (L-694,458, 1), a potent human leukocyte elastase inhibitor, was achieved via chiral synthesis of key intermediates: (S)-3,3-diethyl-4-[4'-[(N-methylpiperazin-1-yl)carbonylphenoxy]-2-azetidinone (2) and (R)-alpha-propylpiperonyl isocyanate (3). Synthesis of beta-lactam 2 was achieved by a novel enantioselective lipase hydrolysis of ester 5 to produce (S)-3,3-diethyl-4-(4'-carboxyphenoxy)-2-azetidinone (6) (60% yield, three cycles, 93% ee) with isolation, epimerization, and recycling of the undesired (R)-ester 5. Isocyanate 3 was prepared by chiral addition of Zn(n-Pr)(2) to piperonal (98% yield, 99.2% ee), azide displacement and reduction to (R)-alpha-propylpiperonylamine (11) (58% yield, 85% ee), crystallization as the D-pyroglutamic acid salt (92% yield, 98.2% ee), and isocyanate formation (98% yield) with phosgene.     

Eu(III)-cyclen-phen conjugate as a luminescent copper sensor: the formation of mixed polymetallic macrocyclic complexes in water

The cationic cyclen based Eu(III)-phen conjugated 1.Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)-Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1.Eu conjugates.     

Secondary deuterium kinetic isotope effects in irreversible additions of allyl reagents to benzaldehyde

The competitive kinetics of additions of allyl to benzaldehyde-h and -d from allyltributyl tin, from diisopropyltartrylallyl boronate, and from allyl bromide and zinc dust in aqueous tetrahydrofuran have inverse secondary deuterium kinetic isotope effects, SDKIEs. These inverse SDKIEs are in contrast to the normal SDKIEs that were obtained with allyl lithium and allyl Grignard, suggesting rate-determining single-electron transfer in these cases. By various MO calculations the transition state for addition of allyl boronate occurs with substantial B-O bond formation and little C-C bond formation. The magnitudes of the SDKIEs with the other two allylating reagents, when compared with reasonable equilibrium isotope effects for the addition, suggest transition states with substantial C-C bond formation.