Substituted 1,2-Thiazetidine 1,1-Dioxides. Synthesis of (<Emphasis Type="Italic">RS</Emphasis>)- and (<Emphasis Type="Italic">S</Emphasis>)-1,2-Thiazetidine-3-acetic Acid 1,1-Dioxide and its Reactions with Amino Acids and Dipeptides

Summary. (RS)-2-tert-Butyldimethylsilyl-1,2-thiazetidine-3-acetic acid 1,1-dioxide prepared from (RS)-S-benzyl--homocysteine was condensed via DCC/NHS with various L-amino acid esters or dipeptide esters yielding N-silylated -sultam peptides. A -sultam active ester was isolated as an intermediate. Desilylation with TBAF in THF yielded stable N-unsubstituted products, and deprotection of the benzyl esters was achieved by catalytic hydrogenation. (S)-S-Benzyl--homocysteine was obtained by fractional crystallization of the brucine salt of the racemate and transformed into benzyl (S)-1,2-thiazetidine-3-acetate, which was on the other hand synthesized by an enantiospecific route from -benzyl Boc-L-aspartate. Some -sultam peptides were prepared from the (S)-enantiomer, and finally some -sultam peptides containing D-Ala units were obtained.     

Liquid chromatographic/electrospray mass spectrometric determination (LC/ESI-MS) of chelerythrine and dihydrochelerythrine in near-critical CO2 extracts from real and spiked plasma samples

Two polar benzo[c]phenanthridine alkaloids, chelerythrine (CHE) and dihydrochelerythrine (DHCHE), were extracted at 35 °C and 10 MPa (15 MPa for real samples) from real and spiked plasma samples with acceptable recoveries (95.1% and 81.0%, respectively) using near-critical CO₂ modified with aqueous (1:1, v/v) methanol. The alkaloids were quantified by a liquid chromatographic/electrospray mass spectrometric (LC/ESI-MS) method on a Zorbax SB-CN column (75 mm × 4.6 mm, 3.5 μm particle size) using methanol (organic phase) and 50 mM ammonium formiate (aqueous phase) as a mobile phase. A linear gradient 0-1 min, isocratic at 60% organic phase (v/v); from 1.0 to 7.0 min, 60-71% organic phase (v/v); from 7.0 to 18.0 min, 71-60% organic phase (v/v) was applied. The limit of detection was 1.22 ng (3.50 pmol) for CHE and 0.95 ng (2.72 pmol) for DHCHE per 1 ml of the sample. The linearity of the calibration curves was satisfactory as indicated by coefficients of determination 0.9979 and 0.9995 for CHE and DHCHE, respectively. Repeatability and intermediate precision (average R.S.D.s) were 1.0-1.5%, accuracy was in the range 99.7-100.3%. Average recovery was 100.1% for both, standard solutions and spiked plasma extracts. Three samples of real rat plasma were extracted and analysed to test the method.     

Protective properties of redox polymer film deposited on stainless steel

We propose a novel composite organic-inorganic coating in the form of a redox polymer film for protection of stainless steel against general corrosion in strong acid medium (2 M H₂SO₄). We utilize an anion exchange polymer, protonated poly(4-vinylpyridine), into which hexacyanoferrate anions have been introduced. Owing to the presence of Fe(CN)₆^3–/4– at the interface formed by the film and the steel, a sparingly soluble metal hexacyanoferrate (mostly Prussian blue, PB) is formed as an overcoating on the steels surface, presumably on the passive (metal oxide) layer. The redox polymer film on the steel seems to act as a composite three-dimensional bilayer-type coating in which hexacyanoferrate(III,II) anions (that are capable of effective charge storage) exist in the outer portions of the film, whereas the inner PB layer improves the systems overall adherence and stability. By analogy to a conducting polymer (e.g. polyaniline, polypyrrole), introduction of the redox polymer composite film leads to stabilization of the steel substrates potential within the passive range.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Determination of phosphoamino acids by micellar electrokinetic capillary chromatography with laser-induced fluorescence detection

A micellar electrokinetic capillary chromatography (MEKC) method with laser-induced fluorescence detection (LIF) was developed for analyzing three phosphoamino acids including phosphotyrosine (P-Tyr), phosphoserine (P-Ser), and phosphothreonine (P-Thr). 3-(2-Furoyl)quinoline-2-carboxaldehyde (FQ), a fluorogenic dye, was employed for derivatization of these phosphoamino acids. Results indicated that the complete baseline resolution of each phosphoamino acid was obtained within 10 min, using 20 mmol l⁻¹ sodium borate buffer (pH 9.35) containing 20 mmol l⁻¹sodium deoxycholate (SDC) and 10 mmol l⁻¹ Brij35. Other common amino acids, especially Glu and Asp, did not disturb the assay of these phosphoamino acids. There was a linear relationship between the peak area for analyte and its concentration, with correlation coefficients in the range of 0.9966-0.9996. The concentration detection limits (signal-to-noise = 3) for P-Tyr, P-Ser, and P-Thr were 10, 40, and 75 nmol l⁻¹, respectively. The developed method was successfully applied for determining phosphoamino acids in the hydrolysis sample of a phosphorylated protein kinase.     

NEMCA effect: why are the work function changes of the gas exposed catalyst-electrode surface one-to-one related to the changes in the catalyst working electrode potential?

In the present work, an important point concerning the NEMCA effect is addressed. We analyse the reasons why the changes in the work function of the gas exposed catalyst-electrode surface are one to one related to the changes in the catalyst working electrode potential E with respect to a reference electrode. It is concluded that this is due to the unique properties of the catalyst/solid electrolyte interface: the structure of the double layer in this region is very different from that in liquid electrolytes, being the potential difference at this interface mainly determined by the specific adsorption of the mobile species in the solid electrolyte.

Mixed-mode reversed-phase and ion-exchange separations of cationic analytes on polybutadiene-coated zirconia

The retention and selectivity of the chromatographic separation of basic (cationic) analytes on a polybutadiene-coated zirconia (PBD-ZrO2) stationary phase have been studied in greater detail than in previous studies. These separations are strongly influenced by the chemistry of the accessible surface of zirconia. In the presence of buffers which contain hard Lewis bases (e.g., phosphate, fluoride, carboxylic acids) zirconia's surface becomes negatively charged due to adsorption of the buffer anion at the hard Lewis acid sites. Consequently, under most conditions (e.g., neutral pH), cationic analytes undergo both hydrophobic and cation-exchange interactions. This mixed-mode retention process generally leads to greater retention factors for cations relative to those on silica-based reversed phases despite the lower surface areas of the zirconia phase, but, more importantly, adsorption of hard Lewis bases can be used to control the chromatographic selectivity for cationic analytes on these zirconia-based stationary phases. In contrast to our prior work, here we show that when mixed-mode retention takes place, both retention and selectivity are easily adjusted by changing the type of hard Lewis base buffer anion, the type of buffer counter-ion (e.g., sodium, potassium, ammonium), the pH, and the ionic strength of the eluent as well as the type and amount of organic modifier.     

Electrochemical behavior and determination of epinephrine at a mercaptoacetic acid self-assembled gold electrode

The electrochemical behavior of epinephrine (EP) at a mercaptoacetic acid (MAA) self-assembled monolayer modified gold electrode was studied. The MAA/Au electrode is demonstrated to promote the electrochemical response of epinephrine by cyclic voltammetry. The possible reaction mechanism is also discussed. The diffusion coefficient D of EP is 6.85 × 10⁻⁶ cm² s⁻¹. In 0.1 mol L⁻¹ phosphate buffer (pH 7.20), a sensitive oxidation peak was observed at 0.177 V, and the peak current is proportional to the concentration of EP in the range of 1.0 × 10⁻⁵–2.0 × 10⁻⁴ mol L⁻¹ and 1.0 × 10⁻⁷–1.0 × 10⁻⁶ mol L⁻¹. The detection limit is 5 × 10⁻⁸ mol L⁻¹. The modified electrode is highly stable and can be applied to the determination of EP in practical injection samples. The method is simple, quick, sensitive and accurate.     

Partitioning of <Emphasis Type="Italic">π</Emphasis> -electrons in rings of polycyclic conjugated hydrocarbons: Part 6. Comparison with other methods for estimating the local aromaticity of rings in polycyclic benzenoids

By adopting the convention that shared double bonds in polycyclic conjugated hydrocarbons contribute with one -electron and unshared ones with two -electrons, a partition of -electrons in each ring (-electron content, EC) can be obtained by averaging over all Kekulé structures, which are assumed to have equal weights. This affords a simple measure of local aromaticity that is comparable with other such local aromaticity indices in polycyclic benzenoids.     

Intramolecular Dynamics and Molecular Structure of Europium(III) Chelate Complexes with Crown Ethers as Studied by NMR Spectroscopy

Earlier the intramolecular inversion of the 18-crown-6 molecules was found in the complex ion pairs [Ln(ptfa)₂ (18-crown-6)]⁺ [Ln(ptfa)₄]⁻ (H₂O)₄ where Ln = La(1), Ce (2), Pr (3), Nd (4), and ptfa is 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione. In this work the peculiarities of the molecular structure and dynamics were studied for [Eu(ptfa)₂ (18-crown-6)]⁺ [Eu(ptfa)₄]⁻ (H₂O)₄ (5) by NMR spectroscopy techniques. Through VT-NMR spectra analysis the temperature dependence was obtained for the rate constant. The free energy ΔG^‡(320) of 18-crown-6 ring inversion activation was found to be 65 ± 5 kJ mol⁻¹ for 5 in CDCl₃. This result is comparable with the earlier data [S.P. Babailov and D.A. Mainichev: J. Inclusion Phenom. Macrocyclic Chem. 43, 187–193 (2002)] for complexes 2, 3, 4 in deuterated toluene (ΔG^‡(320)=65 ± 9, 64 ± 9, 64 ± 9 kJ mol⁻¹ respectively). It was found by relaxation NMR spectroscopy that the effective distance between Ln and protons of the crown molecule is 4.5 ± 0.2 Å. The analysis of structural parameters testifies that the crown ether and chelated anions are in the first coordination sphere of a Ln cation. Obtained geometrical parameters show that the complex cations of Eu, Ce and Pr have similar spatial structures.This revised version was published online in July 2005 with a corrected issue number.     

Thermal analysis study of the effect of molecular weight on constrained amorphous phase in poly(phenylene sulfide)

We report a thermal analysis study of the effect of molecular weight on the amorphous phase structure of poly(phenylene sulfide), PPS, crystallized at temperatures just above the glass transition temperature. Thermal properties of Fortron PPS, having viscosity average molecular weights of 30000 to 91000, were characterized using temperature modulated differential scanning calorimetry (MDSC). We find that while crystallinity varies little with molecular weight, the heat capacity increment at the glass transition decreases as molecular weight decreases. This leads to a smaller liquid-like amorphous phase, and a larger rigid amorphous fraction, in the lower molecular weight PPS. For all molecular weights, constrained fraction decreases as the scan rate decreases.This research is supported by the U. S. Army Research Office through contract DAAH04-96-1-0009. The authors thank Hoechst Celanese for providing different molecular weight Fortron samples and Dr. George Collins for providing sample information. The authors acknowledge the assistance of Elizabeth Oyebode and Leonardo Grimaldi with sample preparation and MDSC work.