Landau theory of the finite temperature mott transition

In the context of the dynamical mean-field theory of the Hubbard model, we identify microscopically an order parameter for the finite temperature Mott end point. We derive a Landau functional of the order parameter. We then use the order parameter theory to elucidate the singular behavior of various physical quantities which are experimentally accessible.     

Solid-state NMR spectroscopy applied to a chimeric potassium channel in lipid bilayers

We show that solid-state NMR can be used to investigate the structure and dynamics of a chimeric potassium channel, KcsA-Kv1.3, in lipid bilayers. Sequential resonance assignments were obtained using a combination of (15)N- (13)C and (13)C- (13)C correlation experiments conducted on fully labeled and reverse-labeled as well as C-terminally truncated samples. Comparison of our results with those from X-ray crystallography and solution-state NMR in micelles on the closely related KcsA K (+) channel provides insight into the mechanism of ion channel selectivity and underlines the important role of the lipid environment for membrane protein structure and function.     

Cathodic stripping voltammetric determination of selenium(IV) at a thin-film mercury electrode in a thiocyanate-containing electrolyte

The well-known method for the determination of selenium(IV), which is based on the cathodic stripping voltammetry of copper(I) selenide, has been adapted for application at the thin-film mercury electrode on glassy carbon (TFME). Insufficient reproducibility and sensitivity have been overcome by using a 0.1 mol/L HClO₄ electrolyte solution containing 0.02 mol/L thiocyanate ions. Thiocyanate ions have been found to increase the peak height of the selenium response and shift it to more positive potentials. This behaviour is explained by an adsorption of SCN^– at the interface glassy carbon/Cu₂Se and its action as an electron transfer catalyst between glassy carbon and copper(I) selenide. A 3σ-detection limit of 75 ng/L Se^(IV) has been achieved. The relative standard deviation is 5.2% at 5 μg/L selenium(IV). The influence of cadmium(II), arsenic(III), zinc(II), iron(III) and lead(II) ions on the selenium response has been studied. In case of lead ions, a new signal occurred at more negative potentials than the reduction of Cu₂Se. This signal, which is probably due to the reduction of PbSe, can also be used for the determination of selenium(IV). Received: 13 November 1996 / Revised: 19 December 1996 / Accepted: 24 December 1996     

High field imaging of the normal pancreas

Fifty MR scans conducted at 1.5 T were evaluated to assess how well the pancreas could be identified. A total of 128 sets of images were reviewed. The pancreatic head was identified in 81%, 82% and 74% of scans with T1, intermediate and T2-weighting, respectively. The body was identified in 100%, 96% and 70% and tail in 96%, 87% and 54% on those respective sequences. Relative contrast was calculated between pancreas and: liver, spleen, muscle, fat, stomach and small bowel. The variability seen in contrast compared to stomach and small bowel suggests that consistent MR visualization of the pancreas will probably necessitate the use of an oral contrast agent.     

Bicyclic [b]‐heteroannulated pyridazine derivatives. 9. cyclization reactions of 4,4‐dimethyl‐ and 4‐phenyltetrahydropyridazine‐3,6‐dione 3‐hydrazones with esters of keto dicarboxylic acids

Bis(ethoxycarbonyl)alkylidene derivatives 4 and 5 of the respective title hydrazones were obtained in the reactions with diethyl oxomalonate, diethyl oxosuccinate, diethyl 2‐oxoglutarate, and diethyl oxalo‐propionate as mixtures of geometric isomers with high predominance of one of them. On heating at 160‐200° without any solvent or on refluxing in ethanol 4 cyclized to yield the corresponding pyri‐dazino[6, 1‐c]triazines 6 , whereas heating of 5 gave, depending on the chain length, the corresponding pyra‐zolylpyridazines 8b and 8d or the pyridazinylpyridazine 8c . X‐ray analysis was used to determine the structures of 6 and 8 ; the unit cell of 6c was found to accommodate 16 molecules representing four conforma‐tional varieties. The different behavior of 4 and 5 in the cyclization reactions was interpreted in terms of the tautomeric equilibrium which was shifted towards the enamine form in 4 , and towards the imine form, in 5 . Transmission of a long‐range chirality effect in 4d and 5a‐d manifested itself in the ¹H nmr spectra as the magnetic non‐equivalence of the CH₂ protons in one or both ester ethyl groups.     

Unexpected reactions associated with the xanthate‐mediated polymerization of N‐vinylpyrrolidone

The monomer N‐vinylpyrrolidone (NVP) undergoes side reactions in the presence of R group functional xanthates and impurities. The fate of the monomer NVP and a selection of six O‐ethyl xanthates during xanthate‐mediated polymerization were studied via NMR spectroscopy. A high number of by‐products were identified. Significant side reactions affecting NVP include the formation of an unsaturated dimer and hydration products in bulk or in solution in C₆D₆. In addition, the xanthate adjacent to a NVP unit was found to undergo elimination at moderate temperature (60–70 °C), resulting in unsaturated species and the formation of new xanthate species. The presence of the chlorinated compound α‐chlorophenyl acetic acid, ethyl ester, a precursor in the synthesis of the xanthate S‐(2‐ethyl phenylacetate) O‐ethyl xanthate, resulted in a dramatic increase in the rate of side reactions such as unsaturated dimer formation and a high ratio of unsaturated chain ends. The conditions for the occurrence of such side reactions are discussed in this article, with relevance to increasing the control over the polymerization kinetics, endgroup functionality, and control over the molar mass distribution. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6575–6593, 2008     

Antioxidant inhibition of porphyrin-induced cellular phototoxicity.

Porphyrins such as protoporphyrin IX (PP IX) and uroporphyrin I (UP I) can be phototoxic to human cells. To study the protective ability of antioxidants (beta-carotene, lycopene, ascorbic acid and alpha-tocopherol), against such porphyrin phototoxicity, membrane destruction experiments (Jurkat cells) and human cell cultures (fibroblasts) were performed. Both beta-carotene and lycopene and also the combination of beta-carotene, ascorbic acid and alpha-tocopherol offered cell protection against PP IX phototoxicity. Investigations of both cell membrane protection and of cell growth showed differences in terms of the protection afforded by the anti-oxidants. Thus, for PP IX, carotenoids alone, and in combination with ascorbic acid and alpha-tocopherol, showed higher protection factors in general than UP I. However, for membrane protection there was significant protection against UP I by the combination of beta-carotene, ascorbic acid and alpha-tocopherol but not by any of these anti-oxidants alone. The membrane protection against PP IX by beta-carotene, and especially lycopene, is significant presumably because of the high lipophilicity of all these molecules. However, the hydrophilic UP I will cause phototoxicity mainly via H(2)O(2), radical or singlet oxygen production in the aqueous phase, and these reactive species may be generated some distance from the cell membrane. This may lead to the little or no protection observed for UP I by the individual antioxidants. Nevertheless, a combination of beta-carotene, ascorbic acid and alpha-tocopherol offers membrane protection against the phototoxicity of both porphyrins. This is believed to occur as a result of synergistic processes. Our results suggest that the treatment of porphyria cutanea tarda and erythropoietic protoporphyria may be improved by the use of a combination of the antioxidants studied.