Kinetics and mechanism of H+(aq)-assisted aquation of [Cr(N3)(S-pdtra)]− and [Cr(N3(edtrp)]−ions

Two new complex anions, [Cr(N3)(S-pdtra)]– and [Cr(N3)(edtrp)]–, were obtained in solution by N3–/HN3 anation of the aqua analogues (S-pdtra = S-propane-1,2-diamine-N,N,N-triacetate, edtrp = ethylenediamine-N,N,N-tripropionate). Aquation of these species in acidic media leads to the same geometrical isomers as those used for the synthesis. The aquation rate is strongly dependent upon [H] and is substantially higher in D2O than in H2O. Protonation of the coordinated azide was not observed spectrophotometrically. The rate law and activation parameters have been determined and discussed.     

Comparative Action Spectrum for Ultraviolet Light Killing of Mouse Melanocytes from Different Genetic Coat Color Backgrounds

The photobiology of mouse melanocyte lines with different pigment genotypes was studied by measuring colony-forming ability after irradiation. The cell lines were wild-type black (melan-a) and the mutants brown (melan-b) and albino (melan-c). Four lamps emitting various UV wavelengths were used. These were germicidal (UVC, 200–280 Dm), 82.3% output at 254 nm, TL01 (UVB, 280–320 nm), 64.2% at 310–311 nm, FS20, broadband with peak output at 312 nm and Alisun-S (UVA, 320–400 nm), broadband with peak output at 350–354 nm. Appropriate filtration reduced the contaminating UVC to nonlethal levels for the longer waverange lamps. Wild-type melan-a was resistant to UVC and UVA compared to the other two cell lines, but the differences were small. The melan-c cell line was more resistant to UVB and markedly more resistant to FS20 than the pigmented lines. With the exception of FS20 responses, melan-b was more sensitive than melan-a to killing by the various UV lamps. There were more pyrimidine dimers (cyclobutane dimers and 6–4 photoproducts) produced in melan-a than in melan-c cells by UVC, UVB and FS20 lamps. Unlike melan-c, melan-a and melan-b showed a strong free radical signal of melanin character with a detectable contribution of pheomelanin-like centers. The contribution of pheome-lanin was higher in melan-b than in melan-a, while the total melanin content in these two cell lines was comparable. The abundant melanin granules of wild-type melan-a melanocytes were well melanized and ellipsoidal, whereas those of melan-b melanocytes tended to be spherical. In the albino line (melan-c) the melanocytes contained only early-stage melanosomes, all of which were devoid of melanin. The results indicate that pigment does not protect against direct effect DNA damage in the form of pyrimidine dimers nor does it necessarily protect against cell death. High pigment content is not very protective against killing by UVC and UVA, and it may photosensitize in UVB the very wavelength range that is of greatest concern with respect to the rising incidence in skin cancer, especially melanoma. It is clear from these studies that, in pigment cells, monochromatic results cannot predict polychromatic responses and that cell death from solar irradiations is a complex phenomenon that depends on more than DNA damage.     

Kinetics and mechanism of aquation of [Cr(NCS)(edtrp)]-, [Cr(NCS)(R-pdtrp)]-, [Cr(edtrp)(NCSHg)]+ and [Cr(R-pdtrp) (NCSHg)]+ complexes. Unexpected rate retardation in strongly acidic media for NCS- ligand release

Three new complex ions, [Cr(NCS)(R-pdtrp)]^-, [Cr(R-pdtrp)(NCSHg)]⁺ and [Cr(edtrp)(NCSHg)]⁺, that are derivatives of the trans-equatorial isomers of [Cr(R-pdtrp)(H₂O)]⁰ and [Cr(edtrp)(H₂O)]⁰ (edtrp= ethylenediamine-N,N,N-tripropionate, R-pdtrp= R-propane-1,2-diamine-N,N,N-tripropionate) have been obtained and characterized in solution. Rate constants and activation parameters, including, in two cases, volumes of activation, have been determined. Rate retardation for NCS^- ligand release has been observed with increasing acidity within the pH 0–2 range. The mechanism of the reactions has been discussed.     

Design and synthesis of pyrankacin: a pyranmycin class of broad-spectrum aminoglycoside antibiotic

[STRUCTURE: SEE TEXT] A novel broad-spectrum aminoglycoside antibiotic, pyrankacin, has been prepared. In addition to the synthetic innovation in dideoxygenation and regioselective Staudinger reduction, we have obtained prominent antibacterial activity against several clinically important pathogens in the course of this work.     

Creatine phosphate--creatine kinase in enzymatic synthesis of glycoconjugates

[reaction: see text] Enzymatic production of glycoconjugates is hampered by expensive phosphagens such as acetyl phosphate (AcP) and phosphoenolpyruvate (PEP). Here, we introduce creatine phosphate--creatine kinase system as a novel and practical energy source in carbohydrate synthesis. This system was successfully demonstrated in the production of bioactive oligosaccharides with different sugar nucleotide regeneration systems.     

The Surface Tension and Density of Liquid Ag–Bi, Ag–Sn, and Bi–Sn Alloys

Summary. The sessile drop method has been used to measure density and surface tension for pure Ag, Bi, Sn, and their mixtures. For pure metals and Bi–Sn alloys negative temperature coefficients of surface tension have been obtained. In case of Ag–Bi and Ag–Sn alloys the temperature coefficients of surface tension take negative or positive values depending on composition. Experimental values of the surface tension for Ag–Bi, Ag–Sn, and Bi–Sn are compared with those computed from Butler’s model. A relatively good agreement is observed.     

Comparison of modifiers for determination of arsenic, antimony and selenium by atomic absorption spectrometry with atomization in graphite tube or hydride generation and in-situ preconcentration in graphite tube

The study was performed to compare the effect of magnesium modifier (magnesium nitrate) with that of other modifiers (palladium nitrate and nickel nitrate) in determination of arsenic, antimony and selenium by atomic absorption spectroscopy with atomization in a graphite tube, with generation of hydrides and in situ preconcentration in a graphite tube. The assumed criterion of a modifier performance was the magnitude of the analytical signal. It was found that in determinations with atomization in a graphite furnace the effects of all these modifiers were comparable, while in those with hydride generation and in situ preconcentration in a graphite tube the magnesium modifier showed poorer performance (25% decrease of the analytical signal). In determinations of arsenic and selenium the analytical signal obtained with magnesium salt as a modifier was comparable with those obtained in the presence of all other modifiers.     

Cationic copolymerization of tetrahydrofuran with ethylene oxide in the presence of diols: Composition,microstructure, and properties of copolymers

Cationic copolymerization of tetrahydrofuran (THF) with ethylene oxide (EO) in the presence of diols leads to dihydroxy terminated telechelic copolymers. In the present article the influence of copolymerization conditions on the copolymer structure was studied in view of conclusions derived from studies of copolymerization kinetics and mechanism. It was shown that according to established copolymerization mechanism, the number average molecular weights increase linearly with conversion up to M_n ≅ 2500, hydroxyl end groups are bound exclusively to EO units and copolymers are composed of [EO]–[THF]_y segments. Microstructure of copolymers may be to some extent regulated by changing reaction conditions. Some physical properties of copolymers also were studied. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3455–3463, 1999     

Polyethers containing stable ionic end-groups

Polyethers of low and medium molecular weight containing at both ends stable ionic groups (phosphonium ions) were obtained by living cationic polymerization of tetrahydrofuran initiated by difunctional initiator, trifluoromethanesulfonic anhydride, followed by termination with triphenylphosphine. It was shown, that products contain significant quantities of low molecular weight diphosphonium salt; a plausible explanation is presented. Alternative approach to the synthesis of diionically terminated polyethers was based on the conversion of hydroxyl end-groups of polyether diols, which can be obtained by cationic Activated Monomer (AM) polymerization of e.g. oxiranes, into phosphonium ion end-groups. Using this approach, poly(ethylene oxide)'s with M_n ranging from 300 to 3400, terminated at both ends with stable ionic groups, were prepared and characterized. Measurements of NMR relaxation times and viscosity measurements provide the evidence for the aggregation of ionic end-groups. The potential applications of inter- and intramolecular aggregation are discussed. It is shown, that intramolecular aggregation of ionic terminal groups in low molecular weight poly(ethylene oxide) leads to cyclic structures resembling crown ethers and showing comparable efficiency for complexing cations.