The slow molecular mobility in amorphous trehalose.

The molecular mobility in amorphous trehalose is studied by thermally stimulated depolarisation currents (TSDC). The effect of aging on the sub-T(g) motional processes was analysed during annealing at a given aging temperature, some degrees below the calorimetric glass transition temperature T(g)=115 degrees C. The features of different motional components of the secondary relaxation are monitored as a function of time as the glass structurally relaxes on aging. The faster components of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes show a significant dependence on aging consisting of some kind of local motions with some intermolecular nature. The dielectric strength of this relaxation decreases with increasing aging time, and there is no evidence for any modification with aging of the relaxation time of this local mobility. The TSDC study of the molecular mobility of amorphous trehalose in the temperature region of the glass transformation provides the unexpected result that no glass transition signal is observable in this temperature region.     

1H and 13C NMR spectral assignment of androstane derivatives

(1)H and (13)C NMR spectroscopic data for 5alpha-androstanes and halo-5alpha-androstanes with different substituents at positions C-3, C-9, C-11 and C-17 were examined and assigned by a combination of 1D and 2D NMR experiments. The substituent effects on the (13)C chemical shifts were compared with those of epi-androsterone, used as a reference compound. The coupling constants (n)J((19)F,(13)C) were measured for compounds 6, 8, 11 and 14.     

DNA cleavage by the photolysis of cyclopentadienyl metal complexes: mechanistic studies and sequence selectivity of strand scission by CpW(CO)3CH3

[Reaction: see text]. The photolysis of CpW(CO)3Me has been shown to produce methyl radicals and to cleave DNA in a single-stranded manner, and preliminary evidence implicated a carbon-centered radical in this process. In this work, the mechanism of strand scission in this reaction was determined to occur by hydrogen atom abstraction from the 4'- and 5'-positions of the deoxyribose moiety of the backbone of DNA. Additionally, in a side reaction that does not lead to frank strand scission, all four bases of DNA are methylated under these conditions; however, none of these base or backbone modifications lead to the formation of abasic sites.     

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