Fluorescence quenching of tryptophan by trifluoroacetamide

Trifluoroacetamide was found to be a good quencher of tryptophan fluorescence, and the quenching was shown to proceed via both a dynamic and a static process. The respective quenching constants were determined by the measurement of the decrease of the fluorescence lifetime in the presence of the quencher. The static and the bimolecular rate quenching constants of N-acetyltryptophanamide are equal to 0.34 1·mol^?1 and 1.9·10⁹ 1·mol^?1·s^?1, respectively. These values indicate that trifluoroacetamide is an efficient quencher of tryptophan fluorescence. This conclusion is also supported by a complete quenching of bovine serum albumin and wheat germ agglutinin fluorescence. In the case of lysozyme, trifluoroacetamide quenches the fluorescence of tryptophan residues which fluoresce with a maximum at 348 nm but not the buried tryptophan residues which fluoresce with a maximum at 333 nm. Trifluoroacetamide quenching of wheat germ agglutinin emission confirms the homogeneity and the high accessibility of emitting tryptophan residues, in agreement with a previous report (Privat, J.P. and Monsigny, M. (1975) Eur. J. Biochem. 60, 555–567). The tryptophan fluorescence decay of wheat germ agglutinin is biexponential even in the presence of the quencher; the static and bimolecular rate quenching constants are equal to 0.22 1·mol^?1 and 092·10⁹ 1·mol^?1·^?1, respectively. In the presence of a specific lectin ligand, the methyldi-N,N′-trifluoroacetyl-β- chitobioside, the quenching of wheat germ agglutinin fluorescence involves a direct contact between tryptophan residues and trifluoroacetamido groups of the ligand and in contrast with the quenching induced by free trifluoroacetamide shows that the tryptophan fluorescence is not fully quenched.     

Long-term study (1974-1998) of seasonal changes in the phytoplankton in Lake Geneva: a multi-table approach

The monitoring of Lake Geneva began after one decade of eutrophicationand has provided a uniform set of phytoplankton data. This studyaimed to define the mean annual pattern of the seasonal successionsof phytoplankton species, and to determine whether inter-annualdistortions in this seasonal structure occur. We analysed the25 annual patterns using the STATIS multi-table method. Thephytoplankton successions in the first part of the year fittedwell with the pattern predicted by the Plankton Ecology Group(PEG) model. But the temporal evolution in the summer phytoplanktoncommunity differed from the PEG model, and was subject to between-yearsdifferences. We identified three homogeneous periods for theannual patterns: 1974–1985, 1986–1991 (except 1988)and 1992–1996 (including 1988). During the first period,phytoplankton succession followed the reference annual pattern,the typical autumnal community was missing during the secondperiod, and this autumn community developed earlier during thethird period. This study reflects the good ability of the phytoplanktoncommunity of Lake Geneva to resist both inter-annual fluctuationsand brutal shifts in the annual functioning of the system. Webelieve these shifts resulted from a gradual change in environmentalparameters, including the deepening of the phosphorus-depletedlayer and a change in its timing as a result of the long-termmeteorological changes.     

Grignard addition-reactions to glycosulose derivatives

The course of Grignard addition-reactions to 1,2:5,6-di-O-isopropylidene-α-D-ribo-hexofuranos-3-ulose (1) has been examined as a function of the nature of the reagent, the solvent, the halide, and the temperature. Ethylmagnesium bromide in ether at — 14° converted 1 into 60% of the 3-C-ethyl-D-allo adduct 2. The latter was convertible in 90% yield into the 3-benzyl ether 6, despite the tertiary nature of the hydroxyl group. The use of tetrahydrofuran (THF) or THF-ether at higher temperatures, or of ethylmagnesium iodide, lowered the yield of 2 and gave substantial proportions of such side products as 1,2:5,6-di-O-isopropylidene-α-D-allofuranose (3). 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose (4), and the hydrate (5) of the starting ketone 1. Phenylmagnesium bromide in ether or THF converted 1 into the 3-C-phenyl-D-allo derivative 7 in 84% yield, accompanied by only minor proportions of side products; the latter were the 3-C-phenyl-D-gluco adduct 8 and the product (9) of 5,6-dioxolane ring-opening. The structures of 8 and 9 were confirmed by an acetylation-deacetylation sequence, and by n.m.r. spectroscopy. The 3-C-phenyl-D- allo derivative 7 could be converted in 95% yield into its 3-benzyl ether 10. Cyclohexylmagnesium bromide reacted with 1 in ether or THF at various temperatures to give 3-C-cyclohexyl-1,2:5,6-di-O-isopropylidene-α-D)-allofuranose (11) in low yields; the main product generally encountered was 3. with variable proportions of 4, 1,2-O-isopropylidene-α-D-allofuranose (18), the hydrate 5, and a dimeric product 19 (further characterized as its oxime 20). Compound 11 was, however, obtainable in >95% yield by reducing 7 with hydrogen in the presence of rhodium-on-alumina. Phenylmagnesium bromide reacted with the 4-ketone derivative 25 in THF at 0° to give 83% of 1,6-anhydro-2,3-O-isopropylidene-3-C-phenyl-β-D-talopyranose (26), and no side-products were detected.