Biomedical subjects
Publications and source records attributed to Anne Lesage.
Using solid-state NMR carbon-proton dipolar correlation spectroscopy, we observed hydrogen exchange on the millisecond time scale between water molecules and protein protons in a solid sample. These interactions are shown to be related to important structural features of the protein such as hydrogen-bonding or salt-bridge networks.
We show how high-resolution NMR spectra can be obtained for solids for which the spectra are normally broadened due to structural disorder. The method relies on correlations in the chemical shifts between pairs of coupled spins. It is found experimentally that there are strong correlations in the chemical shifts between neighboring spins in both phosphorus-31 and carbon-13 spectra. These correlations can be exploited not only to provide resolution in two-dimensional spectra, but also to yield "chains" of correlated chemical shifts, constituting a valuable new source of structural information for disordered materials.
The molecular complex [Re(=CtBu)(=CHtBu)(CH2tBu)2] 1 reacts with a silica partially dehydroxylated at 700 degrees C to give syn-2, [(=SiO)Re(=CtBu)(=CHtBu)(CH2tBu)], as a single isomer according to mass-balance analysis, IR, and solid-state NMR spectroscopy. 1D and 2D solid-state NMR (HETCOR and long-range HETCOR) on a 13C-labeled-2 has allowed us to observe the chemical shifts of all carbons (including those that are not labeled) and ascertain their assignments. Moreover, EXAFS data are consistent with the presence of two carbons at a relatively short distance (1.79 A), which cannot be deconvoluted, but which are consistent with the presence of alkylidene and alkylidyne carbons along with two other first neighbors at a longer distance (2.01 A), the alkyl carbon and the O atom by which the Re is attached to the surface. Moreover, the data also suggest the presence of a siloxane bridge of the silica surface at 2.4 A in the coordination sphere of the Re center. Thermal and photochemical treatment allow us to observe the anti isomer, which was also fully characterized by 1D and 2D solid-state NMR. This behavior parallels the reactivity of molecular Re complexes, and their respective 1H and 13C chemical shifts match those of the corresponding molecular analogues syn- and anti-2m and n. Finally, the grafting of 1 onto silica involves the reaction of both the alkyl and the alkylidene ligand with an equiprobability, leaving the alkylidyne as a spectator ligand. Noteworthy is the formation of 2 [(=SiO)Re(=CtBu)(=CHtBu)(CH2tBu)], rather than the corresponding trisneopentyl-neopentylidyne Re complex, monografted on silica, [(=SiO)Re(=CtBu)(CH2tBu)3], which would have been expected from the reactivity of 1 with various molecular Brönsted acids and which also suggests that a proximal siloxane bridge forces the alpha-H abstraction process, leading to syn-2a.
The experimental lineshapes of the carboxyl and methyl carbon resonances of fully 13C enriched L-Alanine are studied in detail at different MAS frequencies and decoupling field strengths. Complex lineshapes at intermediate spinning speeds were explained by the joint effect of off rotational resonance and coherent CSA-dipolar cross-correlation. Whereas off rotational-resonance effects lead to complex lineshapes due to a splitting of some energy levels, coherent CSA-dipolar cross-correlation introduces either a differential intensity and/or a differential broadening of the lines of the J-multiplet. The conditions which lead to such effects are explained and experimentally verified. Additional simulations show that these effects can be expected over a wide range of static magnetic fields and are not restricted to L-Alanine.
In this paper we demonstrate experimentally that the continuously phase-modulated homonuclear decoupling sequence DUMBO-1 is suitable for high-resolution proton NMR spectroscopy of rigid solids. Over a wide range of experimental conditions, we show on the model sample L-alanine as well as on small peptides that proton linewidths of less than 0.5 ppm can be obtained under DUMBO-1 decoupling. In particular the DUMBO-1 sequence yields well resolved proton spectra both at slow and fast MAS. The DUMBO-1 decoupling scheme can in principle be inserted in any multi-nuclear or multi-dimensional solid-state NMR experiment which requires a high-resolution 1H dimension. An example is provided with the 13C-1H MAS-J-HMQC experiment.
Spin-state selection in solid-state NMR is demonstrated, using similar pulse sequences as used in liquid-state NMR. The different transitions of all three carbon resonances in fully 13C-labeled L-alanine are separated in different spectra. By selecting spin-states, the contribution of the J-coupling to the linewidth is removed, leading to a considerable enhancement in resolution. The spin-state-selective technique is demonstrated for magic-angle spinning frequencies from 6 to 35kHz. Other experimental conditions affecting the sensitivity of the experiments are discussed. Sensitivity losses due to the introduction of the spin-state-selective filter are shown to be acceptable. Finally, spin-state selection was used to experimentally confirm the differential broadening expected for the two transitions of the CH3 resonance.
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Biological assay guided fractionation of a dichloromethane extract of Synaptolepis kirkii led to the isolation of four new and five known daphnane-type diterpene orthoesters, whose structure was established by spectroscopic data. Full spectroscopic data of the new and known natural products are reported here for the first time. Pronounced neurotrophic and substantial antileukaemia activities of these compounds were found in in vitro assays.
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