NMR analysis of carbohydrates with model-free spectral densities: the dispersion range revisited |
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Authors: | L Catoire I Braccini N Bouchemal-Chibani L Jullien C Herve du Penhoat S Perez |
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Institution: | (1) Dpt de Chimie CNRS URA 1679, Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris Cedex 05, France;(2) Ingenierie Moleculaire, INRA, BP527, 44316 Nantes |
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Abstract: | Over the past decade molecular mechanics and molecular dynamics studies have demonstrated considerable flexibility for carbohydrates.
In order to interpret the corresponding NMR parameters, which correspond to a time-averaged or 'virtual' conformer, it is
necessary to simulate the experimental data using the averaged geometrical representation obtained with molecular modelling
methods. This structural information can be transformed into theoretical NMR data using empirical Karplus-type equations for
the scalar coupling constants and the appropriate formalism for the relaxation parameters. In the case of relaxation data,
the 'model-free' spectral densities have been widely used in order to account for the internal motions in sugars. Several
studies have been conducted with truncated model-free spectral densities based on the assumption that internal motion is very
fast with respect to overall tumbling.
In this report we present experimental and theoretical evidence that suggests that this approach is not justified. Indeed,
recent results show that even in the case of moderate-sized carbohydrates internal motions are occurring on the same timescale
as molecular reorientation. Simulations of relaxation parameters (NOESY volumes, proton cross-relaxation rates, carbon T1
and nOe values) in the dispersion range (0.1<Tc<5 ns) show that rates of internal motion can be fairly precisely defined with
respect to overall tumbling. Experimental data for a variety of oligosaccharides clearly indicate similar timescales for internal
and overall motion.
This revised version was published online in August 2006 with corrections to the Cover Date. |
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Keywords: | model-free spectral densities correlation time relaxation |
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