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Interactions du cation sodium avec des molécules d'intérêt biologique acides aminés et oligopeptides.

Abstract : There exists, today, many and effective methods of three-dimensional structural characterization of biological compounds in the condensed phase (NMR, X-ray diffraction or circular dichroism). In the last ten years, this field has extended to the gas phase. This work falls in this context and considers the structuring role of Na+ on the ! amino acids Gly and Pro and on oligo-peptides of Gly and Ala, ! by combi ning theoretical calculations and infra-red spectroscopy of gaseous ions by multiphoton dissociation (IRMPD), coupled to mass spectrometry. Experimental IRMPD spectra of the sodiated amino acid complexes, enabled us to identify the exclusive presence of the zwitterionic form in the case of Pro-Na+ and the presence of the non-zwitterionic form in the case of Gly-Na+, in accordance with quantum chemical results. Thus we provided the first direct demonstration of the presence of a zwitterion of an amino acid in gas phase. These were the first infra-red spectra of biological ions in the gas phase. The theoretical study of the Glyn-Na+ and Alan-Na+ complexes showed that for n<=5, the lower energy conformers maximize the electrostatic interaction of the metal ion with the n carbonyl groups, with or without the terminal amine. This behavior was confirmed on the one hand by IRMPD spectroscopy for n=2,3 and on the other hand by the determination of the binding energies of these complexes, by the Cooks kinetic method (n=2-4). For the theoretical study of Glyn-Na+, 5<=n<=10, we coupled Monte-Carlo conformational searches using the AMBER force field, to geometry optimizations using the ri-BLYP density functional, using the approximation of the "resolution of the identity". This approach made it possible to explore in detail very complex potential surfaces. We can distinguish two limiting conformer classes, one in which the peptide is globular and another where it adopts an alpha or 310 helical conformation. We showed! that the lowest energy structures generally display a tetradentate complexation with a strong auto-solvation. All of these structures are globular for n<10. In the case of Gly10-Na+, the most stable conformer has a globular structure around sodium plus five 310-helical residues
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https://pastel.archives-ouvertes.fr/pastel-00001365
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Submitted on : Tuesday, July 27, 2010 - 2:36:26 PM
Last modification on : Wednesday, March 20, 2019 - 12:12:02 PM
Long-term archiving on: : Thursday, October 28, 2010 - 11:31:48 AM

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  • HAL Id : pastel-00001365, version 1

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Catherine Kapota. Interactions du cation sodium avec des molécules d'intérêt biologique acides aminés et oligopeptides.. Génie chimique. Ecole Polytechnique X, 2005. Français. ⟨pastel-00001365⟩

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