13C NMR chemical shifts of the triclinic and monoclinic crystal forms of valinomycin

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Two different crystalline polymorphs of valinomycin, the triclinic and monoclinic forms, have been studied by high resolution, solid state 13C CP-MAS NMR spectroscopy. Although the two polymorphs of the crystal are remarkably similar, there are distinct differences in the isotropic chemical shifts between the two spectra. For the triclinic form, the carbon chemical shift tensor components for the alpha carbons adjacent to oxygen in the lactic acid and hydroxyisovaleric acid residues and the ester carbonyls of the valine residue were obtained using the FIREMAT experiment. From the measured components, it was found that the behavior of the isotropic chemical shift, δiso, for valine residue ester carbonyl carbons is predominately influenced by the intermediate component, δ22. Additionally it was found that the smallest shift component, δ33, for the L-lactic acid (L-Lac) and D-α-hydroxyisovaleric acid (D-Hyi) Cα-O carbon was significantly displaced depending upon the nature of individual amino acid residues, and it is the δ33 component that governs the behavior of δiso in these alpha carbons.

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