APRAMYCIN - COMPLETE H-1 AND C-13 NMR ASSIGNMENTS AND STUDY OF THE SOLUTION CONFORMATION BY ROESY MEASUREMENTS

Complete and self-consistent assignments have been achieved, through the concerted use of various 2D techniques, for the H-1 and C-13 NMR spectra of the free base and protonated forms, respectively, of apramycin (1) in D2O solution. It is shown that J-relayed dipolar (or J/R) and coherent transfer (HOHAHA) processes in ROESY experiments can hardly be eliminated with spin systems such as that studied here. True ROE cross-peaks can, nevertheless, be identified by varying the experimental conditions. The solution conformation of 1 has been characterized based on the ROESY data, proton-proton coupling constants and C-13 T1 measurements. The 4-aminoglucose unit appears to be locked together with the octodiose fragment in a conformation satisfying the requirements of the exo-anomeric effects around the C-1"--O--C-8' bond; the 2-deoxystreptamine unit, on the other hand, enjoys more rotational freedom around the respective glycosidic bond. The undistorted chair conformations of all six-membered rings in 1, and the overall conformation of the molecule, are totally independent of the pH, i.e. the protonation state of the molecule.