High-resolution proton NMR studies of gangliosides. III. elucidation of the structure of ganglioside GM3 lactone

Ganglioside GM3 lactone (1) was prepared in 95 % yield from the parent ganglioside by incubation at 25°C for 4 days in glacial acetic acid. Inspection of the 500 MHz proton NMR spectra of 1 and its precursor in dimethylsulfoxide-d₆-deuterium oxide at 30°C revealed a large deshielding (+1.42 ppm) of the H-2 resonance of the galactosyl residue. This suggests that 1 must be the lactone formed by esterification of the sialic acid carboxyl group with the C-2 hydroxyl of the galactosyl residue. Consideration of all the NMR data leads to a specific structure proposal in which 1 has a highly rigid structure. Interesting features of the structure include a hydro phobic inner surface and a semicircular outer edge of seven-oxygen atoms, which may have physiological importance.