Abstract
A comparison of the 1H and 13C nmr parameters for the disaccharide derivatives βDGal(1→ 4)βDGlcO(CH2)8COOCH3 (1), βDGalNAc(1 → 4)βDGalO(CH2)8COOCH3 (2), and βDGal(1 → 3)βDGalNAcO(CH2)2CH3 (3) allowed assignments of the signals observed for the asialo-GM2- and asialo-GM1-related synthetic haptens, βDGalNAc(1 → 4)βDGal(1→4)βDGlcO(CH2)8COOCH3 (4) and βDGal(1 → 3)βDGalNAc(1 → 4)βDGal(1 → 4)βDGlcO(CH2)8COOCH3 (5). 1H nuclear Overhauser enhancement studies confirmed the conformational preferences for 2 and 3 predicted by HSEA calculation. Comparison of the 1H and 13C spectra indicate that these preferences and that previously known for the lactoside (1) are closely maintained in the haptens 4 and 5. HSEA calculation indicates that the methyl group of methyl N-acetyl-α-D-neuraminic acid (10) prefers the orientation wherein the carboxyl group is near anti-periplanar to the methyl group. However, this orientation was not confirmed by saturation of the methyl group since no enhancement of either Ha-3 or He-3 was observed. On the other hand, saturation of Ha-3c of the αDNeuAc unit of the GM1-related pentasaccharide, βDGal(1 → 3)βDGalNAc(1 → 4)[αDNeuAc(2 → 3)]βDGal(1→4)α,βDGlc, caused strong enhancement of the signal for the aglyconic hydrogen, H-3bof the βDGal(1→4) unit. This observation, along with those of other nuclear Overhauser experiments, established that this pentasaccharide has the carboxyl group anti-periplanar to C-3b. Furthermore, the C-7c to C-9c chain of the αDNeuAc unit adopts nearly the same conformation for the compound in aqueous solution as exists for N-acetylneuraminic acid in the crystalline state. The HSEA calculation indicates important attractive interaction between this chain and the βDGalNAc unit of the aglycon. These conlcusions were extended to the GM2 and GM1 gangliosides by the comparison of chemical shift data. In these compounds, the carboxyl group projects over the hydrophobic α-side of the βDGalNAc unit and the polar OH-4c, NAc-5c, OH-7c groups project over the hydrophobic β-side of the lactose unit. Comments are made on the possible bearing of these topographical features on certain biological properties of gangliosides.
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