FURTHER DEFINITION OF THE SIZE OF THE BLOOD GROUP-I ANTIGENIC DETERMINANT USING A CHEMICALLY SYNTHESIZED OCTASACCHARIDE OF POLY-N-ACETYLLACTOSAMINE TYPE

In earlier studies, the minimum structure which inhibited the binding of anti-i to an i-active glycoprotein was the linear trisaccharide, beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> 3)-D-Gal. There was an increasing hierarchy of inhibitory activities in the linear tetrasaccharide, beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> 3)-beta-D-Galp-(1 -->4)-beta-D-GlcNAc, its methyl beta-glycoside, and in the methyl beta-glycoside of the hexasaccharide. The linear octasaccharide methyl beta-glycoside in this series is approximately only half as active as the hexasaccharide methyl beta-glycoside. Analyses by high resolution H-1-n.m.r. of these two oligosaccharides indicated that they have similar conformations in solution, and there is no evidence for the occurrence of inter-molecular interactions which might partially hinder the binding of anti-i to the octasaccharide methyl beta-glycoside. These results are consistent with the size of the i antigen being in the region of a hexasaccharide. It is proposed that the methyl aglycon group of the hexasaccharide methyl beta-glycoside confers an above normal activity by presenting a hydrophobic area for additional contact in the vicinity of the antibody-combining site.