Synthesis and antigenic properties of C-7-modified Kdo mono- and disaccharide ligands and Kdo disaccharide interresidue lactones.

In order to define binding interactions of Kdo-specific monoclonal antibodies directed against the chlamydial alpha-(2-->8)-linked Kdo disaccharide epitope on a molecular level, modifications at the 7-position of the proximal and distal Kdo unit were investigated. The synthesis of 7-O-methyl and 7-azido-7-deoxy-7-epi-Kdo monosaccharide derivatives was achieved via an 8-O-TBS protected derivative, whereas methylation of O-7 at the proximal Kdo unit of the alpha-(2-->8)-linked Kdo disaccharide was conveniently accomplished via a 4,5; 4',5'; 7',8'-tri-O-carbonyl-protected disaccharide intermediate. Attempted epimerization at C-5 of the inner unit of a alpha-(2-->4)-linked Kdo disaccharide, however, resulted in formation of the corresponding 5,6-dehydro derivative, which was fully deprotected. Treatment of unprotected alpha-(2-->8)- as well as alpha-(2-->4)-linked Kdo disaccharides in neat acetic acid furnished the corresponding interresidue lactone derivatives. The lactones displayed limited stability under neutral conditions and were hydrolyzed at pH 7 within 3 days. Access to the lactones, however, provides a means for selective derivatization of the carboxylic group located at the distal Kdo residue, which was demonstrated by methanolysis of the lactone to afford the monomethyl ester of the alpha-(2-->8)-linked Kdo disaccharide. ELISA inhibition experiments of the ligands with two Kdo-specific monoclonal antibodies showed slightly reduced reactivity for the binding of the alpha-(2-->8) Kdo-specific antibody S25-2, whereas the 7-O-methyl disaccharide antigen displayed high binding affinity toward the Kdo monosaccharide-specific antibody S67-27.