Kinetics of binding of oligosaccharides to a homogeneous pneumococcal antibody: dependence on antigen chain length suggests a labile intermediate complex

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Abstract
Temperature-jump experiments were performed with di-, tetra-, and hexasaccharides derived from type III pneumococcal polysaccharide using a homogeneous corresponding [rabbit] antibody Ig[immunoglobulin]G 45-394. A decrease in stability of the oligosaccharide-antibody complexes with decreasing chain length was observed and was entirely reflected in the decrease of the association rate constants which were 1.7 .times. 104 M-1 s-1 for the di-, 3.7 .times. 105 M-1 s-1 for the tetra-, and 1.1 .times. 106 M-1 s-1 for the hexasaccharide at 23.degree. C. The dissociation rate constants for all oligomers were about 12 s-1. This marked chain-length dependence of the association rate constants as well as their low values are unexpected for a single binding step. A mechanism is proposed which consists of a fast formation of a labile oligosaccharide-antibody precomplex followed by a slow isomerization step which is induced by the oligosaccharide ligands but which is chain-length independent.