Antigen valence and Fc-localized secondary forces in antibody precipitation

A tetravalent dinitrophenyl hapten, 1,6,11,16-tetra-(?-N-Dnp)-l-lysine₁₆3 was synthesized. Upon interaction with specific antibody of moderate affinity (K_t = 6 × 10⁶ M^?1), a maximum of 80% precipitation occurred. Low pH or 2.5 M guanidine hydrochloride completely inhibited precipitation without significantly affecting hapten binding. Anti-Dnp (Fab')₂ fragments also formed precipitates when mixed with the tetravalent hapten, but these were not soluble in 2.5 M guanidine hydrochloride, and the precipitation itself occurred over a much narrower range of hapten/antibody ratios than observed with intact molecules. These results suggest that a site located in the Fc region assists aggregation and preciptation in intact antibody molecules.