The relation of the rates of flocculation and amounts of precipitate in precipitin reactions to the concentration of hydrogen ion and of neutral salts.

The effects of concentration of hydrogen ion and of neutral salts on the amounts of precipitate and rates of flocculation have been studied with several antigen-antibody systems, in particular those of bovine serum albumin (BSA) and lysozyme (L) with their rabbit antibodies. In all the systems studied the amount of precipitate and rate of flocculation were maximum over a H range extending on both sides of H 7. The range of H, over which the rate of flocculation was maximum, was narrower than the range over which the amount of precipitate was maximum; outside this range the rate of flocculation fell more steeply than the amount of precipitate. The rate of flocculation, but not the amount of precipitate formed with protein antigens, was reduced in concentrations of sodium chloride above 0.15 N; the reduction was greater in solutions of magnesium chloride and less in solutions of sodium sulphate than in sodium chloride solutions of equivalent concentration. The precipitates formed by protein antigens, that have relatively small molecular weights, with the corresponding antibodies, were more reduced by changes of hydrogen ion concentration than those formed by bovine serum albumin with antibody; the rates of flocculation of these small protein antigens with antibody were also more affected by change of concentration of hydrogen ion and neutral salts. It is suggested that this difference is due to the presence of fewer combining sites on the smaller molecules than on serum albumin. The reduction of the amounts of precipitate, formed by protein antigens, as the hydrogen ion concentration increased, is compatible with the hypothesis that a negatively charged group, with H near 5, on each antibody receptor site is essential for combination with antigen. The reduction of the amount of precipitate through a range of H over which amino groups lose their positive charge suggests that positively charged amino groups are essential for the combination of antigen with antibody; other evidence, however, indicates that positively charged amino groups of the protein antigens are not essential. The effect of increased hydrogen ion concentration on the precipitin curves of Type III pneumococcal polysaccharide with rabbit antibody is consistent with the hypothesis that the presence of negatively charged groups on the combining sites of the antigen are essential for its reactions with antibody. Experiments indicate that when the salt concentration is low it is mainly the later stages of aggregation that are delayed.