Ionic properties of beef brain gangliosides

The titration characteristics and the reactions of aqueous systems of beef brain gangliosides with cations were studied. When free of calcium, all the sialic acid groups can be titrated with roughly half neutralization at pH approx. 4. In the presence of metallic chlorides the titrations indicated that some of the acid groups were bound to metal ions. In 1 mM CaCl₂ this effect was extremely pronounced. The reactions with metallic cations and some amines were studied by measuring the cation-proton exchange at pH 4.0. In 1 mM CaCl₂ this exchange was greater than in 50 mM univalent chlorides. The amines, l-norepinephrine, dl-dopamine and procaine also exchanged protons similar to the alkali metals while the reaction of acetylcholine was different. Analysis of the titration data indicated that a configurational change may occur in the course of protonation. Mixed systems of beef brain gangliosides with egg lecithin titrated like gangliosides alone, however, the reactions with metallic cations were decreased. It is suggested that a barrier to metallic ions may exist in these systems.