The use of gadolinium as a probe in the Fc region of a homogeneous anti-(type-III pneumococcal polysaccharide) antibody

The binding of gadolinium [Gd(III)] to a homogeneous rabbit anti-(type-III pneumococcal polysaccharide) IgG (immunoglobulin G) and its Fab (N-terminal half of heavy and light chain) and Fc (C-terminal half of heavy-chain dimer) fragments was demonstrated by measurements of solvent-water proton relaxation rates in the appropriate Gd(III) solutions. At pH 5.5 the binding of Gd(III) to the Fc fragment is much tighter (KD approx. 5 micronM) than binding to the Fab fragment (KD approx. 250 micronM). The binding of Gd(III) to the whole IgG molecule (KD approx. 4 micronM) is very similar to that for the Fc fragment alone. This specificity of binding to the Fc region allows the use of Gd(III) as a probe of the Fc conformation. The environment of the Gd(III) in the Fc region of whole IgG is not affected by the presence of octasaccharide derived by hydrolysis of type-III pneumococcal polysaccharide, but the corresponding 28-unit saccharide does cause detectable changes. The addition of 16-unit saccharide to anti-(SIII polysaccharide) IgG in the presence of Gd(III) does not change the solvent water proton relaxation rate, although aggregation does occur. The effects of the 28-unit saccharide may be explained therefore by a change in the tumbling time of the IgG. From a study of the effect of various antigen/antibody ratios, it is concluded that the 28-unit-saccharide-induced changes in the Gd(III) environment in the Fc region are caused by the specific geometrical structure of the antigen-antibody complexes formed, and not simply by occupancy of the combining sites on the antibody.