Hapten-Induced Structural Changes in Rabbit Immunoglobulin G with Specifically Mercuriated Inter-Heavy-Chain Disulfide

Rabbit antipolyalanyl immunoglobulin G was specifically modified by insertion of a mercuric ion between the cysteinyl sulfurs of its reduced single inter-heavy-chain disulfide bond. This mercuriated derivative was used for investigation of the effect of binding a divalent specific hapten to the antibody. Nuclear magnetic resonance of 35Cl- exchanging between the mercury coordination sphere and the solvent was used to monitor the hapten-induced change in the environment of mercury in the protein. Upon binding the divalent hapten the 35Cl line width was reduced. This spectral change was interpreted as hapten-induced alteration of steric relations within the hinge region. The measurements of the circularly polarized component of protein tryptophyl fluorescence (CPL) revealed no difference between the native and mercuriated protein, but reduced and alkylated species had distinctly different CPL, especially at longer wavelengths (350-370 nm). A pronounced change in the CPL induced by binding the divalent hapten was apparent below 350 nm and was of identical magnitude for the native and mercuriated protein yet rather small for the reduced and alkylated derivative. In agreement with earlier studies it is proposed that fluorophores emitting below 350 nm are probably related to the binding site, while those emitting above 350 nm are more sensitive to other domains of the protein. The interdependence of the intactness of the hinge disulfide and the chirality of fluorophores within the binding site was demonstrated.