Thermodynamic and functional characterization of a stable IgG conformer obtained by renaturation from a partially structured low pH-induced state.

At pH2, rabbit IgG adopts a partially structured state that exhibits loss of thermal unfolding transition, tentatively assigned to the CH2 domain, whilst retaining a well-defined tertiary structure for the rest of the molecule and extensive secondary structure. Renaturation of IgG from this state yields a stable conformer that differs from native IgG by a lower degree of interaction between the CH2 and CH3 domains, and stronger interaction between the CH1 and CH2 domains, as judged by differential scanning calorimetry and probing the IgG conformation with specific ligands (C1q component of complement, protein A and monospecific antibodies to the CH2 domain and hinge region.