Large increase in thermal stability of the CH2 domain of rabbit IgG after acid treatment as evidenced by differential scanning calorimetry

Rabbit IgG after exposure to 0.05 M glycine-HCl, pH 2.0, and native IgG were compared by diffential scanning calorimetry (DSC) at pH 3.5 and C1q binding studies at pH 7.8. For acid-treated IgG, a large increase (by approx. 12–15°C) in thermal stability of the CH2 domain occurs and this domain no longer demonstrates a separate and thermodynamically independent unfolding at 56°C seen for native IgG. The results suggest that stabilization of the CH2 domain in acid-treated IgG arises from stronger, relative to the native protein, interaction of the CH2 domain with adjacent and more stable IgG domain(s). Conformational differences of the two forms of IgG were confirmed at neutral pH by a 4-fold increase of C1q-binding affinity of acid-treated IgG.