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 Ig G were compared by differential scanning calorimetry (DSC) at pH 3.5 a nd C1q binding studies at pH 7.8. For acid-treated IgG, a large increa se (by approx. 12-15 degrees C) in thermal stability of the CH2 domain occurs and this domain no longer demonstrates a separate and thermody namically independent unfolding at 56 degrees C seen for native IgG. T he results suggest that stabilization of the CH2 domain in acid-treate d IgG arises from stronger, relative to the native protein, interactio n of the CH2 domain with adjacent and more stable IgG domain(s). Confo rmational differences of the two forms of IgG were confirmed at neutra l pH by a 4-fold increase of C1q-binding affinity of acid-treated IgG.