Changes in calorimetric parameters and solvent accessibility of hydrophobic groups in native and chemically modified immunoglobulin G

Structural rearrangements and aggregation resulting from covalent modificat ion of human IgG by caprylic ester of N-hydroxysuccinimide were studied by differential scanning calorimetry, dynamic light scattering and ANS-binding spectrofluorimetry. The thermogram for the native IgG displays only one tr ansition peak (T-max = 68 degrees C, Delta H = 20.9 +/- 0.3 J/g). For the c hemically modified IgG the temperatures corresponding to the initial and ma ximal deviations of the heat now as well as the area under the transition p eak decreased as the number of attached alkyl groups increases. This findin g may be explained by weakening the intramolecular interactions responsible for the rigidity of the IgG molecular structure and by an increase in the protein-protein interactions for the modified IgG. Dynamic light scattering data indicate spontaneous aggregation of the modified IgG molecules in aqu eous solution; the size of aggregates depends on the modification degree. T hese data correlate with a drastic increase in the surface hydrophobicity i ndex for IgG molecules with an increase in the number of attached alkyl cha ins.