The Role of Interchain Disulphide Bridges in the Conformational Stability of Human Immunoglobulin G1 Subclass

The hydrogen‐deuterium exchange data of human immunoglobulin G1 (IgG 1) are interpreted by assuming fast fluctuations of the protein conformation, through which the peptide groups become exposed to the solvent. The probability of solvent exposure of peptide hydrogens reflects a rather loose conformation for native IgG in comparison with other globular proteins. The probability of solvent exposure is greater than 10 ‐3 for half of the peptide groups, which shows that the conformational transitions by which these groups are exposed to the solvent are accompanied by changes in standard free energy less than 17 kJ/mol (4 kcal/mol). In the range of pH 6.2–8.45, at 25 °C no gross conformational changes are reflected in the hydrogen‐deuterium exchange behaviour of the native, the reduced‐nonalkylated‐reassociated and the reduced‐S‐alkylated‐reassociated IgG 1. No difference could be detected in the conformational stability of the native and reoxidised reassociated IgG1 proteins. The lack of inter‐subunit disulphide bridges in S‐alkylated‐reassociated molecules results in all increased conformational motility. This destabilization of protein conformation affects about 90% of the peptide groups covered by the measurements, and corresponds to changes in standard free energy of 8 kJ/mol on the average.