CONFORMATIONAL STUDIES OF IMMUNOGLOBULIN G AND ITS SUBUNITS BY METHODS OF HYDROGEN-DEUTERIUM EXCHANGE AND INFRARED SPECTROSCOPY

1. 1. A comparison of the infrared spectra (Amide A, B, I and II bands) of immunoglobulin G (IgG) with those of proteins of known conformation suggests that the secondary structure of IgG involves mainly irregular and β conformation. The change of the shape of the Amide II band in the course of 1H → 2 exchange shows that the regions of the protein molecule that have different structures are characterized by different conformational motilities. 2. 2. At pH 5.1-9.5, the shape of the infrared bands and the pH dependence of the 1H → 2H exchange rate of the isolated light chains and Fab and Fc fragments are similar to those of the intact IgG molecule. This indicates that at neutral pH the light chains and the Fab and Fc fragments are similar to the parent IgG molecules in their relative contents of β structure and/or irregular conformation as well as in compactness. The peptide NH groups seem to make no significant contribution to the interaction between the light and heavy chains. 3. 3. At pH < 5.1, the native structure of the Fc fragment is destroyed whereas the conformational properties and the compactness of the Fab fragments and of the light chains do not change significantly even at pH 2. The conformational changes that are seen in intact IgG molecules at pH about 3.9 seem to occur preferentially in their Fc components. © 1969.