Conformation of the hinge region and various fragments of human IgG3

A study of the circular dichroic (CD) spectra of various fragments of human IgG3, including the isolated hinge region, Fh, has shown that the hinge region has a high degree of an unusual secondary structure, unique within immunoglobulin material recorded to date. This structure appears to be rigid and aperiodic throughout the hinge region and is compatible with a repeated amino acid sequence. The conformation of the isolated Fh fragment is the same as that of the bound hinge region; also, there is no substantial conformational interaction between the hinge region and the Fab or Fc fragments of human Igtg3. a comparison of the CD spectra of Fc and pFc fragments isolated from an IgG1 and an IgG3 myeloma protein has shown that subclass differences of amino acid sequence do not substantially alter the conformation of these fragments.