[Study of the irreversible conformation change of immunoglobulin M by spin-labeling at the carbohydrate and peptide moieties of the molecule].

The irreversible conformational change of the immunoglobulin M (IgM) molecule (Waldenström disease) at pH approximately 3 was studied by means of spin-labels introduced in the carbohydrate (2,2,6,6,-tetramethyl-4-aminopiperidine-1-oxyl) and peptide (2,2,5,5,-tetramethyl-3-(dichloro-symm.-triazinylamino)-pyrrolidine-1-oxyl) moieties of the molecule. A marked rise of structure density of IgM especially in the (Fc)5-region and some minor local conformational changes in the Fab-regions were found. Comparison of our findings with the published data shows that Fab-regions of the principal immunoglobulins are rigid structures. Steric hindrance for Fab-regions increases markedly in the row Fab--F(ab')2--IgG--IgA--IgM restricting their spatial mobility. Monomeric Fc-regions of IgM are evidently flexible and one of the domains is especially mobile. It is supposed that oligosaccharide groups of IgM are of two types which differ in their spatial mobility. It was found by ammonium sulfate precipitation of IgM spin-labeled at the peptide moiety that the relative mobility of amino acid residues coupled with spin-label is strongly restricted.