[Modification of surface residues of lysine in immunoglobulin G using the spin marker 2,2,5,5-tetramethyl-3-maleimidopyrrolidine-1-oxyl].

Exposed lysine residues of human IgG were modified by a spin-label, 2,2,5,5-tetramethyl-3-male-imidopyrrolidine-1-oxyl at pH 9.2. Under these conditions, the degree of modification was about 10 lysine residues per protein molecule. The ESR spectrum of the spin-labeled immunoglobulin was much more mobile than that of spin-labeled immunoglobulin with the modification degree of about 1 residue that was obtained at pH 7.0. Thus, the sharp increase in the modification degree due to the increase in pH by two units leads to a marked loosening of the tertiary structure of the protein in solution, which is just indicated by the mobile ESR spectrum. Lithium chloride added to the solution of spin-labeled immunoglobulin induces a similar "immobilization" of its ESR spectra as sucrose.