Va. Lapuk et al., Modification of surface lysine residues in immunoglobulin G by spin-label 2,2,5,5-tetramethyl-3-maleimidopyrrolidine-1-oxyl, BIOFIZIKA, 44(5), 1999, pp. 806-810
Exposed lysine residues of human IgG were modified by a spin-label, 2,2,5,
5-tetramethyl-3-maleimidopyrrolidine-1-oxyl at pH 9.2. Under these conditio
ns, the degree of modification was about 10 lysine residues per protein mol
ecule. The ESR spectrum of the spin-labeled immunoglobulin was much more mo
bile 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, whic
h is just indicated by the mobile ESR spectrum. Lithium chloride added to t
he solution of spin-labeled immunoglobulin induces a similar <<immobilizati
on>> of its ESR spectra as sucrose.