DIFFERENTIATION OF THE SPATIAL MOBILITY OF LYSINE RESIDUES IN IMMUNOGLOBULIN-G BY THE SPIN-LABEL METHOD

Some dynamic and conformational characteristics of human IgG molecules were studied by spin-labeling with -oxyl-2,2,5,5-tetramethyl-3-pyrrol idinyl)maleimide with selectively modifies the lysine residues (Lys) i n this protein. The rotational correlation time tau is found to be 26/-2 nsec, which is typical for the spin-labeling of IgG peptides. Base d on their spatial mobility, spin-labeled Lys can be divided at a 3 : 1 ratio into two groups of more mobile and less mobile residues, respe ctively. A supposition is substantiated that some Lys in IgG have lowe r mobility because they are involved in H-bond formation. The temperat ure dependence of the ordering parameter S attests to pronounced immob ilization of the less mobile Lys. Spin-labeled IgG stored at 4 degrees C for a long time developed EPR spectral peaks similar to those of fr ee (nonbound) spin label. This was caused by autolysis of the samples, which could be entirely suppressed by adding phenylmethylsulfonylfluo ride.