Circular Dichroism of Immune Complexes, IgG and Faby with Unique Antigenic Determinants from Rheumatoid Serum

IgG-lgG and IgG-IgM complexes were isolated from one rheumatoid arthritis (RA) serum by affinity chromatography to immobilized F(ab′)2γ of specific antibodies against unique determinants in the complexes. Both IgG und IgM, when isolated from these complexes, contained the unique determinants. The circular dichroism (CD) spectrum of IgG differed from that of normal IgG at neutral pH. At pH 3 both IgG and IgM displayed normal CD spectra, and only one third of the molecules now had affinity for the immobilized ligand. The molecules with affinity at pH 3 exhibited an abnormal CD spectrum at pH 3, and a normal CD spectrum was obtained only of those components that lacked affinity. Onethird of the Fabγ isolated from the IgG and IgG complexes with the unique determinants contained the unique determinants that were lacking in the rest of the Fabγ and in all of the Fcγ fragments. The CD of the two Fabγ preparations showed the same principal differences as the CD of the molecules with and without affinity to the ligand at acidic pH.