Secondary structural properties of the oligomeric forms of mouse IgA and the unusual effect of guanidine hydrochloride.

The circular dichroism spectra of the monomer, dimer and tetramer forms of MOPC 167 mouse IgA are reported. The spectra were very similar in the aromatic region (250-310 nm) but the intensity of the negative band at 217 nm, arising from the peptide backbone, was higher in the dimer and tetramer (-4600 degree cm2 dmol-1) than in the monomer (-3400 degree cm2 dmol-1). When monomer and dimer IgA were exposed to concentrations of guanidine hydrochloride around 1.3 m, the 217 nm band increased in intensity, twofold in the case of the monomer, and solutions of the dimer became turbid, indicating aggregation of the proteins and formation of a cross-beta-structure. At concentrations of guanidine hydrochloride of above 2.3 M both proteins were similarly denatured, indicating that dimerization through J chain does not alter the stability of IgA. The aggregating effect of guanidine hydrochloride did not occur with samples of human IgA1 and IgA2 and may be related to the unusual CH1 domain of mouse IgA.