Conformational changes induced in antipoly( l-prolyl) antibodies by oligoproline haptens of different sizes

Abstract

Difference absorption spectra, circular dichroism and fluorescence of anti-poly( l-prolyl) antibodies of the IgG and IgM class and the papain derived F ab fragment of anti-poly( l-prolyl) IgG were studied as function of the binding of a series of oligo- l-proline haptens. Optical changes as well as inhibition of passive micro-hemagglutination could be observed upon interaction with tetra, penta and hexa- l-proline but not with tri- l-proline. The difference absorption spectra of anti-poly( l-prolyl) antibody-hapten complexes exhibited two maxima at 284 nm and 292.5 nm. In the circular dichroism band at 290.5 nm the positive ellipticity was enhanced by about 20%, upon addition of excess penta- or hexa- l-proline, whereas the papain derived F ab fragment showed no change in that same band. No shift in the fluorescence emission maximum (340 nm) of the antibodies was observed upon addition of tetra-, penta- and hexa- l-proline. The ligand tetra- l-proline caused an increase of 13.5% in fluorescence intensity of the intact IgG antibodies and 12% in that of the F ab fragment. The same enhancement was observed upon addition of penta- and hexa- l-proline to F ab. The initial increase in fluorescence intensity of anti-poly( l-prolyl) IgG and IgM upon titration with these larger haptens was followed by a decrease at higher concentrations of oligopeptide. We conclude that the helical penta- and hexa- l-proline are monovalent antigenic determinants which cause a conformational transition in the intact antibody molecule following the primary step of its binding.