CONFORMATIONAL-CHANGES INDUCED IN ANTI-POLY(L-PROLYL) ANTIBODIES BY OLIGOPROLINE HAPTENS OF DIFFERENT SIZES

Difference absorption spectra, circular dichroism and fluorescence of anti-poly(l-prolyl) antibodies of the IgG and IgM class and the papain derived Fab 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 Fab 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 Fab fragment. The same enhancement was observed upon addition of penta- and hexa-l-proline to Fab. 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. © 1979.