Supramolecularity creates nonstandard protein ligands

Congo red and a group of structurally related dyes long used to stain amylo id proteins are known to associate in water solutions. The self-association of some dyes belonging to this group appears particularly strong. In water solutions their molecules are arranged in ribbon-like micellar forms with liquid crystalline properties. These compounds have recently been found to form complexes with some native proteins in a non-standard way. Gaps formed by the local distribution of beta-sheets in proteins probably represent th e receptor sites for these dye ligands. They may result from higher structu ral instability in unfolding conditions, but also may appear as long: range cooperative fluctuations generated by ligand binding. Immunoglobulins G we re chosen as model binding proteins to check the mechanism of binding of th ese dyes. The sites of Structural changes generated by antigen binding in a ntibodies, believed to act as a signal Propagated to distant parts of the m olecule, were assumed to be suitable sites for the complexation of liquid-c rystalline dyes. This assumption was confirmed by proving that antibodies e ngaged in immune complexation really do bind these dyes; as expected, this binding affects their function by significantly enhancing antigen binding a nd simultaneously inhibiting C1q attachment. Binding of these supramolecula r dyes by some other native proteins including serpins and their natural co mplexes was also shown. The strict dependence of the ligation properties on strong self-assembling and the particular arrangement of dye molecules ind icate that supramolecularity is the feature that creates non-standard prote in ligands, with potential uses in medicine and experimental science.