The structure and protein binding of amyloid-specific dye reagents

The self-assembling tendency and protein complexation capability of dyes related to Congo red and also some dyes of different structure were compared to explain the mechanism of Congo red binding and the reason for its specific affinity for /3-struc- ture. Complexation with proteins was measured directly and expressed as the num­ber of dye molecules bound to heat-aggregated IgG and to two light chains with dif­ferent structural stability. Binding of dyes to rabbit antibodies was measured indi­rectly as the enhancement effect of the dye on immune complex formation. Self-as­sembling was tested using dynamic light scattering to measure the size of the supra- molecular assemblies. In general the results show that the supramolecular form of a dye is the main factor determining its complexation capability. Dyes that in their compact supramolecular organization are ribbon-shaped may adhere to polypeptides of /3-conformation due to the architectural compatibility in this unique structural form. The optimal fit in complexation seems to depend on two contradictory factors involving, on the one hand, the compactness of the non-covalently stabilized supra- molecular ligand, and the dynamic character producing its plasticity on the other. As a result, the highest protein binding capability is shown by dyes with a moderate self-assembling tendency, while those arranging into either very rigid or very unsta­ble supramolecular entities are less able to bind.