The conformational characteristics of Congo red, Evans blue and Trypan blue

The structures of the closely related bis-azo dyes Evans blue, Trypan blue and Congo red, which appeared to have different self-assembly properties an d correspondingly different abilities to form complexes with amyloids and s ome other proteins, were compared in this work. Ab initio and semi-empirica l methods were used to find the optimal structures and partial charge distr ibutions of the dyes. The optimal structures were searched using different widely used programs. The structures of Congo red and evans blue were found to be planar, except for the torsion on the central diphenyl bond connecti ng the two halves of the dye. Both symmetrical parts of the molecules appea red very close to planarity. However, Trypan blue exhibits non planarity on the di-azo bonds, as well as on the central bond between the symmetrical p arts of the dye. In a consequence, the non planarity of this molecule is hi gher than in the case of its isomer, Evans blue and co ngo red as well. The extra rotation around the azo bonds extorted by the close proximity of the sulfonic groups may be the direct cause of its poor self-assembling and co mplexation properties versus Evans blue. (C) 2000 Elsevier Science Ltd. All rights reserved.