Erythromycin A as a supramolecular receptor: Association with dyes

Abstract

Several organic compounds, such as p-methyl red (1), o-methyl red (2), p-nitroaniline (3), sulforhodamine B (4), and rhodamine 6G (5) form molecular complexes in non-aqueous solutions with the antibiotic erythromycin A (E). The association constants (K_Ass) for these complexes were determined by spectrophotometric titrations. Enthalpies and entropies of complexation in chloroform were calculated from the temperature dependence of K_A for 3, 4, and 5. The association process appears to be mainly entropy controlled. In chloroform solution, p-methyl red associates with E while o-methyl red does not. On the other hand both compounds show a weak interaction in dioxane. The differences in behavior in the two solvents are attributed to a change in conformation of E which allows a favorable interaction with 2 in dioxane but not in chloroform. Rhodamine 6G forms dimers in dioxane and its association with E destroys them.