Interaction of a Triaryl Methane Dye with Cucurbit[7]uril and Bovine Serum Albumin: A Perspective of Cooperative versus Competitive Bindings

Interactions of a triaryl methane dye, crystal violet (CV), with cucurbit[7]uril (CB7) and bovine serum albumin (BSA) hosts have been investigated using various photophysical studies. While due to its structural flexibility free CV is very weakly fluorescent its binding to CB7 and BSA hosts leads to significant fluorescence enhancements, the effect being very larger with BSA host than CB7. In both CV-BSA and CV-CB7 systems, CV binds through 1:1 inclusion complex formation, the binding constant (K-b) being as large as similar to 3.4x10(5) M-1 for the former as compared to a moderate K-b of similar to 6.3x10(3) M-1 for the latter system. In CV-CB7-BSA ternary system, where measurements were done keeping one of the host concentration constant and significantly higher and gradually increasing the concentration of the other host in the solution, the changes observed in the absorption and fluorescence characteristics of the dye as well as that observed in the binding isotherms clearly indicate a competitive binding interaction of CV with both CB7 and BSA hosts simultaneously present in the solution. To be emphasized that the competitive binding interaction observed for the CV-CB7-BSA ternary system is similar to the commonly encountered interaction in many dye-host-biomacromolecule kind of ternary systems but differs strikingly from the unique cooperative interaction reported earlier for an analogous dye, brilliant green (BG), with the same CB7 and BSA hosts.