Capillary electrophoretic investigations on the interactions between rhodamine 6G and poly(vinyl sulfate)

Abstract

In capillary electrophoresis for rhodamine 6G (R6G) solution containing poly(vinyl sulfate) (PVS), which was also added to the running buffer, a signal of R6G was found to be shaped like a square wave. As the R6G concentration was increased at a fixed concentration of PVS, the width of the square-wave type signal increased, although the signal intensity remained constant. In the increase of the signal width, the migration time on the falling edge of the square-wave type signal was lengthened, whereas nearly the same migration time was observed for the rising edge of the signal. When the concentration of PVS decreased at a fixed concentration of R6G, the width of the square-wave type signal decreased, accompanied by an increase in the signal intensity. A simulation based on a model where the binding constant of R6G toward PVS has either a large or small value, depending on the number of R6G molecules in a small domain, has well reproduced the dependence of the signal shape on the R6G concentration as well as the PVS concentration. The assumption in the simulation indicates the cooperative binding of R6G to PVS. Under the conditions that the sum of the concentrations of 1,1′-di- n-heptyl-4,4′-bipyridinium dibromide and R6G has been kept constant, the increase in the 1,1′-di- n-heptyl-4,4′-bipyridinium dibromide concentration has resulted in a decrease in the signal intensity of R6G, without a change in the signal width. This finding is explained by the competitive binding of the R6G and 1,1′-di- n-heptyl-4,4′-bipyridinium cations to PVS.