Amylose–dye complexes in cationic micelles: an optical spectroscopy study

Abstract

The formation of amylose complexes with rose bengal (RB), erythrosine B (ER), and phenolphthalein (PP) in the presence of the cationic detergent tetradecyltrimethylammonium bromide (TTABr) was studied using optical spectroscopy methods. Absorption spectroscopy, steady-state fluorescence spectroscopy and picosecond time-resolved fluorescence spectroscopy were used to derive association constants k s of the dyes, critical micelle concentration (CMC) values and structural information on the complexes formed. It seems that PP fits very well into amylose sites, where it forms an efficient inclusion complex with k s=44,500 M −1. The molecular diameter of RB is too big to fit the amylose cavity. Only part of the xanthene unit may be adopted in the helical cavity of amylose, whereas most of the interaction occurs through electrostatic and/or dipole–dipole interactions with the amylose chain. The ER molecule is an intermediate case, because it may fit the amylose cavity or adsorb on the amylose surface to form a complex. The presence of a surfactant in the amylose–ligand system increases the association constant for all dyes. In the presence of amylose, a decrease of the detergent CMC value of about one order of magnitude is observed. It is probable that the increased number of micelles incorporate more dyes into the amylose vicinity, which finally changes the structure of the amylose chain. On a macro scale, it was noted that the samples with dyes and detergent have a lower tendency to precipitate and the gelation process is delayed compared to that in water.