Quantum chemical study of the spectral characteristics of fluorescein dyes bound to chitosan

Recently, there has been an increased interest in natural polysaccharides, in particular, chitosan, which are widely used in medicine and industry. Chitosan labeled with fluorescein dyes acquires additional optical properties that can be used in sensing and delivery systems. Mechanism of binding of a polymer to a label largely determines the field of its possible applications. The quantum chemical calculation using the B3LYP/aug-cc-pVDZ theory level has been made in order to contribute to the understanding of intermolecular interactions. The geometry of fluorescein, eosin Y, and erythrosin B in the dianionic, monoanionic, and neutral quinoid forms interacting with chitosan has been optimized and the absorption spectra have been calculated using the time-dependent density functional theory taking into account the solvent. The comparison of the calculated absorption spectra with the experimental data has shown a major role of the electrostatic mechanism in binding of anionic dyes to the protonated chitosan groups.