The interaction of 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) with chitosan (CS) is investigated by studying the changes that HPTS produces on the properties of CS. HPTS specifically binds to the free amino groups of CS modifying its zeta potential, isoelectric point, water solubility, aggregation state, and the viscosity of the solutions. The association of HPTS to CS is a positive cooperative process described by the Hill equation. In a first stage, HPTS causes a screening that parallels the one caused by NaCl. A subsequent increase of the HPTS concentration results in conformational changes of CS favoring the union of the HPTS and a massive CS aggregation. These facts, not observed in the presence of NaCl, cannot be explained merely by an electrostatic screening effect caused by HPTS and a cross-linking mechanism is suggested. The effect of the acidic dye pyranine on chitosan and the physicochemical properties of the resulting complex that they form are investigated. The strong electrostatic interaction among the sulfonate and the amino groups of pyranine and chitosan, respectively, results in a cooperative association of the dye to the macromolecule. The dye causes the screening of the chitosan charge and promotes its aggregation, probably, acting as a cross-linker. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
- cooperative association