Pectin gelation with chlorhexidine: Physico-chemical studies in dilute solutions

• Pectin gelation was observed with a pharmaceutical ingredient, chlorhexidine.
• Binding behaviours of pectin with Ca2+ ions and chlorhexidine are partly similar.
• Binding interactions to pectin are stronger with chlorhexidine than with Ca2+ ions.

Low methoxyl pectin is known to gel with divalent cations (e.g. Ca2+, Zn2+). In this study, a new way of pectin gelation in the presence of an active pharmaceutical ingredient, chlorhexidine (CX), was highlighted. Thus chlorhexidine interactions with pectin were investigated and compared with the well-known pectin/Ca2+ binding model. Gelation mechanisms were studied by several physico-chemical methods such as zeta potential, viscosity, size measurements and binding isotherm was determined by Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR). The binding process exhibited similar first two steps for both divalent ions: a stoichiometric monocomplexation of the polymer followed by a dimerization step. However, stronger interactions were observed between pectin and chlorhexidine. Moreover, the dimerization step occurred under stoichiometric conditions with chlorhexidine whereas non-stoichiometric conditions were involved with calcium ions. In the case of chlorhexidine, an additional intermolecular binding occurred in a third step.