The effect of pH and surfactant on the aggregation behavior of chlorin p6: a fluorescence spectroscopic study.

Steady state and time-resolved fluorescence properties of chlorin P6, a potential drug for photodynamic therapy, have been investigated as functions of pH. A decrease in pH of the medium has been shown to cause protonation of the ionizable carboxylic acid side chain, leading to an increase in hydrophobicity and consequent aggregation. The aggregates dissociate on further protonation. The dissociation is explained in terms of formation of cations and their mutual repulsion. A synchronous fluorescence spectroscopic study revealed the presence of two anionic forms in equilibrium at physiological pH, with a shift in the equilibrium on slight decrease in the pH. The anionic nature of chlorin P6 in aqueous solutions at physiological pH has been confirmed by complexation with surfactants. The nature of the charge on the headgroups of the surfactants has been found to govern the formation of chlorin-surfactant complexes.