Photophysical properties of erythrosin ester derivatives in ionic and non-ionic micelles

Abstract

The xanthene dye erythrosin B (ERY) has high molar absorptivity and singlet oxygen quantum yield (ΦΔO21), which make it a potential photosensitizer for photodynamic applications. However, ERY is very hydrophilic in water at physiological pH due to its two negative charges, which results in low interaction with biological membranes. In this work we synthesized ester derivatives of ERY: methyl (ERYMET), butyl (ERYBUT) and decyl (ERYDEC) esters. The light absorption process was not greatly affected by the insertion of the alkyl group. The ester hydrophilic–lipophilic balance was evaluated by partition in a 1-octanol/water mixture. The insertion of the alkyl chain and the loss of one charge increased the hydrophobicity and, as expected, ERYDEC was the most hydrophobic dye. The interaction of the esters with the biological membranes was simulated in biomimetic models, micelles of: CTAB (cationic), SDS (anionic) and the polymeric surfactant pluronic F-127® and P-123® (non-charged). The order of the binding constants was ERY < ERYMET < ERYBUT < ERYDEC in all micelles investigated, equal to the hydrophobicity sequence. The localization of the dyes in micelles determined by quenching fluorescence measurements using iodide as a suppressor was consistent with their charge and hydrophobic characteristics. CTAB micelles showed a biphasic interaction due to the association of the cationic monomers of the surfactant and the anionic dyes. The fluorescence quantum yields of the esters in buffered water were equal to those of ERY, with low values of ∼0.02 and a slight increase to ∼0.1 in micellar media, the value of which was still small. The quantum yields of singlet oxygen of all xanthenes investigated were high in micellar systems and similar to each other, independent of the alkyl group (0.6–0.7). ERYDEC showed low ΦΔO21 of 0.38 in buffered water due to the self-aggregation process, but high values in micellar systems. These results in micelles are very important for formulated medications in photodynamic therapy and photodynamic inactivation of microorganisms. Ester derivatives of erythrosin show a potential as photosensitizers in photodynamic clinical applications, especially those formulated with biocompatible surfactants, such as polymeric pluronics.