Influence of pH on aggregation and photoproperties of n-(2-hydroxypropyl)methacrylamide copolymer–meso-chlorin e6 conjugates

Abstract

The influence of pH on the aggregation and photoproperties of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers containing meso-chlorin e6 monoethylenediamine (Mce6) attached to the copolymer via either nonbiodegradable G or biodegradable GFLG side chains was studied. Dynamic light scattering, UVIVIS and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, and fluorescence quenching techniques were used. The photosensitizing efficiencies of these conjugates were also determined. The dynamic light-scattering data indicate that the intermolecular aggregation of Mce6 species within the copolymer conjugates is not significant and is not affected by pH or loading of Mce6 to copolymer at 5 × 10–4 g/mL of copolymer conjugate concentration. However, intramolecular aggregation of the Mce6 species within the copolymer conjugates does occur in aqueous buffers, as demonstrated by absorption and fluorescence measurements in ethanol–buffer mixtures. The fluorescence lifetime of excited Mce6 was influenced by aggregation, mainly attributed to the pH and copolymer side-chain hydrophobicity. The Stern-Volmer collisional quenching constant, Ksv iodide anion with Mce6 species was found to be a function of pH, reflecting both the electrostatic repulsion between negatively charged Mce6 species and iodide anions and the intramolecular aggregation of Mce6 moieties. The extent of aggregation was found to be a function of solvent pH, loading of Mce6 to copolymer, and side-chain hydrophobicity. The photosensitizing efficiency of the copolymer bound Mce6, as determined through the photooxidation of furfuryl alcohol, was dominated by Mce6 loading to copolymer and side-chain hydrophobicity, but was only slightly pH dependent. Evidently, the Mce6 aggregation only weakly influenced the charge transfer in the process of oxygen generation.