Glycosylated derivatives of tetraphenyl porphyrin: photophysical characterization, self-aggregation and membrane binding

Abstract

A series of glucose- or galactose-residue-bearing tetraphenyl porphyrins (TPPs) has recently been synthesized with the aim of studying the structural dependence of porphyrin cellular localization and efficiency in photodynamic therapy (PDT). For the present investigation, four derivatives with different (spherical or planar) configurations and/or different hydrophobicity have been selected. As a first step, singlet-state spectroscopic properties such as spectral characteristics, lifetime and quantum yield in different solvents are determined. It is found that besides the solvent composition, the configuration of the molecule critically influences the singlet-state spectroscopic properties. Possible self-aggregation processes are investigated at room temperature in phosphate buffer (pH = 7.4). Dimerization equilibrium constants are determined. These values are found to be dependent on the chemical structure of substituents on the tetraphenyl groups and only slightly on the conformation of the whole molecule. The association of non-aggregated forms of porphyrins to unilamellar liposomes, modelling the lipid bilayer of a biological membrane, is studied by fluorescence spectroscopy at neutral pH. On mixing with liposomes, amphiphilic porphyrin derivatives exhibit an increase in their fluorescence intensity and lifetime. The monomer-liposome binding constants are determined for these derivatives. The localization of liposome-bound dyes is studied by fluorescence labelling of (a) the lipid region in interaction with both lipid chains and headgroups or (b) the carbohydrate chain region of lipids. Alterations in fluorescence intensities of porphyrin derivatives in the presence of liposomes and changes in the lifetime and fluorescence excitation anisotropy of fluorescent markers in liposomes are detected only for unsymmetrically substituted amphiphilic porphyrin derivatives. Our results suggest that the presence of at least one apolar substituent on tetrapyrrolic ring is required for the localization in the lipid phase.