Lay summary
The current research project is concerned with the study of porphyrin model membrane interactions utilizing NMR spectroscopy as main tool.
Porphyrinic compounds are known to localize and accumulate in rapidly dividing tissues. Based on these and other advantageous characteristics, such as their light absorption properties, porphyrins are studied as sensitizers in Photodynamic therapy (PDT). Primary localization sites of porphyrinic PDT drugs are cellular membranes. However, uptake mechanisms and the reasons for selectivity of porphyrin uptake are still not clear.
The long-term goal of this project is therefore to enhance the understanding of the principles underlying porphyrin - membrane interactions which may in turn contribute to improved, target-oriented drug design for PDT. Moreover, this project points out the potential of NMR spectroscopy as powerful tool to study porphyrin membrane interactions.
Unilamellar dioleoylphosphatidyl choline (DOPC) vesicles serving as model membranes were investigated systematically in the presence of the porphyrinic compounds chlorin e6 (CE) and monoaspartyl-chlorin e6 (MACE).
Analysis of the model membrane-1H spectra in the presence of chlorin derivatives provided information on i) whether the compound incorporates or not, ii) its average location at the model membrane, and iii) if and how fast the porphyrin distributes across the phospholipid bilayer. The NMR studies revealed that porphyrin membrane interactions strongly depend on factors such as the degree of ionization, pH, porphyrin aggregation behavior, and porphyrin concentration. Time-dependent porphyrin distribution between the phospholipid monolayers preceded exponentially, was accelarated at low pH and high porphyrin concentration, and strongly correlated with porphyrin structure, i.e. side chains imposing a certain polarity. NMR-diffusion and dynamic light scattering measurements revealed that the integrity and homogeneity of the supramolecular model membrane structure were retained upon porphyrin association. Both, diffusion data and complementary results obtained with Magic Angle Spinning (MAS) NMR suggested that the porphyrin exists in two different environments, the vesicular phase and porphyrin self-associates in the aqueous bulk phase.
Further investigations of this ongoing project are aimed at a more detailed mapping of the membrane-associated porphyrin at the submolecular level and will also include structurally different porphyrins and various membrane modifications. The expected results may help to evaluate conclusions drawn from our previous studies and may provide a basis for elaborating general principles for porphyrin membrane interactions.