Polarized light spectroscopy studies of model membranes and DNA

Doctoral thesis, 2014

Over the past decades Linear Dichroism (LD) spectroscopy has proven to be a powerful tool for studies of structure and functioning of different cell constituents, including DNA, cell membranes and proteins. Due to structural complexity of cell constituents, the LD studies of biomolecules depend on development of simplified model systems, which can mimic the original structure and functionality of the cell components. The results presented in thesis concern investigation of LD model systems and include two parts. The main part of this thesis (Papers I-IV) presents investigations of cell membrane models, while the final part of this thesis (Paper V) introduces a model system that can be used for DNA studies. Papers I and II deepen the knowledge concerning one of the most common membrane mimic - DOPC liposomes. Paper I rapports the impact of the viscosity and applied shear force on the behaviour of this membrane host. Paper II demonstrated how, by using theoretical orientational models, the microscopic orientation of the sample can be monitored. Further, in papers III-IV a new lipid model, so called bicelles, is introduced for LD studies. This lipid system can form, at certain conditions, disc-like bilayers, which we find can be aligned by shear flow, resulting in a system having low light scattering and greater orientation than liposomes. The final part of this thesis introduces a particular kind of imperfectly paired DNA as a model system for studies of the transition-states of DNA. Probing of DNA was performed by monitoring the DNA-interaction with a bidppz-bridged binuclear ruthenium complex (ΔΔ-P) that has a known selective affinity for mismatched regions of DNA. While for native DNA, the intercalation of ΔΔ-P complex takes hours at high temperature to complete, it is demonstrated here that the ruthenium complex undergoes very rapid intercalation into re-annealed DNA already at room temperature, indicating that introduction of the static imperfectly paired DNA-structures greatly decreases the activation energy for the threading process.