Stefan Bössinger
Coupling the thermodynamic state with the dynamics of lipid interface layers - and its possible role for biology
Supervisor: Prof. Dr. Matthias Schneider [Experimental physics I]
Date of oral examination: 09/18/2012
126 pages, english
Hydrated interfaces and lipid membranes are ubiquitous throughout the biological world. Although these systems have been thoroughly studied under static conditions over the past decades, their dynamic properties are only poorly understood. In particular, longitudinal waves in lipid monolayers have not received much attention in biology. However, it has been proposed recently that such waves might be the underlying principle for the propagation of action potentials in nerves. The central topic of this thesis is therefore to investigate the properties of sound wave propagating in lipid-based interfaces. Experimentally this is accomplished by the excitation of pressure pulses resulting from the fusion of a solvent drop with the air-water interface of a lipid monolayer. The initial development of a comprehensive understanding for the excitation process of longitudinal lipid monolayer waves delivers a sound foundation for further experiments conducted within the scope of this thesis. For this purpose, pressure pulses were excited by fusion of solvent droplets with the aqueous interface of a lipid monolayer. Despite the highly non-linear compressibility profile of the lipid membrane, a linear stress-strain relation was found for the amplitude of the wave. In combination with the elucidated influence of the type of solvent molecules, a comprehensive picture for the excitation process was found, which allows to precisely determine the excitability of the monolayer. In the next step a detailed study about wave excitation by the vapor phase exerted from solvent drops was conducted. With this contactless excitation method it was possible to induce long-term auto-oscillations of membrane tension. Investigation of the relaxation characteristics of the showed coinciding maxima of relaxation time and monolayer compressibility. In order to study the propagation of waves in an even more realistic model of the cell membrane, monolayers formed from lipids with a poly(ethylene glycol)-conjugated headgroup were used. The latter represents a highly hydrophilic moiety which closely mimics carbohydrate residues that constitute the glycocalyx of cells. The increased interaction with the supporting aqueous sub phase led to augmented damping of the waves and linearized their velocity profile. In subsequent experiments, a drastic reduction of membrane excitability was found upon the introduction of Gadolinium ions to the sub phase. Even submillimolar concentrations were sufficient to abolish excitability of the interface. A similar effect was found for other lanthanides e.g. Lanthanum and Neodymium. It seems very likely that the blockage of monolayer waves by these highly charged ions is due to a complexation of phospholipid molecules. Moreover, the resulting solidification of the membrane could represent a potential explanation as to why lanthanides are also potent blockers of mechanogated ion channels in nerves and muscles. Finally, the effects upon collision of longitudinal monolayer waves was investigated. In a nerve cell, two colliding action potentials annihilate each other. In contrast, two colliding monolayer waves create interference patterns in surface tension. By considering the path length for each pulse, these patterns can be calculated, proofing the classical wave behavior of the phenomenon observed. The experimental results on the properties of acoustic monolayer waves presented in this thesis will contribute to a better understanding of pulse propagation in biological systems and in particular of action potential propagation in nerves.