Dynamics of biological membranes and associated water.

Licentiate thesis, 2008

An ingenious assembly of biomolecules and water constitute what we call biological tissue. The presence of water in this assembly is fundamental for the physiological processes and can make the difference between life and death. This thesis is concerned with fundamental questions related to the dynamics of water and its presence around all living cells. The cell is surrounded by a membrane that not only serve as a covering, within which the cell can function, but also by means of transport for life essential species. The capacity of membrane to cooperate with its surrounding is life essential and made possible by to proteins and polar lipids, essentially phospholipids that make the membranes soft. Phospholipids are so called amphipathic molecules, i.e. they have a hydrophobic part (repels water) and a hydrophilic (water loving) part. The hydrophilic region is usually referred to as the head group, and the hydrophobic part is known as the tails. The amphipathic character of the phospholipids enable them to form lamellar structures called bilayers. In this work hundreds of parallell lipid bilayers on a surface have served as our model system. One bilayer is composed of two layers of lipids arranged so that their hydrocarbon tails face one another, while their charged head groups face the water on either side of the bilayer. We have studied the influence of water in two different membrane systems. One is naturally existing in the plasma membrane of Halobacterium salinarum, which is a bacterium that lives in extremely salty conditions. The other membrane system we have used is from the lecithin group of lipids whose lipid head is made of the alcohol choline (CH3)3N+CH2CH2OH. The lecithins are involved in the transport of other lipids and exist abundantly in the liver and in egg yolk. At biological temperatures there is a mix of many fast and slow motions which makes it diffcult to find out different motions and their possible interrelations. For moderate hydration levels there is no crystallization of the water within the lipid bilayers (not even at temperaures as low as 80K), however there is a substantial slowing-down of the dynamics with decreasing temperature and below a certain temperature the lipids are frozen in a glassy phase. Starting at such low temperatures the onset of different motions of the water and in the lipid systems were probed at successively higher temperatures by using broadband dielectric spectroscopy, differential scanning calorimetry and quasielastic neutron scattering. We found that the molecular motions of the lipids is similar to the molecular motions of glass forming liquids at low temperatures and this behavior was dependent on the amount of water in the lipid system. Transport of charge carriers was also found to be strongly dependent on the water content and this charge transport was changed at the gel-to-liquid transition of the lipids. Furthermore, it was shown that already at 120K (−153◦C) there is local fast motions of the lipid tails, that should be independent of the water content. This work highlights a complex cooperation between water activity and motions in membranes. By shedding light on the coupling between water and membrane motions new insights into the role of water for membrane properties can be gained.