Formation And Dynamics Of Myelin Of Charged Lipids With Added Electrolytes

Charged lipids constitute a substantial fraction of bio-membranes. The structure and function of the bio-membranes greatly depend on the type and content of the charged lipids. Due to the ubiquitous nature of electrolytes in cells, the electrostatic interaction between bio-membranes and other molecules is important for behaviors of bio-membranes. Therefore, the study of charged lipid systems can advance the understanding of such interactions. So far, several theoretical models have been set up for studying electrostatics of bio-membranes and a lot of experiments have been done to investigate the phase separation, and deformation of charged bio-membranes as well as interactions between charged bio-membranes and other molecules. However, only few studies involve in myelin formation of charged lipids in electrolyte solutions. In this thesis, we investigate the myelin formation of charged lipids with emphasis on morphology evolution and stability. The thesis is organized as follows1) In Chapter 2, we study the effect of electrolytes on the assembling structures of cardiolipin. It is shown that the assembling structures critically depend on the concentration and valency of cations, rather than their types or ionic radii. Anions have negligible effects on the assembling structures. Myelin figures are observed in certain concentration range of univalent cations and in neutral or alkalic medium. The minimum diameter of the myelin structures does not depend on the thickness of the dried lipid membranes and the concentration of electrolytes, and the maximum diameter increases with increasing the thickness of the dried membranes and decreases with increasing the concentration of the electrolytes.2) In Chapter 3, in order to explain the experimental findings of the added salt effect on myelin formation in charged lipid systems, we extend the argument for the neutral lipid systems proposed by Huang-Zou-Witten to the charged lipid systems with added salts by taking into account the electrostatic interaction. The calculations indicate that when the water layer thickness is less than the threshold (or the pressure between the membrane is larger than its threshold), myelin structures can form. Increasing repulsion or decreasing attraction between the membranes, say increasing electrolyte repulsion, hydration repulsion, and decreasing Van der Waals attraction, will induce myelin formation. As the Debye-Huckel screening length increases (corresponding to decreasing the concentration of the electrolyte), myelin will be easier to form due to the increased repulsion between the membranes. When the radius of the water core (R(?)) is small, myelin structures are hard to form as the bilayer number N is small and become easier to form with increasing bilayer number, but when R(?) is large, the bilayer number has little effect. Our calculations agree with most of experimental observations.3) In Chapter 4, we study the growth dynamics of the myelin structures formed from charged lipids. The results indicate that for most myelin structures, the scaling relationship between the myelin length L and its growing time t follows L∝t at the initial stage of the growth, and L∝t1/2 at the following stage; but some myelin structures exhibit a third stage with scaling relationship L∝tp(p<1/2); Besides, there is a small amount of myelin structures only showing one scaling relationship L∝tp(p≈1) through the whole growing process. The type and concentration of the electrolytes do not influence the scaling relationship between the myelin length L and its growing time t. But the growth rate increases with increasing concentration of the electrolytes.4) In Chapter 5, we study the coil instability of myelin figures. During myelin formation, besides straight myelin structures, lots of coiling structures form due to instability. The coiling instability of myelin structures greatly depends on the concentration of the electrolytes, and only happens in certain concentration of the electrolytes. The types of the electrolytes have little effect on such coiling instability. The coiling myelin structures include single, double, and super helical structures. The coiling instability happen ether during the myelin growth or after the myelin stops growing. The scaling relationship between the length of coiling structure L and its growing time t is similar to the myelin growth without coiling.