【作者】 林美玉；

【导师】 杨玉良；

【作者基本信息】 复旦大学 ， 高分子化学与物理， 2007， 博士

【摘要】 带电磷脂是生物细胞膜的重要组成成份。细胞膜的结构性质及许多重要的生物功能,都与膜中带电磷脂的种类、含量直接相关。由于细胞膜所处的环境是电解质溶液,所以普遍存在的膜与膜之间或膜与各种分子之间的静电相互作用对生物膜的结构和性能起了关键作用。因此在电解质溶液中研究带电磷脂膜的形态和生长动力学有助于深入理解这类相互作用。关于带电双分子层膜的静电性质以及电解质对其影响,理论学家们已建立了各种理论模型,实验上较多关注的是电解质对含带电磷脂多组分膜的分相、形变的影响,以及带电磷脂膜与某些生物大分子的作用。而对电解质溶液中带电磷脂的多层管状囊泡的形成及动力学的研究却非常少。因此,我们主要围绕着这个问题展开研究,主要内容及结果如下：1)研究了电解质种类、价态、浓度对带电磷脂聚集形态的影响。结果表明,阳离子的价态、浓度对心磷脂的聚集形态有很大影响,而离子半径和同一价态下不同离子种类的影响较小；阴离子种类、价态对带电磷脂聚集形态的影响很小。在一定的一价阳离子电解质溶液中,心磷脂的主要组装形态是同中心多层管状囊泡(myelin结构)。系统考察了其它实验条件对myelin形成及结构的影响。结果表明,在固定电解质浓度下,中性pH值较酸性的容易形成myelin。Myelin的最小直径基本不受干膜的厚度和电解质浓度的影响,而最大直径随干膜厚度的增加而增大,随电解质浓度的减小而减小。2)为了解释为什么带电磷脂在电解质溶液中能形成myelin结构,我们将Witten关于非带电膜myelin的形成理论推广到带电膜体系,即膜间相互作用除了范德瓦尔斯力、疏水排斥力外,还增加了静电排斥力。计算了与实验相对应的各个参数刘myelin形成的影响,结果表明,当多层膜间的压力超过临界值(或膜间距小于临界值时),myelin结构的能量低,体系主要以myelin形式存在。增加膜间排斥力或减小膜间吸引力,如减小范德瓦尔斯力,增加疏水排斥力、静电排斥力等均有助于myelin的形成,反之,不利于myelin的形成；增加myelin的曲率能不利于myelin的形成。随着电解质浓度的减小,膜与膜之间的排斥力越来越大,越有利于myelin的形成。当myelin的中心水腔(R(?))很小时,层数N对myelin的形成有很大的影响,层数越多越有利于myelin的形成；而当中心水腔(R(?))比较大时,N对myelin形成的影响很小。这些计算结果与实验现象基本相符。3)研究了电解质溶液中带电磷脂的myelin的生长动力学,以及电解质种类、浓度对其影响。结果表明,对于大多数myelin,其生长初期基本符合L∝t,但持续时间较短(一·般为2～8秒),而生长后期符合L∝t1/2(持续十几至几十秒)；部分(?)nyelin除了以上这两个阶段外,还有第三阶段,在第三阶段中,L∝tp且p<1／2；极少数myelin在生长的整个过程符合一个标度关系,即L∝tp(p≈1)。这种标度关系基本上不受电解质的种类和浓度影响,但随着电解质浓度的增加,myelin的生长速率常数增大。4)考察电解质种类、浓度对螺旋结构myelin形成的影响。结果表明,螺旋结构的形成基本不受电解质种类的影响,但受电解质的浓度影响比较大,只有在一定的电解质浓度范围内,才能形成螺旋结构。螺旋结构有规则的和不规则的,包括单螺旋、双螺旋和超螺旋结构。螺旋结构的形成主要有两种方式,其一是伴随着myelin的生长,即一边生长,一边盘绕,对于从一开始就以麻花状结构生长的双螺旋,其长度与时间的标度关系与非螺旋结构的myelin的相似；其二是先形成myelin后再盘绕,这种方式形成螺旋结构的速率很快,一般在几秒甚至更短时间内完成。对于单根myelin形成的双螺旋,麻花状的双螺旋结构比一段是直的、另一段盘绕在上面的双螺旋结构稳定。更多还原

【Abstract】 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.更多还原