Hauser H
Laboratorium für Biochemie, Eidgenössische Technische Hochschule Zürich, Switzerland.
Chem Phys Lipids. 1991 Mar;57(2-3):309-25. doi: 10.1016/0009-3084(91)90083-n.
The effect of protons and cations on the crystal (gel)-to-liquid crystal transition temperature Tm of isoelectric and negatively charged phospholipids are summarized. The general trends emerging are as follows: Tm depends on the state of ionization of the phospholipid in that Tm-vs-pH-curves parallel the titration curve of the phospholipid. Protonation of phospholipids causes Tm to increase, deprotonation or ionization has the opposite effect. The effects of cations on the Tm of phospholipids may be grouped into non-specific and specific effects. Unspecific effects of cations such as the screening of negative charges of the phospholipid polar group are qualitatively similar to protonation: Tm increases, in the order monovalent less than divalent less than trivalent cations and the effects on negatively charged phospholipids are larger than those on isoelectric phospholipids. Unspecific, electrostatic effects on Tm are reasonably well accounted for by the Gouy-Chapman theory. If, however, specific binding comes into play and/or electrostatic effects are accompanied by changes in phospholipid structure, simple, electrostatic theories fail to explain the observed changes in Tm. The crystal (gel)-to-liquid crystal transition is also a function of the degree of hydration: Tm generally decreases with increasing hydration reaching a plateau in excess H2O. In addition to screening of electric charges, ions may exert yet another non-specific effect: ions may affect Tm indirectly by competing with the phospholipid polar group for water of hydration. This indirect effect plays a role at high ionic strength and/or at low hydration of the phospholipid. Specific binding of cations to negatively charged phospholipids can lead to tight associations of the metal ion with the lipid polar group. Isothermal crystallization of the phospholipid bilayer is induced that is accompanied by a total or partial loss of water of hydration resulting in a marked increase in Tm. For instance, in crystalline Ca2(+)-phosphatidylserine complexes Tm is increased by more than 100 degrees C.
总结了质子和阳离子对等电及带负电荷磷脂的晶体（凝胶）-液晶转变温度Tm的影响。呈现出的一般趋势如下：Tm取决于磷脂的电离状态，即Tm与pH曲线平行于磷脂的滴定曲线。磷脂的质子化导致Tm升高，去质子化或电离则有相反的效果。阳离子对磷脂Tm的影响可分为非特异性和特异性效应。阳离子的非特异性效应，如对磷脂极性基团负电荷的屏蔽，在性质上与质子化相似：Tm升高，顺序为单价阳离子小于二价阳离子小于三价阳离子，且对带负电荷磷脂的影响大于对等电磷脂的影响。关于Tm的非特异性静电效应，Gouy-Chapman理论能较好地解释。然而，如果发生特异性结合和/或静电效应伴随着磷脂结构的变化，简单的静电理论就无法解释观察到的Tm变化。晶体（凝胶）-液晶转变也是水合程度的函数：Tm通常随水合程度增加而降低，在过量H2O中达到平稳状态。除了电荷屏蔽外，离子可能还会产生另一种非特异性效应：离子可能通过与磷脂极性基团竞争水合水而间接影响Tm。这种间接效应在高离子强度和/或磷脂低水合状态下起作用。阳离子与带负电荷磷脂的特异性结合可导致金属离子与脂质极性基团紧密结合。诱导磷脂双层的等温结晶，伴随着水合水的全部或部分损失，导致Tm显著升高。例如，在结晶的Ca2（+）-磷脂酰丝氨酸复合物中，Tm升高超过100℃。
