Lewis R N, McElhaney R N
Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Biophys J. 2000 Oct;79(4):2043-55. doi: 10.1016/S0006-3495(00)76452-6.
The thermotropic phase behavior of lipid bilayer model membranes composed of the even-numbered, N-saturated 1,2-diacyl phosphatidylserines was studied by differential scanning calorimetry and by Fourier-transform infrared and (31)P-nuclear magnetic resonance spectroscopy. At pH 7.0, 0.1 M NaCl and in the absence of divalent cations, aqueous dispersions of these lipids, which have not been incubated at low temperature, exhibit a single calorimetrically detectable phase transition that is fully reversible, highly cooperative, and relatively energetic, and the transition temperatures and enthalpies increase progressively with increases in hydrocarbon chain length. Our spectroscopic observations confirm that this thermal event is a lamellar gel (L(beta))-to-lamellar liquid crystalline (L(alpha)) phase transition. However, after low temperature incubation, the L(beta)/L(alpha) phase transition of dilauroyl phosphatidylserine is replaced by a higher temperature, more enthalpic, and less cooperative phase transition, and an additional lower temperature, less enthalpic, and less cooperative phase transition appears in the longer chain phosphatidylserines. Our spectroscopic results indicate that this change in thermotropic phase behavior when incubated at low temperatures results from the conversion of the L(beta) phase to a highly ordered lamellar crystalline (L(c)) phase. Upon heating, the L(c) phase of dilauroyl phosphatidylserine converts directly to the L(alpha) phase at a temperature slightly higher than that of its original L(beta)/L(alpha) phase transition. Calorimetrically, this process is manifested by a less cooperative but considerably more energetic, higher-temperature phase transition, which replaces the weaker L(beta)/L(alpha) phase transition alluded to above. However, with the longer chain compounds, the L(c) phase first converts to the L(beta) phase at temperatures some 10-25 degrees C below that at which the L(beta) phase converts to the L(alpha) phase. Our results also suggest that shorter chain homologues form L(c) phases that are structurally related to, but more ordered than, those formed by the longer chain homologues, but that these L(c) phases are less ordered than those formed by other phospholipids. These studies also suggest that polar/apolar interfaces of the phosphatidylserine bilayers are more hydrated than those of other glycerolipid bilayers, possibly because of interactions between the polar headgroup and carbonyl groups of the fatty acyl chains.
通过差示扫描量热法、傅里叶变换红外光谱和（31）P-核磁共振光谱研究了由偶数、N-饱和的1,2-二酰基磷脂酰丝氨酸组成的脂质双层模型膜的热致相行为。在pH 7.0、0.1 M NaCl且不存在二价阳离子的条件下，这些未在低温下孵育的脂质的水分散体表现出单一的、可通过量热法检测到的、完全可逆、高度协同且能量相对较高的相变，并且转变温度和焓随着烃链长度的增加而逐渐升高。我们的光谱观察证实，这一热事件是层状凝胶（Lβ）到层状液晶（Lα）的相变。然而，在低温孵育后，二月桂酰磷脂酰丝氨酸的Lβ/Lα相变被一个更高温度、焓更高且协同性更低的相变所取代，并且在长链磷脂酰丝氨酸中出现了一个额外的更低温度、焓更低且协同性更低的相变。我们的光谱结果表明，在低温下孵育时热致相行为的这种变化是由于Lβ相转变为高度有序的层状晶体（Lc）相所致。加热时，二月桂酰磷脂酰丝氨酸的Lc相在略高于其原始Lβ/Lα相变温度的温度下直接转变为Lα相。从量热法来看，这个过程表现为一个协同性较低但能量相当高的高温相变，它取代了上述较弱的Lβ/Lα相变。然而，对于长链化合物，Lc相首先在比Lβ相转变为Lα相的温度低约10 - 25℃的温度下转变为Lβ相。我们的结果还表明，较短链的同系物形成的Lc相比长链同系物形成的Lc相在结构上相关但更有序，但这些Lc相比其他磷脂形成的Lc相有序性更低。这些研究还表明，磷脂酰丝氨酸双层的极性/非极性界面比其他甘油脂质双层的更水合，这可能是由于极性头部基团与脂肪酰链的羰基之间的相互作用。
