Lewis R N, McElhaney R N
Department of Biochemistry, University of Alberta, Edmonton, Canada.
Biophys J. 1993 Nov;65(5):1866-77. doi: 10.1016/S0006-3495(93)81251-7.
The mixed interdigitated gel phases of unlabeled, specifically 13C = O-labeled, and specifically chain-perdeuterated samples of 1-O-eicosanoyl, 2-O-lauroyl phosphatidylcholine and 1-O-decanoyl, 2-O-docosanoyl phosphatidylcholine were studied by infrared spectroscopy. Our results suggest that at the liquid-crystalline/gel phase transition temperatures of these lipids, there is a greater redistribution in the populations of free and hydrogen-bonded ester carbonyl groups than is commonly observed with symmetric chain n-saturated diacyl phosphatidylcholines. The formation of the mixed interdigitated gel phase coincides with the appearance of a marked asymmetry in the contours of the C = O stretching band, a process which becomes more pronounced as the temperature is reduced. This asymmetry is ascribed to the emergence of a predominant lipid population consisting of free sn1- and hydrogen-bonded (hydrated) sn2-ester carbonyl groups. This suggests that the region of the mixed interdigitated bilayer polar/apolar interface near to the sn1-ester carbonyl group is less hydrated than is the case with the noninterdigitated gel-phase bilayers formed by normal symmetric chain phosphatidylcholines. In the methylene deformation region of the spectrum, the unlabeled lipids exhibit a pronounced splitting of the CH2 scissoring bands. This splitting is significantly attenuated when the short chains are perdeuterated and collapses completely upon perdeuteration of the long chains, irrespective of whether the long (or short) chains are esterified to the sn1 or sn2 positions of the glycerol backbone. These results are consistent with a global hydrocarbon chain packing motif in which the zigzag planes of the hydrocarbon chains are perpendicular to each other and the sites occupied by long chains are twice as numerous as those occupied by short chains. The experimental support for this chain-packing motif enabled more detailed considerations of the possible ways in which these lipid molecules are assembled in the mixed interdigitated gel phase. Generally, our results are compatible with a previously proposed model in which the mixed interdigitated gel phase is an assembly of repeat units which consists of two phosphatidylcholine molecules forming a triple-chain structure with the long chains traversing the bilayer and with the methyl termini of the shorter chains opposed at the bilayer center. Our data also suggest that the packing format which is most consistent with our results and previously published work is one in which the hydrocarbon chains of each repeat unit are parallel to each other with the repeat units themselves being perpendicularly packed.
采用红外光谱法研究了1-O-二十碳酰基-2-O-月桂酰基磷脂酰胆碱和1-O-癸酰基-2-O-二十二碳酰基磷脂酰胆碱的未标记样品、特定的13C=O标记样品以及特定的全氘代链样品的混合叉指状凝胶相。我们的结果表明，在这些脂质的液晶/凝胶相转变温度下，游离和氢键结合的酯羰基群体中的重新分布比通常在对称链n-饱和二酰基磷脂酰胆碱中观察到的更为显著。混合叉指状凝胶相的形成与C=O伸缩带轮廓中明显的不对称现象的出现相吻合，随着温度降低，这一过程变得更加明显。这种不对称归因于由游离的sn1-酯羰基和氢键结合（水合）的sn2-酯羰基组成的主要脂质群体的出现。这表明，与正常对称链磷脂酰胆碱形成的非叉指状凝胶相双层相比，靠近sn1-酯羰基的混合叉指状双层极性/非极性界面区域的水合程度较低。在光谱的亚甲基变形区域，未标记的脂质表现出CH2剪式振动带的明显分裂。当短链全氘代时，这种分裂明显减弱，而当长链全氘代时，分裂完全消失，无论长（或短）链是酯化在甘油主链的sn1还是sn2位置。这些结果与一种整体烃链堆积模式一致，即烃链的锯齿平面相互垂直，长链占据的位点数量是短链占据位点数量的两倍。对这种链堆积模式的实验支持使得能够更详细地考虑这些脂质分子在混合叉指状凝胶相中组装的可能方式。一般来说，我们的结果与先前提出的模型相符，在该模型中，混合叉指状凝胶相是由两个磷脂酰胆碱分子形成三链结构的重复单元的组装体，长链穿过双层，较短链的甲基末端在双层中心相对。我们的数据还表明，与我们的结果和先前发表的工作最一致的堆积形式是每个重复单元的烃链相互平行，而重复单元本身垂直堆积。
