Small D M
J Lipid Res. 1984 Dec 15;25(13):1490-500.
The aliphatic chains of many biologically important lipids are heterogeneous and often related to the functions of the molecules. Certain phospholipids containing arachidonic acid may serve as precursors for prostaglandins, certain diglycerides may serve as second messengers for certain membrane-triggered reactions (43), and other phospholipids containing a very short chain in the two position may serve as vasoactive hormones (44). The packing of such molecules is of interest. The evidence is quite clear from both the conformation of saturated and unsaturated molecules and from mixing experiments in the solid state that long and short chains don't mix well, nor do unsaturated and saturated chains, even if they are of the same chain length. There is even some evidence to indicate that some degree of chain segregation occurs even in the liquid state. However, different chains are often associated through covalent bonds, e.g., in wax esters, diacylglycerols, triacylglycerols, and phospholipids. A variety of possibilities for chain segregation are present in the neat phases of wax esters, ceramides, diacylglycerols, and triacylglycerols. However, in the unique case of membrane lipids like phospholipids or sphingolipids, the two chains are forced to lie side by side by virtue of the interaction of the polar group with water, and thus interactions between different chains must occur. Most of the evidence suggests that, when a solid phase results in these systems, the nonspecific chain packing mode (hexagonal chain packing) is preferred. In fact, for all of the phospholipids studied thus far, clearcut evidence of specific chain-chain interaction in molecules having both unsaturated and saturated chains has never been observed. However, for mixed chain triacylglycerols, evidence of specific chain-chain interactions (beta' and even beta) has been found and some suggestions have been given as to how this might occur through chain segregation mechanisms in the neat state. The literature suggests that further work needs to be done on the interaction of different chains that are covalently linked to the same molecule. Such studies will lead to a better understanding of the structure of lipid bilayers, membranes, lipoproteins, and lipid deposits.
许多具有重要生物学意义的脂质的脂肪链是异质的，并且通常与分子的功能相关。某些含有花生四烯酸的磷脂可能作为前列腺素的前体，某些甘油二酯可能作为某些膜触发反应的第二信使（43），而在二位含有非常短链的其他磷脂可能作为血管活性激素（44）。此类分子的堆积情况很有意思。从饱和与不饱和分子的构象以及固态混合实验中得到的证据都很清楚，长链和短链混合得不好，不饱和链和饱和链也一样，即使它们的链长相同。甚至有一些证据表明，即使在液态时也会发生一定程度的链分离。然而，不同的链常常通过共价键相连，例如在蜡酯、二酰基甘油、三酰基甘油和磷脂中。蜡酯、神经酰胺、二酰基甘油和三酰基甘油的纯相中存在多种链分离的可能性。然而，在磷脂或鞘脂等膜脂这种独特的情况下，由于极性基团与水的相互作用，两条链被迫并排排列，因此不同链之间必然会发生相互作用。大多数证据表明，当这些体系形成固相时，非特异性链堆积模式（六方链堆积）是首选。事实上，对于迄今为止研究的所有磷脂，从未观察到同时含有不饱和链和饱和链的分子中有明确的特异性链-链相互作用的证据。然而，对于混合链三酰基甘油，已发现特异性链-链相互作用（β'甚至β）的证据，并就其在纯态下如何通过链分离机制发生给出了一些推测。文献表明，需要对共价连接到同一分子上的不同链之间的相互作用进行进一步研究。此类研究将有助于更好地理解脂质双层、膜、脂蛋白和脂质沉积物的结构。
