Li S, Lin H N, Wang Z Q, Huang C
Department of Biochemistry, University of Virginia, Charlottesville 22908.
Biophys J. 1994 Jun;66(6):2005-18. doi: 10.1016/S0006-3495(94)80993-2.
As a cis carbon-carbon double bond (delta) is introduced into the middle of an isolated all-trans hydrocarbon chain, it can be shown by molecular graphics that this delta-bond makes a bend of 130 degrees in the chain axis, thus producing a boomerang-like conformation. Such a bent structure, indeed, has been detected experimentally for oleic acid by x-ray crystallography (Abrahamson and Ryderstedt-Nahringbaur, 1962). Membrane diacyl phospholipids are largely mixed-chain lipids containing a saturated sn-1 acyl chain and an unsaturated sn-2 acyl chain. 1-Palmitoyl-2-oleoyl-phosphatidylcholine (POPC), the most abundant phospholipid in animal cell membranes, is a typical example in which the sn-2 acyl chain is the acyl chain of an oleic acid. However, this sn-2 acyl chain of POPC is unlikely to adopt a boomerang-like configuration in the gel-state lipid bilayer due to the steric hindrance imposed by neighboring chains. Instead, it has been suggested that the oleate chain in POPC is kinked in the shape of a crankshaft in the gel-state bilayer (Huang, 1977; Lagaly et al., 1977), because POPC with such a kinked sn-2 acyl chain, which is denoted here as the secondary structural element or motif, can pack efficiently against other neighboring phospholipids. In this communication, 16 different types of secondary structural elements or motifs are derived for POPC at T < Tm based on a single protocol guided by two-dimensional steric contour maps and computer-based molecular graphics. After subjecting these derived molecular species to energy minimization using the molecular mechanics method, the number of the secondary structural motifs is reduced to 13 as a result of conformational degeneracy. The structure and steric energy of each of the energy-minimized lipid rotomers are presented in this communication. Furthermore, these rotomers packed in small clusters are also simulated to mimic the lipid bilayer structure of 1-palmitoyl-2-oleoyl-phosphatidylcholines at T < Tm.
当一个顺式碳 - 碳双键（δ）引入到孤立的全反式烃链中间时，通过分子图形可以显示，这个δ键在链轴上产生130度的弯曲，从而形成一种回飞棒状的构象。事实上，通过X射线晶体学已经在油酸中实验检测到了这种弯曲结构（亚伯拉罕森和赖德施泰特 - 纳林鲍尔，1962年）。膜二酰基磷脂主要是混合链脂质，含有饱和的sn - 1酰基链和不饱和的sn - 2酰基链。1 - 棕榈酰 - 2 - 油酰 - 磷脂酰胆碱（POPC）是动物细胞膜中最丰富的磷脂，是一个典型例子，其中sn - 2酰基链是油酸的酰基链。然而，由于相邻链的空间位阻，POPC的这个sn - 2酰基链在凝胶态脂质双层中不太可能采用回飞棒状构型。相反，有人提出POPC中的油酸盐链在凝胶态双层中呈曲轴状扭结（黄，1977年；拉加利等人，1977年），因为具有这种扭结的sn - 2酰基链的POPC（在此表示为二级结构元件或基序）可以有效地与其他相邻磷脂堆积。在本通讯中，基于二维空间轮廓图和计算机辅助分子图形引导的单一方案，在T < Tm时为POPC推导了16种不同类型的二级结构元件或基序。使用分子力学方法对这些推导的分子物种进行能量最小化后，由于构象简并，二级结构基序的数量减少到13个。本通讯展示了每个能量最小化的脂质旋转异构体的结构和空间能量。此外，还模拟了这些以小簇形式堆积的旋转异构体，以模拟T < Tm时1 - 棕榈酰 - 2 - 油酰 - 磷脂酰胆碱的脂质双层结构。
