Di L, Small D M
Department of Biophysics, Boston University School of Medicine, Massachusetts 02118-2394, USA.
Biochemistry. 1995 Dec 26;34(51):16672-7. doi: 10.1021/bi00051a015.
Phospholipids containing a saturated fatty acid in the primary position and an unsaturated fatty acid in the secondary position are a major structural part of biological membranes. The mixed-chain hydrophobic core of the membranes is the diacylglycerol part. To better understand the core properties of membranes we have studied the physical behavior of 1-stearoyl-2-linoleoyl-sn-glycerol (SLDG) by X-ray diffraction and differential scanning calorimetry (DSC) in the dry and hydrated states. Dry SLDG has four polymorphic phases: alpha (transition temperature, 11.6 degrees C; delta H = 7.5 kcal/mol); sub-alpha 1 (3.0 degrees C; 0.6 kcal/mol); sub-alpha 2(-1.0 degrees C; 0.5 kcal/mol); and beta' (16.1 degrees C; 15.4 kcal/mol). The alpha, sub-alpha 1, and sub-alpha 2 phases are metastable with a probable extended bilayer structure (d001 approximately 59.5 A). The chain packing of the alpha phase is hexagonal, while sub-alpha 1 and sub-alpha 2 have pseudohexagonal chain packing. The beta' phase has a tilted bilayer structure (46.9 A) with strong wide-angle diffractions, suggesting elements of orthorhombic perpendicular packing. Compared to saturated 1,2-diacylglycerols, SLDG packs much less efficiently, but, when compared to 1-stearoyl-2-oleoyl-sn-glycerol, it appears to pack somewhat more efficiently. Thus polyunsaturated linoleate chains appear to pack marginally more effectively with the saturated stearate chains than do monounsaturated chains. SLDG hydrates with 0.5 mol of H2O, which prevents the beta' phase from forming. Only one hydrated alpha phase (alpha w) and two hydrated sub-alpha (sub-alpha w1, sub-alpha w2) phases are formed. These phases are similar in structure to the nonhydrated alpha phases, but the bilayer period is increased by about 2 A (d001 approximately 61.5 A). This causes minor changes in polymorphism, including lower melting temperatures and enthalpy. A comparison of diacylglycerols to phosphatidylcholines with the same chains shows that the addition of a strong polar group (e.g., phosphocholine) to the free hydroxyl of the glycerol depresses chain melting and prevents the more efficient packing of the SLDG core of the bilayer.
在主位含有饱和脂肪酸且在次位含有不饱和脂肪酸的磷脂是生物膜的主要结构部分。膜的混合链疏水核心是二酰基甘油部分。为了更好地理解膜的核心性质，我们通过X射线衍射和差示扫描量热法（DSC）研究了1-硬脂酰-2-亚油酰基-sn-甘油（SLDG）在干燥和水合状态下的物理行为。干燥的SLDG有四个多晶型相：α相（转变温度为11.6℃；ΔH = 7.5千卡/摩尔）；亚α1相（3.0℃；0.6千卡/摩尔）；亚α2相（-1.0℃；0.5千卡/摩尔）；以及β'相（16.1℃；15.4千卡/摩尔）。α相、亚α1相和亚α2相是亚稳的，可能具有延伸的双层结构（d001约为59.5埃）。α相的链堆积是六方的，而亚α1相和亚α2相具有假六方链堆积。β'相具有倾斜的双层结构（46.9埃），有强烈的广角衍射，表明存在正交垂直堆积的元素。与饱和的1,2-二酰基甘油相比，SLDG的堆积效率低得多，但与1-硬脂酰-2-油酰基-sn-甘油相比，它似乎堆积效率略高。因此，与单不饱和链相比，多不饱和的亚油酸酯链与饱和的硬脂酸酯链堆积时似乎略更有效。SLDG与0.5摩尔的H2O水合，这阻止了β'相的形成。只形成了一个水合α相（αw）和两个水合亚α相（亚αw1、亚αw2）。这些相在结构上与非水合α相相似，但双层周期增加了约2埃（d001约为61.5埃）。这导致多晶型发生微小变化，包括较低的熔点和焓。将二酰基甘油与具有相同链的磷脂酰胆碱进行比较表明，在甘油的游离羟基上添加一个强极性基团（如磷酸胆碱）会降低链的熔点，并阻止双层中SLDG核心更有效的堆积。
