Morrow M R, Singh D, Lu D, Grant C W
Department of Physics, Memorial University of Newfoundland, St. John's, Canada.
Biophys J. 1993 Mar;64(3):654-64. doi: 10.1016/S0006-3495(93)81424-3.
The glycosphingolipid, galactosyl ceramide (GalCer), was studied by 2H nuclear magnetic resonance (NMR) in fluid phospholipid bilayer membranes, with regard to arrangement of its acyl chain. For this purpose, species with perdeuterated 18-carbon fatty acid (18:0[d35]GalCer) or with perdeuterated 24-carbon fatty acid (24:0[d47] GalCer) were dispersed in bilayers of the 18-carbon phospholipid, 1-stearoyl-2-oleoyl-phosphatidylcholine (SOPC). For 18:0[d35] GalCer, smoothed profiles of the order parameter, SCD, were found to be very similar to one another over the range of glycolipid concentration, 5-40 mol%. In addition, they were very similar to orientational order parameter profiles well known from the literature on phospholipid and glycolipid acyl chains (which deals in general with membranes of homogeneous chain length in the range 14-18 carbons). Corresponding order parameter profiles for the long-chain species, 24:0[d47] GalCer, were also similar to one another for glycolipid concentrations between 5 and 40 mol%. Their shapes, however, were distinctly different from those of the shorter chain analogues. SCD profiles for the two species were quantitatively similar to a membrane depth of C15. SCD values at C16 and C17 were approximately 20 and 30%, respectively, higher for the long-chain glycosphingolipid than for its short-chain analogue in SOPC. Nitroxide spin labels attached rigidly to C16 of the long-chain glycolipid in SOPC gave electron paramagnetic resonance (EPR) order parameters that were twice as high as for a spin label at C16 on the shorter chain glycolipid. Comparison was made between spectra of 24:0[d47] GalCer in SOPC and fully hydrated bilayers of the pure 24:0[d47] GalCer, a system that is considered to be partially interdigitated in fluid and gel phases. The resultant 2H NMR order parameter profiles displayed similar features, indicating that related organizational properties exist in these fluid systems. Effective chain length of 24:0[d47] GalCer within the SOPC membrane was calculated using the method of Schindler and Seelig (1975. Biochemistry, 14:2283-2287). The result suggested that the long-chain fatty acid should protrude roughly one third of the host matrix chain length across the bilayer midplane. However, a treatment of the same order parameters making very few assumptions about chain conformation indicated a high degree of orientational flexibility for the "extra" length of the long chain fatty acid. It seems likely that a realistic treatment of the long-chain fatty acidin a shorter chain fluid host matrix considers interdigitation as a subset of the conformational possibilities, many of which are rapidly interconverting on the NMR timescale of 10-4_10-5 s and longer lived on the EPR timescale of 10-8_10-9 s.
利用2H核磁共振（NMR）对糖鞘脂半乳糖神经酰胺（GalCer）在流体磷脂双分子层膜中的酰基链排列进行了研究。为此，将含有全氘代18碳脂肪酸（18:0[d35]GalCer）或全氘代24碳脂肪酸（24:0[d47]GalCer）的物质分散在18碳磷脂1-硬脂酰-2-油酰磷脂酰胆碱（SOPC）的双分子层中。对于18:0[d35]GalCer，在糖脂浓度为5 - 40 mol%的范围内，序参数SCD的平滑曲线彼此非常相似。此外，它们与磷脂和糖脂酰基链文献中熟知的取向序参数曲线非常相似（这些文献一般涉及链长在14 - 18碳范围内的均一链长的膜）。对于长链物质24:0[d47]GalCer，在糖脂浓度为5至40 mol%之间时，相应的序参数曲线也彼此相似。然而，它们的形状与短链类似物明显不同。两种物质的SCD曲线在C15膜深度处定量相似。在SOPC中，长链糖鞘脂在C16和C17处的SCD值分别比其短链类似物高约20%和30%。刚性连接到SOPC中长链糖脂C16上的氮氧化物自旋标记给出的电子顺磁共振（EPR）序参数是短链糖脂C16上自旋标记的两倍。对SOPC中24:0[d47]GalCer的光谱与纯24:0[d47]GalCer的完全水合双分子层的光谱进行了比较，纯24:0[d47]GalCer体系在流体相和凝胶相中被认为是部分交叉排列的。所得的2H NMR序参数曲线显示出相似的特征，表明在这些流体体系中存在相关的组织特性。使用Schindler和Seelig（1975年，《生物化学》，14:2283 - 2287）的方法计算了SOPC膜内24:0[d47]GalCer的有效链长。结果表明，长链脂肪酸应在双分子层中平面上突出宿主基质链长的大约三分之一。然而，对相同序参数进行处理时，对链构象的假设极少，这表明长链脂肪酸的“额外”长度具有高度的取向灵活性。在较短链流体宿主基质中对长链脂肪酸进行实际处理时，似乎交叉排列是构象可能性的一个子集，其中许多在10 - 4_10 - 5 s的NMR时间尺度上快速相互转换，而在10 - 8_10 - 9 s的EPR时间尺度上寿命更长。
