Structure and polymorphism of 1,2-dioleoyl-3-acyl-sn-glycerols. Three- and six-layered structures.
Structure and polymorphism of 1,2-dioleoyl-3-acyl-sn-glycerols. Three- and six-layered structures.
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1,2-二油酰基-3-酰基-sn-甘油的结构和多态性。
DOI：
10.1021/bi00335a051
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发表时间：
1985
期刊：
影响因子：
2.9
通讯作者：
Redgrave,TG
中科院分区：
文献类型：
--
作者：
Fahey,DA;Small,DM;Kodali,DR;Atkinson,D;Redgrave,TG
三酰基甘油通常含有至少一种不饱和脂肪酸，是储存生物脂质的最重要形式，包括甲壳动物、哺乳动物和大多数植物。由于人们对这种混合链三酰甘油的物理性质知之甚少，因此已经开始对此类化合物进行系统研究。合成了具有14-24个碳长度的均匀碳饱和脂肪酰基链的立体特异性甘油中的1，2-二油酰基-3-酰基。通过差示扫描量热法和 X 射线粉末衍射检查了它们的多晶型行为。热行为揭示了根据饱和链长度的一到四个主要多晶型转变。最稳定多晶型物的熔化热和熵与碳数的关系图在整个系列中呈线性，斜率分别为每个碳原子 1.0 kcal/mol 和每个碳原子 2.6 cal/(mol K)。这些斜率表明饱和链堆积在有序的紧密堆积的晶格中。当化合物快速冷却至 5°C 时，X 射线粉末衍射显示出强的 ß'（约 3.8 和 4.2 Á）反射和弱的 ß（约 4.6 Á）反射。当化合物被加热到最高熔化多晶型物的熔化温度 5°C 以内时，β 亚晶胞反射增强。任何化合物在 30 分钟的 X 射线照射下均未发现 a 相的证据。在所提出的堆积布置中，饱和链和不饱和链被分成层。所有化合物的稳定形式均表现出三层堆积模式，其中油酰链双层与饱和链叉指层分离。垂直于基面的甘油主链的厚度约为 4.1 Á。饱和链倾斜 55.8°。相对于基准平面。该倾斜度接近从油酸单晶研究中获得的值56.5，这可能表明饱和链从甘油主链突出，就像油酰链前九个碳的线性延伸一样。此外，还确定了 1,2-二油酰基-3-肉豆蔻酰基-J «-甘油的亚稳态六层堆积模式。虽然一些复杂的多态性涉及链堆积的调整，但其他多态性可能与从不稳定的六层结构到稳定的三层结构的转变有关..^^。甘油是脂质代谢的关键分子。三酰基甘油，其中甘油的所有三个羟基都被酯化为脂肪酸，是高等动物和大多数植物的主要储存脂质。大多数天然存在的三酰甘油表现出光学活性，表明脂肪酸在 1 位和 3 位的具体分布是不同的。此外，大多数天然存在的三酰基甘油具有至少一种且通常两种不饱和脂肪酸。特定的三酰甘油结构对三酰甘油的物理性质具有重要意义（Small，1985k），并且可能对酶水解和随后的代谢等生理特性具有重要意义（Morley 等人，1974）。固态三酰基甘油的一个特征是多晶型，因为相似晶格能量可能存在许多不同的堆积排列。这种多态性一直是不同工作者进行了近一个多世纪的大量研究的主题[参见Bailey (1950)、Chapman (1962)、Lutton (1950)、Malkin (1954)和Small (1985a)的评论]。在大多数早期的三酰甘油研究中，研究的是外消旋混合物；然而，外消旋混合物的性质不一定与特定异构体的性质相同（Larsson，1964）。
Triacylglycerols, which usually contain at least one unsaturated fatty acid, are the most important forms of stored biological lipids inteleosts, mammals, and most plants. Since the physical properties of such mixed-chain triacylglycerols are poorly understood, a systematic study of such compounds has been initiated. Stereospecific l, 2-dioleoyl-3-acyl-in-glycerols were synthesized with even carbon saturated fatty acyl chains of 14-24 carbons in length. Their polymorphic behavior was examined by differential scanning calorimetry and X-ray powder diffraction. The thermal behavior revealed from one to four major polymorphic transitions depending upon saturated chain length. Plots of enthalpy of fusion and entropy vs. carbon number for njelting of the most stable polymorph were linear throughout the series with slopes of 1.0 kcal/mol per carbon atom and 2.6 cal/(mol K) per carbón atom, respectively. These slopes indicatethat the saturated chains are packed in a well-ordered tightly packed lattice. When the compounds were rapidly cooled to 5 C, X-ray powder diffraction revealed strong ß'(ca. 3.8 and 4.2 Á) reflections and weak ß (ca. 4.6 Á) reflections. The ß subcell reflections intensified when the compounds were heated to within 5 C of the meltihg temperature of the highest melting polymorph. Evidence of an a phase was not seen on 30-min X-ray exposures for any of the compounds. In the proposed packing arrangement the saturated and unsaturated chains are segregated into layers. The stable form of all compounds exhibits a triple layer packing mode in which a bilayer of oleoylchains is segregated from an interdigitated layer of saturated chains. The thickness of the glycerol backbone normal to the base plane is about 4.1 Á. The saturatedchains are tilted 55.8 with. respect to the base plane. This tilt is close to the value of 56.5 obtained from the single crystal study of oleic acid, which may indicatethat the saturated chains projectfrom the glycerol backbone like a linear extension of the first nine carbons of the oleoyl chain. In addition, a metastable six-layer packing mode was identified for l, 2-dioleoyl-3-myristoyl-J «-glycerol. While some of the complex polymorphisms involve adjustments in chain packing, others may relate to transitions from unstable six-layer to stable three-layer structures..^^. cylglycerols are key molecules in lipid metabolism. Tri-acylglycerols, in which all three hydroxyls of the glycerol are esterified to fatty acids, represent the major storage lipid of higher animals andmost plants. Most naturally occurring triacylglycerols show optical activity, indicating that the specific distributions of fatty acids at the 1-and 3-positions are different. Furthermore, most naturally occurring tri-acylglycerols have at least one and often two unsaturáted fatty acids. A specific triacylglycerol structure has important implications for the physical properties of the triacylglycerols (Small, 1985k) and also perhaps for physiological charac-teristics such as enzyme hydrolysis and subsequent metabolism (Morley et al., 1974). A characteristicfeature of triacyl-glycerols in the solid state is polymorphism because of the possibility of many different packing arrangements of similar lattice energy. This polymorphism has been the subject of numerous studies by different workersextending almost over a century [see reviews by Bailey (1950), Chapman (1962), Lutton (1950), Malkin (1954), and Small (1985a)]. In most earlier studies of tri'acylglycerols raCemic mixtures were studied; however, properties of the racemic mixtures are not necessarily the same as those of the specific isomers (Larsson, 1964).