Schmidt Walter F, Barone Justin R, Francis Barry, Reeves James B
Animal and Natural Resources Institute, Beltsville Agricultural Research Center, ARS, USDA, Beltsville, MD 20705, United States.
Chem Phys Lipids. 2006 Jul;142(1-2):23-32. doi: 10.1016/j.chemphyslip.2006.02.016. Epub 2006 Mar 24.
Stearic acid (SA) is highly soluble in structurally diverse solvents. SA/solvent packing within a (24.8 A)3 cubic volume explains the stoichiometry of SA solubility at multiple temperatures in multiple solvents. In the absence of solvent, the cubic volume contains 25 molecules at van der Waals distances from each other. At 55 degrees C, SA occupied half the cubic volume in saturated solution of four structurally diverse solvents. Below 4% SA/volume (e.g. in acetonitrile), the head and foot of each SA molecules on average is more than one solvent molecule away from the head and foot of a neighboring SA molecule. At 50% SA/cubic volume, -CH2- groups on SA molecules are separated from neighboring -CH2- groups on SA molecules by a monolayer of solvent molecules. Lowering the temperature from 55 to 25 degrees C, the volume fraction of SA decreased by a factor of 2 (or more) for every 6 degrees C. Lowering temperature increased the relative number of column of solvent molecules in the cubic phase, and correspondingly, the distance between SA molecules within the cubic volume increased. In three of five solvents, molecular mechanics calculations demonstrated the van der Waals stabilization that occurs from SA/SA affinity in the absence of solvent is similar in magnitude to the van der Waals stabilization from SA/solvent affinity. Methyl-t-butyl ether was less stabilized than hexane, acetone or methanol because the more bulky molecules packed less efficiently within the cubic volume. The most efficient/most stable packing however was still as columns of solvent between columns of SA. The efficiency and stability of SA and solvent packing optimal within the (24.8 A)3 cubic volume. Between 100 and 8% SA, multiple SA molecules present within the cubic volume function as SA aggregates. Both inter- and intra-cubic (phase) volume properties of SA aggregates coexist. Although acetonitrile and SA at the molecular level are both rod shaped, acetonitrile disrupted the packing of SA molecules within the cubic phase. The disrupted packing explains the much lower solubility of SA in acetonitrile than in the other solvents. The same molecular structures (e.g. methanol) can either stabilize or disrupt the packing of aggregated SA molecules, depending upon temperature. The mechanisms of aggregation within cubic volumes could also occur with structurally more complicated lipids. Aggregation and dispersion from such cubic phases could also be present in more complex chemical and/or macromolecular environments.
硬脂酸（SA）在结构多样的溶剂中具有高度溶解性。在一个（24.8 Å）³ 的立方体积内，SA/溶剂的堆积情况解释了SA在多种溶剂中多个温度下的溶解度化学计量关系。在没有溶剂的情况下，该立方体积内包含25个彼此处于范德华距离的分子。在55℃时，SA在四种结构不同的溶剂的饱和溶液中占据了一半的立方体积。在SA/体积低于4%（如在乙腈中）时，每个SA分子的头部和尾部平均与相邻SA分子的头部和尾部相距超过一个溶剂分子。在SA/立方体积为50%时，SA分子上的 -CH₂- 基团通过一层溶剂分子与相邻SA分子上的 -CH₂- 基团隔开。将温度从55℃降至25℃，每降低6℃，SA的体积分数就降低2倍（或更多）。降低温度增加了立方相中溶剂分子柱的相对数量，相应地，立方体积内SA分子之间的距离也增加了。在五种溶剂中的三种中，分子力学计算表明，在没有溶剂的情况下，SA/SA亲和力产生的范德华稳定作用在大小上与SA/溶剂亲和力产生的范德华稳定作用相似。甲基叔丁基醚的稳定性低于己烷、丙酮或甲醇，因为体积较大的分子在立方体积内堆积效率较低。然而，最有效的/最稳定的堆积方式仍然是SA柱之间的溶剂柱。SA和溶剂在（24.8 Å）³ 立方体积内的堆积效率和稳定性最佳。在SA含量为100%至8%之间，立方体积内存在的多个SA分子作为SA聚集体起作用。SA聚集体的立方相内和立方相之间的体积性质同时存在。尽管乙腈和SA在分子水平上都是棒状的，但乙腈破坏了立方相中SA分子的堆积。这种被破坏的堆积解释了SA在乙腈中的溶解度远低于其他溶剂的原因。相同的分子结构（如甲醇）根据温度的不同，既可以稳定也可以破坏聚集的SA分子的堆积。立方体积内的聚集机制在结构更复杂的脂质中也可能发生。这种立方相的聚集和分散也可能存在于更复杂的化学和/或大分子环境中。
