The fluid-mosaic model, homeoviscous adaptation, and ionic liquids: dramatic lowering of the melting point by side-chain unsaturation.
The fluid-mosaic model, homeoviscous adaptation, and ionic liquids: dramatic lowering of the melting point by side-chain unsaturation.
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DOI：
10.1002/anie.200906169
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发表时间：
2010-04
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通讯作者：
Samuel M. Murray;R. A. O'brien;Kaila M. Mattson;C. Ceccarelli;R. Sykora;Kevin N. West;James H. Davis-James-H.-Davi
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文献类型：
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作者：
Samuel M. Murray;R. A. O'brien;Kaila M. Mattson;C. Ceccarelli;R. Sykora;Kevin N. West;James H. Davis-James-H.-Davi
Singer和Nicolson在1972年提出的流体镶嵌模型认为，磷脂双层是一个动态的二维溶剂环境。其适当的功能与其“流动性”密切相关，这通常通过参照熔点Tm来量化(增加的流动性对应较低的Tm值)。流体马赛克模型高度唤起人们对离子液体(ILS)纳米尺度结构的新兴图景的回忆，[2，3]，正如磷脂双层的功能与Tm值有关，离子液体的效用也是如此。虽然前者通常具有低的Tm值，尽管它是由带长脂肪附属物的带电物种组成的，但当使用逐渐变长的脂肪附属物时，ILS的流动性通常会下降。[4]设计咪唑ILs(最常见的ILs类)是一个挑战，它包含越来越多的亲油结构元素，同时将其熔点保持在室温以下(图1)。[4-9]事实上，一旦附加的N-烷基长度超过七个碳原子，这些ILs的Tm值就开始急剧上升。在这里，我们报道通过使用一种模拟自粘适应(HVA)的方法，可以制备具有非常长的烷基附属物和非常低的Tm值的[10]ILS。这一发现可能对IL在酶催化、润滑剂、传热液、气体存储和分离等应用中的应用具有重要意义。HVA被广泛认为是调节细胞膜熔化温度的一种机制，它是具有“扭结”尾部结构的磷脂进入细胞膜的过程。[10]有人认为，这些磷脂的存在降低了集体膜疏水成分的堆积效率，从而增加了流动性。双硬脂酰磷脂酰胆碱和二油酰磷脂酰胆碱的TM值的比较提供了一个戏剧性的例子，说明这种看似微不足道的差异可以产生多么大的影响。前者的TM值为588C，后者的TM值为±228C；前者的线形饱和C18尾巴的TM值为588C，后者的TM值为±228C。这一效应也是固体三酰甘油(称为脂肪)和那些在室温下呈液体的甘油(称为油)之间TM差异的核心。在这两种情况下，这种影响在性质上可能是熵的，如菲(“线性”，Tm=2178C)和菲(“扭结”，Tm=998C)的情况。[11]因此，我们假设具有长的、不饱和的、脂肪族尾部结构的IL将像相应的磷脂一样，具有显著较低的TM值。为了通过测量它们的TM值来验证我们的假设的有效性，我们以高纯度(99+%)脂肪醇甲磺酸盐、1-甲基咪唑、NaI和NaTf2N为原料，通过三步工艺制备了一系列脂质激发的ILS。[12]每个ILS(方案1)都有一个与天然脂肪酸中相同的长烷基。化合物1、3和8分别具有完全饱和的C16、C18和C20侧链，其
Proposed by Singer and Nicolson in 1972,[1] the fluid-mosaic model holds that the phospholipid bilayer is a dynamic twodimensional solvent milieu. Its proper function is closely tied to its “fluidity”, and that is often quantified by reference to the melting point, Tm (increased fluidity corresponds to a lower Tm value). The fluid-mosaic model is highly evocative of the emerging picture of nanoscale structuring in ionic liquids (ILs),[2, 3] and just as the function of phospholipid bilayers is tied to the Tm value, so too is the utility of ILs. Whereas the former often have low Tm values despite being composed of charged species with long aliphatic appendages, the fluidity of ILs generally decreases when progressively longer aliphatic appendages are used.[4] It is a challenge to design imidazolium ILs (the most common IL class) that incorporate progressively more lipophilic structural elements while keeping their melting points below room temperature (Figure 1).[4–9] Indeed, the Tm values of these ILs begin to rise dramatically once an appended N-alkyl group exceeds seven carbon atoms in length. Herein we report that by using an approach modeled on homeoviscous adaptation (HVA),[10] ILs with very long alkyl appendages and very low Tm values can be prepared. This discovery may have significant implications for IL use in enzymatic catalysis, lubricants, heat-transfer fluids, and gas storage and separation, among other applications. Widely accepted as a mechanism by which the melting temperature of cell membranes is modulated, HVA is the incorporation into cell membranes of phospholipids with “kinked” tail structures.[10] It is argued that the packing efficiency of the collective membrane hydrophobic components is diminished by the presence of these phospholipids and that increased fluidity results. A comparison of the Tm value of distearoylphosphatidylcholine with that of dioleylphosphatidylcholine provides a dramatic example of how much impact this seemingly trivial difference can have. The former, with its linear, saturated C18 tails has a Tm value of 588C; the latter, with its “kinked” C18 tails (each of which incorporates a cis-alkenyl group), has a Tm value of À228C. This effect is also at the heart of the Tm difference between the solid triacyl glycerols called fats, and those that are liquid at room temperature known as oils. In both instances, the effect is probably entropic in nature, as in the case of anthracene (“linear”, Tm= 2178C) and phenanthrene (“kinked”, Tm= 998C).[11] Accordingly, we hypothesized that ILs with long, unsaturated, aliphatic tail structures would, like the corresponding phospholipids, have significantly lower Tm values than their counterparts with saturated appendages. To test the validity of our hypothesis by measuring their Tm values, we prepared a series of lipid-inspired ILs in a threestep process from high-purity (99+%) fatty-alcohol mesylates, 1-methylimidazole, NaI, and NaTf2N.[12] Each of the ILs (Scheme 1) had a long alkyl appendage identical to that in a natural fatty acid. Compounds 1, 3, and 8 feature fully saturated C16, C18, and C20 side chains, respectively, and their