Gaemers S, Bax A
Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, Building 5 Room 126, National Institutes of Health, Bethesda, MD 20892-0520, USA.
J Am Chem Soc. 2001 Dec 12;123(49):12343-52. doi: 10.1021/ja011967l.
The morphologies of three dilute liquid crystalline phases, which are widely used for biological NMR spectroscopy, are investigated by the study of tracer self-diffusion. The aqueous liquid crystalline media investigated include the common phospholipid bicelle medium, a phase consisting of a mixture of pentaethyleneglycol mono dodecyl ether and hexanol, and a medium containing cetylpyridinium bromide and hexanol. Threonine and water were used as tracer molecules for probing the aqueous environment, and tetramethylsilane (TMS) was for probing the lipophilic environment. Pulsed field gradient NMR was used to measure tracer self-diffusion rates in three orthogonal directions. Although results for the water-soluble tracers in bicelle media do not contradict the widely accepted disk-shaped bicelle model, the high TMS diffusion rate observed in the bilayer plane requires extensive transient edge-to-edge contacts of such disks. This morphology is essentially that of a heavily perforated lamellar bilayer phase and explains why this medium remains liquid crystalline well below the Onsager limit for disk-shaped nematogens. Below 25 degrees C, a bicelle mixture consisting of dimyristoyl phosphatidyl choline and dihexanoyl phosphatidyl choline remains isotropic, but tracer diffusion obstruction indicates that the particles are significantly oblate. The diffusion anisotropy in the penta(ethyleneglycol) mono dodecyl ether liquid crystals confirms the previously proposed alpha-lamellar phase. However, weak inhibition of aqueous-phase self-diffusion in the z direction points to the presence of bridge- or caplike obstructions, and the bilayers appear slightly permeable to water. If the previously proposed concentric cylinder superstructure of bilayers applies, the diffusion data indicate that the most outer cylinder must have a diameter greater than 50 microm. The tracer self-diffusion data for the cetylpyridinium bromide/hexanol medium is only compatible with a planar alpha-lamellar phase, with its local director orthogonal to the magnetic field, and a very large domain size over which the director remains parallel.
通过示踪剂自扩散研究，对三种广泛用于生物核磁共振波谱学的稀液晶相的形态进行了研究。所研究的水性液晶介质包括常见的磷脂双分子层介质、由五乙二醇单十二烷基醚和己醇混合物组成的相以及含有十六烷基溴化吡啶和己醇的介质。苏氨酸和水用作探测水性环境的示踪分子，四甲基硅烷（TMS）用于探测亲脂性环境。脉冲场梯度核磁共振用于测量三个正交方向上的示踪剂自扩散速率。虽然双分子层介质中水溶性示踪剂的结果与广泛接受的盘状双分子层模型并不矛盾，但在双层平面中观察到的高TMS扩散速率要求这些盘之间存在广泛的瞬态边对边接触。这种形态本质上是一种有大量穿孔的层状双层相，这解释了为什么这种介质在远低于盘状向列相的昂萨格极限温度下仍保持液晶态。在25摄氏度以下，由二肉豆蔻酰磷脂酰胆碱和二己酰磷脂酰胆碱组成的双分子层混合物保持各向同性，但示踪剂扩散阻碍表明颗粒明显呈扁球形。五（乙二醇）单十二烷基醚液晶中的扩散各向异性证实了先前提出的α-层状相。然而，z方向上对水相自扩散的微弱抑制表明存在桥状或帽状障碍物，并且双层对水似乎有轻微的渗透性。如果先前提出的双层同心圆柱超结构适用，扩散数据表明最外层圆柱的直径必须大于50微米。十六烷基溴化吡啶/己醇介质的示踪剂自扩散数据仅与平面α-层状相兼容，其局部指向矢与磁场正交，并且指向矢在非常大的区域内保持平行。
