Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China.
Langmuir. 2009 Dec 1;25(23):13394-401. doi: 10.1021/la901989j.
Dioctadecyldimethylammonium bromide (DODAB) is known to self-assemble into several lamellar structures in water, existing as either liquid crystalline, gel, or coagel phases. In this work, by using differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction techniques, we have characterized the details of the phase transition mechanisms of the DODAB aqueous dispersions. It was found that the liquid crystalline converts to the coagel phase via a two-step mechanism: first to the gel phase upon cooling and then to the stable coagel phase. Although significant conformational changes in the hydrocarbon tails were observed in both steps, changes in the headgroups of DODAB were only detected in the second step. More interestingly, we found that the lipid tails change prior to the headgroups during the overall liquid crystalline to coagel phase transformation process. This is regarded as a nonsynchronicity phenomenon, which reflects the regional (head/tail) imbalance in molecular interactions. Such a nonsynchronicity phenomenon in the self-assembled aggregates composed of the medium-sized DODAB molecules will shed light on our understanding of the polymorphism and reversibility of amphiphiles including both surfactants and biomembrane phospholipids.
双十八烷基二甲基溴化铵（DODAB）在水中会自组装成几种层状结构，存在液晶、凝胶或凝聚相。在这项工作中，我们通过差示扫描量热法、傅里叶变换红外光谱和 X 射线衍射技术，详细研究了 DODAB 水分散体的相变机制。结果表明，液晶相通过两步机制转变为凝聚相：先冷却到凝胶相，然后转变为稳定的凝聚相。尽管在两个步骤中都观察到了烃尾的显著构象变化，但 DODAB 的头基仅在第二步中发生变化。更有趣的是，我们发现脂质尾在整个液晶到凝聚相转变过程中先于头基发生变化。这被认为是非同步现象，反映了分子相互作用的区域（头/尾）不平衡。这种由中等大小的 DODAB 分子组成的自组装聚集体中的非同步现象，将有助于我们理解包括表面活性剂和生物膜磷脂在内的两亲物的多态性和可逆性。
