Department of Physical Chemistry - Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Poland.
Department of Physical Chemistry - Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Poland.
Adv Colloid Interface Sci. 2015 Aug;222:385-97. doi: 10.1016/j.cis.2014.03.008. Epub 2014 Mar 28.
Biological cell membranes play a crucial role in various biological processes and their functionality to some extent is determined by the hydrophilic/hydrophobic balance. A significant progress in understanding the membrane structure was the discovery of laterally segregated lipid domains, called the lipid rafts. These raft domains are of ordered lamellar liquid-crystalline phase, while rest of the membrane exists in a relatively disordered lamellar liquid-crystalline phase. Moreover, the chemical constitution of the lipid rafts consists of a higher content (up to 50%) of cholesterol (Chol) and sphingomyelin (SM). Sphingomyelin also plays a significant role in the red cells of blood and nerves, in some diseases, as a precursor to ceramides, and other sphingolipid metabolites. In this paper properties of Langmuir and solid supported mixed lipid films of DPPC/SM, DOPC/SM, and Chol/SM are described. Special attention has been paid to wetting properties (hydrophobic/hydrophilic balance) of these films transferred onto a hydrophilic glass surface. To our knowledge such results have not yet been published in the literature. The properties were determined via contact angle measurements and then calculation of the films' apparent surface free energy. The films' wettability and their apparent surface free energy strongly depend on their composition. The energy is affected by both the structure of hydrocarbon chains of glycerophospholipids (DPPC and DOPC) and their interactions with SM. Properties of mixed Chol/SM monolayer depend also on the film stoichiometry. At a low Chol content (XChol=0.25) the interactions between SM and Chol are strong and hence the formation of binary complex is possible. This is accompanied by a decrease in the film surface free energy in comparison to that of pure SM monolayer, contrary to a higher Chol content where the monolayer energy increases. This suggests that cholesterol is excluded from the membrane thus increasing the film hydrophilicity. These results are consistent with the literature data and somehow confirm the hypothesis of lipid raft formation. The roughness of the investigated monolayer surfaces was also determined using optical profilometry. The roughness parameters of the DPPC, SM, and mixed DPPC/SM generally correlate with the changes of their apparent surface free energy, i.e. with the decreasing roughness the apparent surface free energy also decreases. However, this is not the case for mixed DOPC/SM monolayers. Although the roughness increases with SM content the apparent surface free energy decreases. Therefore some other factors, like the presence of unsaturated bonds in the DOPC molecule, influence the film phase state and the energy too. More experiments are needed to explain this hypothesis.
生物细胞膜在各种生物过程中起着至关重要的作用，其功能在某种程度上取决于亲水性/疏水性平衡。对膜结构的理解取得了重大进展，发现了侧向分隔的脂质域，称为脂质筏。这些筏域是有序的层状液晶相，而其余的膜存在于相对无序的层状液晶相中。此外，脂质筏的化学组成包括较高含量（高达 50%）的胆固醇（Chol）和神经酰胺（SM）。鞘磷脂在血液和神经的红细胞中也起着重要作用，在某些疾病中，它是神经酰胺和其他鞘脂代谢物的前体。本文描述了 DPPC/SM、DOPC/SM 和 Chol/SM 的 Langmuir 和固体支撑混合脂质膜的性质。特别关注这些膜转移到亲水玻璃表面后的润湿性质（亲水性/疏水性平衡）。据我们所知，这些结果在文献中尚未发表。通过接触角测量和随后计算膜的表观表面自由能来确定这些性质。膜的润湿性及其表观表面自由能强烈取决于其组成。能量受甘油磷脂（DPPC 和 DOPC）的烃链结构及其与 SM 的相互作用的影响。混合 Chol/SM 单层的性质还取决于膜的化学计量。在低 Chol 含量（XChol=0.25）下，SM 和 Chol 之间的相互作用很强，因此可以形成二元复合物。与纯 SM 单层相比，这伴随着膜表面自由能的降低，而在较高的 Chol 含量下，单层能量增加。这表明胆固醇被排除在膜外，从而增加了膜的亲水性。这些结果与文献数据一致，并且在某种程度上证实了脂质筏形成的假设。还使用光学轮廓法确定了所研究的单层表面的粗糙度。DPPC、SM 和 DPPC/SM 混合单层的粗糙度参数通常与它们的表观表面自由能的变化相关，即随着粗糙度的降低，表观表面自由能也降低。然而，对于混合 DOPC/SM 单层则不然。尽管随着 SM 含量的增加粗糙度增加，但表观表面自由能降低。因此，其他因素，例如 DOPC 分子中存在不饱和键，也会影响膜的相态和能量。需要进行更多的实验来解释这一假设。
