A Study of the Molecular Structure of Phospholipids and the Aggregation of Liposomes Using the Molecular Orbital Method

The development of functional liposomes using phospholipid liposomes has been performed for some time; however, from the perspectives of efficiency and safety, it is important that the fusion or aggregation of liposomes be controlled and regulated. Phospholipids are ubiquitous in biological membranes; among them, phosphatidylcholines (PC) are commonly used in studies on bilayer membranes and functional liposomes. In addition, the aggregation/disaggregation phenomena of PC liposomes are thought to be caused by heat fluctuations in the membrane during gel-liquid crystal transition. Moreover, a dipole is present in the hydrophilic moiety of PC molecules. Therefore, it is considered that the aggregation phenomenon of phospholipid liposomes might be due to heat fluctuations in the membranes and also the effect of local electric forces caused by the dipole. Therefore, in order to examine the three-dimensional conformation of phospholipid molecules, we calculated the total energy of PC molecules using the molecular orbital method, and 3 optimized structures were found. These structures correlated well with those obtained from other experiments. This shows that the analysis of the molecular structure of phospholipids using the molecular orbital method can be an effective tool to elucidate the interactions between membranes and the properties and functions of phospholipid liposomes.