Study of Structural stability and formation mechanisms in DSPC and DPSM liposomes: A coarse-grained molecular dynamics simulation

www.nature.com/scientificreports OPEN Study of Structural stability and formation mechanisms in DSPC and DPSM liposomes: A coarse-grained molecular dynamics simulation H. Hashemzadeh1, H. Javadi2 & M. H. Darvishi2* Liposomes or biological vesicles can be created from cholesterol, phospholipid, and water. Their stability is affected by their phospholipid composition which can influence disease treatment and drug delivery efficacy. In this study, the effect of phospholipid type on the formation and stability of liposomes using coarse-grained molecular dynamics simulations is investigated. For this purpose, the simulation study of the DSPC (1,2-distearoyl-sn-glycero-3-phosphocholine) and DPSM (Egg sphingomyelin) lipids were considered. All simulations were carried out using the Gromacs software and Martini force field 2.2. Energy minimization (3000 steps) model, equilibrium at constant volume to adjust the temperature at 400 Kelvin and equilibrium at constant pressure to adjust the pressure, at atmospheric pressure (1 bar) have been validated. Microsecond simulations, as well as formation analysis including density, radial distribution function, and solvent accessible surface area, demonstrated spherical nanodisc structures for the DPSM and DSPC liposomes. The results revealed that due to the cylindrical geometric structure and small-size head group, the DSPC lipid maintained its perfectly spherical structure. However, the DPSM lipid showed a conical geometric structure with larger head group than other lipids, which allows the liposome to form a micelle structure. Although the DSPC and DPSM lipids used in the laboratory tests exhibit liposome and micelle behaviors, the simulation results revealed their nanodisc structures. Energy analysis including overall energy, Van der Waals interaction energy, and electrostatic interaction energy showed that DPSM liposome is more stable than DSPC liposome. Liposomes, or small vesicles, can be created from non-toxic (normal) cholesterol and phospholipids. Liposomes were introduced in 1965 and initially used as models for studying biological membranes1. Later, Liposomes have been considered as drug delivery systems. Solutions of phospholipids in water exhibit a large variety of aggregation states such as double layer or liposome membrane2,3. Characteristics such as inherent low toxicity, biodegradability and lack of immunogenicity qualify Liposomes as a considerable carrier in novel drug delivery systems4–7. In addition, Liposomes are dynamic, so different structures can be achieved depending on the phospholipids types8,9. It is now possible to engineer a wide variety of sizes, phospholipids, and liposomal superficial properties10. The liposomal surface can be engineered and functionalized by selecting special lipids, in facilitating covalent binding of proteins (e.g. antibodies and proteins to sugars, i.e lectin), glycoproteins and synthetic proteins 11. Liposomes are widely used in vaccines, enzymes and drug (insulin and anticancer drugs) carriers12. In general, the highly unsaturated phospholipid compounds can lead to the instability of the liposome structure13. Lipids derived from biological sources such as eggs and soybeans typically consist of significant levels of unsaturated fatty acids, thus inherently are less stable than their counterparts. While saturated lipids are more stable, they have a higher transition temperature14. Liposomes containing saturated phospholipids showed increased stability and high transition temperature compared to liposomes composed of unsaturated phospholipids. Hence, to liposome synthesizing purposes if unsaturated phospholipids is essential, it is important that keep the transition temperature degree as low as possible. The existence of Polyethylene Glycol (PEG) on the surface of liposomal 1 Nanobiotechnology Department, Faculty of Bioscience, Tarbiat Modares University, Tehran, Iran. Nanobiotechnology Research Center, Baqiyatallah University of Medical Sciences, Tehran, Iran. *email: darvishi@ alumnus.tums.ac.ir 2 Scientific Reports | (2020) 10:1837 | https://doi.org/10.1038/s41598-020-58730-z 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. 1,2-distearoyl-sn-glycero-3-phosphocholine and Egg sphingomyelin phospholipids. delivery systems has shown to increase the half-life in blood-circulation15, while reducing toxicity and exposure of healthy cells to drug toxicity, i.e. drugs in vulnerable tissues such as the liver and kidneys16–19. In addition, the combination of PEG with liposome has resulted in improvement of liposomal stability15,20. The most important obstacle in liposomal technology is their long-term instability, especially when used as drug carriers21. Physical and chemical stability of liposomes are affected by various factors influencing the liposomes stability and the effectiveness of drug penetration22. For this reason, the stability of liposomes is critical for long time circulation. Long-term stability of liposomes containing pharmaceutical substances is strongly influenced by the type of phospholipid’s liposome structure. The main advantage of molecular dynamic simulations is obviously the decreased costs23. Molecular dynamics simulation is useful tools that offer information about biomolecular hydrodynamic behavior9,15. In other words, fundamental understandings about stability and formation mechanisms of lipids especially liposomes could provide a guideline for rational design of them. Molecular dynamics simulation is helpful to open the new opportunities to further investigate liposomes structure and functionality. Coarse-grained (CG) models offer simulation of larger systems like lipids for longer times by decreasing the number of degrees of freedom (df) compared with all-atom models23. In this research, the effect of phospholipid type on the formation and stability of liposomes using coarse-grained molecular simulations is studied. For this purpose, the liposomes of DSPC (1,2-distearoyl-sn-glycero-3-ph osphocholine) and DPSM (Egg sphingomyelin) were simulated. Figure 1 shows the two types of phospholipid used in this study received from the cgmartini and Avantilipids webpages. DSPC and DPSM phospholipids are issued in the laboratory to synthesize spherical liposomes. The DSPC phospholipid creates spherical liposomal structures, while DPSM phospholipid develops a double layer membrane structure. Coarse-grained simulations consider similar atoms close together as a sphere, and allow simulations of systems that are not available at all common atomic time scales24,25. Results and Discussion Today, many theoretical and empirical studies investigate the formation and stability of liposomes, due to their importance as drug carriers, and are experimentally synthesized in laboratories. However, molecular dynamics simulation has the ability to deliver more detailed information in the field of biosynthesis and drug delivering with regards to th (...truncated)