# Molecular Insights into the Interactions Between Human Serum Albumin and Phospholipid Membranes
## 3 Results and Discussion
### The Effect of HSA Binding on Bilayer Shape and Density  
An important feature of phospholipids from a biological perspective is their ability to self-organize, which can, of course, be influenced by the presence of other compounds. The optimized structures of three examples of molecular assemblies containing albumin and considered phospholipid membranes are presented in Figure 1. Additionally, snapshots of the simulation boxes at the conclusion of the simulation are provided in the Supplementary Materials (Figure S1). It is worth noting that at 310 K, a temperature close to physiological conditions, fully hydrated bilayers composed of DPPC and DPPE are in the gel phase [62]. As clearly illustrated in the snapshots, although the obtained membranes maintain a well-ordered structure, as seen in the images, the membrane undergoes a shape change to accommodate HSA. The interaction between phospholipid bilayers and proteins, including albumin, has been extensively studied and is well documented both experimentally and theoretically [63; 64; 65; 66; 67; 68]. These interactions often result in notable surface deformations of the bilayers, driven by the minimization of the overall free energy of the system. This process leads to significant alterations in the curvature and structural integrity of the lipid bilayer at the site of contact with the protein. Albumins,owing to their unique structural properties and affinity for phospholipid liposomes [23], can induce complex changes within the bilayer. It is worth mentioning that experimental research on the systems composed of phosphatidylcholine (PC) and sodium cholate has revealed that the concentration of albumin can significantly affect the size of micelles [69]. Furthermore, [PERSON] [70] has demonstrated the critical role of bovine albumin in the solubilization and phase transitions of systems composed of dipalmitoylphosphatidylcholine (DPPC). These studies suggest the presence of strong, specific interactions between albumin and the membrane surface, which can lead to considerable changes in the physical properties of the membrane. Such findings contribute to a deeper understanding of the dynamic nature of membrane-protein interactions and their implications for cellular processes and membrane functionality. It is suggested that the ability of the cell membrane to transport essential cellular components may be influenced by its strong interaction with albumin [22]. The permeability of membranes can be significantly influenced by the deposition density of phospholipids [71; 72]. It is noteworthy that the effect of reduced liposome packing due to interactions with serum proteins has been described using experimental tools by [PERSON] and [PERSON] [42]. Additionally, research by [PERSON] et al. [72] demonstrated that the presence of calcium salts can affect the packing of phospholipids in the membrane. Figure 1: Preferred orientations of HSA concerning the phospholipid bilayer, illustrated by selected snapshots of complexes in the presence of Ca\({}^{2+}\) cations. The membrane is colored pink, HSA domains: I—green, II—yellow, III—red. Panel (**A**) shows the preferred arrangement of HSA on the DPPC membrane. Panels (**B**) and (**C**) present the two possible orientations of HSA with respect to the DPPE membrane. To characterize the impact of complex formation on structural changes in the phospholipid bilayer, the changes in phospholipid density resulting from the adsorption of HSA on its surface were analyzed. Figure 2 presents selected phospholipid mass profiles as a function of cross-section for selected systems. The plots represent average density values across the entire bilayer. Additionally, the density profiles for the nitrogen atoms in the phospholipid head groups are also depicted. As clearly seen, the formation of a complex with HSA affects the distances between phospholipid molecules in the considered membranes. Interestingly, when focusing on the nitrogen atoms in the phospholipid head groups, only minor variations in density are observed.