# Molecular Insights into the Interactions Between Human Serum Albumin and Phospholipid Membranes
## 1 Introduction  
Phospholipid bilayers play a crucial role in organisms by constituting the basic structure of cell membranes, providing a barrier against environmental factors, and facilitating various cellular processes [1, 2, 3]. Despite their well-known biological role and relatively simple structure, phospholipids remain a subject of scientific interest for their emerging applications and features, such as learning and memory capabilities [3]. Notably, reconstituted proteoliposomes also merit special consideration for their ability to replicate natural biological systems, particularly in bioenergy applications [4] and artificial cell models [5]. Two classes of phospholipids with significant biological relevance are phosphatidylcholine (PC) and phosphatidylethanolamine (PE), with representative examples being 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoryylethanolamine (DPPE). Membranes and liposomes composed of DPPC and DPPE are widely utilized in pharmaceutical and biomedical applications [3; 6; 7; 8; 9; 10; 11; 12]. DPPE membranes exhibit denser packing compared to those formed from DPPC, influencing their interactions with other molecules [13; 14]. A far more complex biomolecule than DPPE and DPPC, which also holds significant biological and biomedical importance, is human serum albumin (HSA). Similar to phospholipids, HSA has the ability to bind with a wide range of both polar and non-polar substances, highlighting its versatile functional properties [15; 16]. Moreover, it plays crucial biological roles, serving as a carrier for various bioactive molecules, maintaining osmotic pressure, pH of blood, and electrolyte homeostasis [17; 18; 19; 20; 21]. The interactions between HSA and phospholipid layers significantly affect molecular transport, as demonstrated in both in vitro and in vivo studies [22]. It is worth mentioning that albumins are known to possess multiple binding sites when interacting with phospholipid liposomes [23]. Notably, even a low concentration of bovine serum albumin can modulate the structural arrangement of phospholipids [24]. Lipid-protein systems have a wide range of applications in various fields, such as encapsulation [25], food packaging, and coatings [26; 27; 28]. The distinctive properties of phospholipid- and albumin-based systems have made them valuable in drug delivery. This has been particularly evident in the development of nanocomplexes and nanoparticles, which enable the controlled and sustained release of active substances [29; 30]. In addition to the well-established biological and pharmaceutical significance of HSA-membrane interactions, their biomechanical implications also warrant attention. Phospholipids, along with HSA, \(\gamma\)-globulin, collagen, lubricin, and hyaluron, are components of articular cartilage and synovial fluid, contributing significantly to joint lubrication and health [31]. In previous works [32; 33; 34; 35; 36; 37], the interactions between phospholipid assemblies and various natural and synthetic macromolecules were examined using molecular dynamics methods. Particularly interesting for understanding the mechanism of binding biologically active compounds to biopolymers and membranes is the role of physiologically occurring ions such as Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), and Ca\({}^{2+}\). It is well-known that they can significantly influence the structural organization and energetic stability of various biomolecular systems [38; 39; 40; 41; 36]. The next phase of the project involves exploring the interactions of HSA with DPPC and DPPE phospholipid bilayers, which contain choline and ethanolamine groups, respectively. Although the ability of albumin and other serum proteins to bind to lipid bilayers has been investigated [42], to the best of our knowledge, there are no available studies on the impact of changes in the phospholipid headgroup structure on albumin affinity to the membrane.