Phospholipids

The unique interfacial properties, enormous diversity, and biocompatibility of phospholipids make them attractive pharmaceutical excipients. These amphiphilic molecules have the property of self-assembling into different structures. The solubility, chemical and structural properties, surface charge and key packaging parameters of phospholipids play a crucial role in formulation design. They serve multiple functions as solubilizers, emulsifiers, surfactants, penetration enhancers, coating agents, sustained release modifiers, and liposomes. BOC Sciences produces phospholipids using advanced synthesis techniques and state-of-the-art equipment, which ensures that phospholipid products are of the highest quality and meet the most stringent industry standards. We have a team of experienced professionals who provide reliable customer support and provide guidance throughout the ordering and product selection process.

What is Phospholipid?

Phospholipids are lipids that contain carbon, hydrogen, oxygen, and phosphate groups. The structure consists of a glycerol backbone that is esterified with fatty acid chains at positions 1 and 2, while the phosphate group at position 3 is further esterified with a polar moiety (alcohol). The polar headgroup imparts hydrophilicity, while the attached fatty acid chains impart hydrophobicity (non-polar regions) to the phospholipid molecule. Therefore, phospholipids are amphipathic. Four different functional groups attached to the second carbon of the glycerol moiety endow the phospholipid molecule with chirality. Variations in attached polar headgroups, main chains, and fatty acid side chains result in different properties. The backbone of phospholipids can be glycerol or sphingosine. Polar headgroups attach to phosphatidic acid to form phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) or phosphatidylglycerol (PG). Sphingosines form the class of sphingomyelin (SM). Some of the fatty acid chains that can be attached are disterol (DS), dipalmitol (DP), dimyristol (DM) and dioleoyl (DO). Fatty acid chains can be saturated or unsaturated.

Fig. 1. Phospholipid bilayer structure (Biochemical Pharmacology. 2022, 206: 115296).

Origin of Phospholipids

Phospholipids can be natural, semi-synthetic or synthetic. Phospholipids are mainly derived from various plants (vegetable oils extracted from soybean, sunflower, corn, and cottonseed) and animal sources (animal tissues such as bovine brain and egg yolk). Phospholipids of vegetable and animal origin contain different amounts and types of lipids. Yolk phospholipids mainly contain sphingomyelin backbone and long-chain polyunsaturated fatty acids. Compared with soybean phospholipids, egg yolk phospholipids have higher saturation, higher PC content and better oxidation stability. The oxidative stability of lipids is directly related to the number of double bonds on the fatty acid side chains. Natural phospholipids can be partially or completely modified by chemical or enzymatic methods to produce semi-synthetic and synthetic phospholipids with desired properties, respectively. Synthetic or semi-synthetic lipids are expensive compared to natural lipids. Where possible, natural phospholipids should be chosen for formulation development because they can be produced in large quantities in a renewable manner at low cost. However, synthetic phospholipids yield stable structures compared to natural phospholipids.

Properties of Phospholipids

Solubility

The solubility of phospholipids depends on the polar head group and fatty acid side chains. According to solubility, they are classified as Class 1: insoluble phospholipids (waxes) that do not absorb water; Class 2: very low solubility phospholipids that swell in water, such as PCs, PEs or SM; Class 3A: soluble phospholipids , forming soluble liquid crystals at low water content, such as ly solecithins; Class 3B: soluble phospholipids, forming micelles, such as saponin.

Surface Charge

The surface charge acquired by phospholipids when dispersed in water depends on the head group and the pH of the medium. At pH 7, phospholipids containing PC and PE headgroups have a neutral charge (zwitterions), while phospholipids containing PS, PI, and PG headgroups have a negative charge. The cationic charge on the phospholipid head facilitates the attraction of the nanoparticles to the cell membrane, increasing the rate of cellular incorporation. Thus, the positively charged nanoparticles on the surface (provided by PE at skin pH) are attracted to cell membranes, thereby enhancing skin penetration.

Phase Transition Temperature (PTT)

The temperature at which phospholipids change from a gel (highly ordered) to a liquid crystalline phase (disordered) is called the phase transition temperature. It depends on the polar head group, the length of the fatty acid side chain, the degree of saturation of the fatty acid side chain and the purity of the phospholipid. The PTT of the PE header group is higher than the PTT of the PC or PG header group. This is related to the stronger head-group interaction of the former. Phospholipids with longer side chains have a higher PTT compared to phospholipids with shorter side chains because more energy is required to break bonds. Likewise, saturated phospholipids show higher PTT. Phospholipids composed of polyunsaturated side chains can exhibit PTTs even below 0℃. Phospholipid bilayer assemblies (liposomes) made of low-PTT lipids (below 37℃) have been reported to be leaky. They are readily taken up by macrophages in the blood. Therefore, lipids with high PTT need to be preferentially selected if longer circulation time and controlled release are desired from these nanoparticles.

Polymorphism

Phospholipids dispersed in water can exist in various forms, depending on the degree and type of hydration of the phospholipids. They can form two-dimensional layered structures or different gel phases. They can exist as three-dimensional spherical, cubic, hexagonal or cylindrical structures. The different polymorphic forms play an important role in modifying the release of the drug and the stability of the formed nanoaggregate. The type of structure formed depends on the size of the polar head group, the degree of saturation of the side chains, the concentration of phospholipids, temperature, ionic strength, pH, the presence of other molecules such as steroids, oils, or divalent cations such as calcium.

Stability of Formed Structures

Nanoparticles formed from phospholipids tend to aggregate. Strong van der Waals attraction is the main cause of uncharged particle aggregation. Charged nanoparticles are considered to be more stable compared to uncharged particles. This can be explained by DLVO theory. A charged surface attracts several counterion shells towards it. These bilayer forces are responsible for the stability of the nanoparticles. Molecular dynamics simulations predict that polar heads have sufficient force to induce bilayer-bilayer repulsion pulses. Although there is a chance that two vesicles will fuse together upon contact.

Reference

1. Morita, S. et al. Regulation of membrane phospholipid biosynthesis in mammalian cells. Biochemical Pharmacology. 2022, 206: 115296.