Phosphatidylcholine (PC) is the most abundant glycerophospholipid in eukaryotic cell membranes, representing approximately 40–50% of the phospholipids in the outer leaflet. It is characterized by a diacylglycerol backbone linked at the sn-3 position to a phosphocholine headgroup (-PO4-CH2CH2N+(CH3)3), which confers zwitterionic properties and a cylindrical molecular geometry. This structure is ideal for stable bilayer formation and contributes critically to membrane integrity.
Molecular Structure
PC follows the general glycerophospholipid scaffold: an sn-glycerol-3-phosphate backbone esterified with two fatty acyl chains (typically totaling 32–38 carbons) and linked to choline via a phosphodiester bond. Common molecular species include POPC (16:0/18:1) and DPPC (16:0/16:0). The large hydrated headgroup (surface area ~70 Ų) balances tail volume (V/aLc ≈ 1), preventing the formation of non-lamellar phases. Variants include lysoPC (containing a single acyl chain) and plasmenyl-PC (sn-1 vinyl ether).
A representative example is 1-oleoyl-2-palmitoyl-phosphatidylcholine, a biological surfactant commonly found in lecithin. Structural diagrams typically highlight the phosphocholine headgroup, the glycerol backbone, and the distinct saturated and unsaturated fatty acyl chains.
Biophysical Properties
The phase transition temperature (Tm) of PC depends strongly on acyl chain saturation. For example, DPPC exhibits a gel-to-liquid crystalline transition at approximately 41°C, whereas DOPC remains fluid with a Tm near -18°C. Cholesterol intercalation broadens these phase transitions and reduces membrane permeability (PL < 10-9 cm/s), contributing to membrane stability.
PC also displays high lateral mobility (~1–10 μm²/s), supporting efficient membrane protein diffusion. Outer leaflet enrichment is maintained through ABC transporter activity, preserving lipid asymmetry and enabling proper cellular signaling and membrane dynamics.
