Phosphatidylglycerol (PG) is an anionic glycerophospholipid characterized by a diacylglycerol backbone esterified at sn-3 to a phosphoglycerol headgroup (-PO4-CH2CH(OH)CH2OH). It typically represents 1–5% of eukaryotic membrane phospholipids but can reach up to 20% in prokaryotes and chloroplasts. Its neutral diol-terminated headgroup mimics water solvation while carrying a net negative charge, enabling unique hydrogen bonding, Ca²⁺-mediated aggregation, and membrane protein stabilization distinct from other glycerophospholipids.
Molecular Structure
PG is derived from sn-glycerol-3-phosphate and commonly contains saturated acyl chains at the sn-1 position (16:0/18:0) and unsaturated chains at sn-2 (e.g., 16:0/18:1 in DPPG or 18:1/18:1 in DOPG). Scattering models describe its headgroup as spanning the carbonyl-glycerol, phosphate, and terminal glycerol moieties. Its conical molecular geometry (head area ~55–65 Ų) promotes negative membrane curvature. In bacteria, PG frequently incorporates branched-chain acyl groups such as 14:0 anteiso.
Biophysical Properties
In the fluid phase, PG exhibits larger molecular areas than phosphatidylcholine (e.g., 68 Ų for DPPG versus 64 Ų for DPPC at 30°C), primarily due to electrostatic repulsion between negatively charged headgroups. PG bilayers display a thickness of ~35–40 Å and a thermal expansivity αA of approximately 0.005/°C. While it predominantly forms lamellar phases, PG can transition into hexagonal HII structures under dehydration or in the presence of Ca²⁺. Charge clustering also enhances interactions with antimicrobial peptides (AMPs).