The results were compared with those obtained with monopolar phospholipids having straight chains (POPC, DPPC) and archaeal-inspired branched-chain phospholipids (DPhPA, DPhPC, DoPhPC, DPhPG) (Figure , Figure ). We found that the GDGTs display significantly higher mean molecular areas (MMA) as compared to monopolar lipids (Figure ). At the lift-off point, the GDGTs display MMA considerably higher than 300 Å 2 while at a surface pressure of 25 mN/m, all GDGTs display MMA of approximately 250 Å 2 . The corresponding values for monopolar lipids are at least 3-4 times smaller. The high MMA values of the GDGTs can be explained by suggesting that these lipids have the conformational flexibility to loop into a U-shaped conformation at the air-water interface -i.e., with both headgroups facing water and the hydrocarbon chains forming an arch (Figure ).