Long-range tilt orientational order in phospholipid monolayers: a comparative study

Monolayers of dipalmitoyl-phosphatidyl-N-monomethylethanolamine (DPP(Me)E) and dipalmitoyl-phosphatidyl-N,N-dimethylethanolamine (DPP(Me2)E) are studied and compared with dipalmitoyl-phosphatidylethanolamine (DPPE), dimyristoyl-phosphatidylethanolamine (DMPE), and dipalmitoyl-phosphatidylcholine (DPPC) to characterize the influence of the headgroup size. The properties of the condensed phases of DPP(Me2)E and DPP(Me)E are between those of DPPC and DMPE or DPPE. DPPC domains are elongated and the orientation changes continuously, whereas DMPE domains are compact and the orientation jumps at curved lines. The domains of DPP(Me2)E and DPP(Me)E are compact, and not elongated. The orientation changes continuously by 360 degrees around a point in the centered domains, and jumps of the orientation occur only in the case of twinning. Furthermore, the size of the headgroup influences the erection of the aliphatic chains. For DPPC and (DPP(Me2)E), no complete erection of chains occurs, whereas for DPP(Me)E the surface pressure required for the complete erection of chains is much higher than for DPPE. The same tendency is found for the collapse. DPPC monolayers do not collapse. DPP(Me2)E monolayers collapse at a much higher surface pressure than those of DPP(Me)E and DPPE.