Pressure induces interdigitation differently in DPPC and DPPG

The phase behaviours of chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62) and chain-perdeuterated dipalmitoylphosphatidylglycerol (DPPG-d62) bilayers were compared using 2H nuclear magnetic resonance spectroscopy and quadrupole echo decay measurements over pressures ranging from ambient to 196 MPa and temperatures ranging from 60 to -25 degrees C. At ambient pressure, the phase behaviours of DPPC-d62 and DPPG-d62 were nearly identical. At 196 MPa, their behaviours were also very similar and both lipids appeared to pass from an interdigitated gel phase at high temperature, through a non-interdigitated gel phase at intermediate temperature, to a chain-immobilized ordered phase at low temperature. At 85 MPa, the behaviour of DPPC-d62 was similar to its ambient pressure behaviour with no evidence of interdigitation. For DPPG-d62, however, the behaviour at 85 MPa was similar to its higher pressure behaviour and spectra characteristic of an interdigitated gel phase were observed. Pressure-temperature phase diagrams for both lipids were compared. While the minimum pressure for DPPC-d62 interdigitation is about 150 MPa, DPPG-d62 was observed to interdigitate at pressures as low as 60 MPa. Given the similarity of their phase behaviours at both higher and lower pressures, this difference reflects the extent to which bilayer phase behaviour depends on the balance between interactions in the headgroup and hydrocarbon regions of the bilayer.