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.

Interaction of toposome from sea-urchin yolk granules with dimyristoyl phosphatidylserine model membranes: a 2H-NMR study.

The yolk granule is the most abundant membrane-bound organelle present in sea urchin eggs and embryos. The major protein component of this organelle, toposome, accounts for approximately 50% of the total yolk protein and has been shown to be localized to the embryonic cell surface. Extensive characterization in several laboratories has defined a role for toposome in mediating membrane-membrane interactions. The current study expands the analysis of toposome-membrane interaction by defining toposome-induced changes to the lipid bilayer. The effect of toposome on the biophysical properties of phosphatidyl serine (PS) multibilayers was investigated using deuterium nuclear magnetic resonance and perdeuterated dimyristoyl PS (DMPS-d(54)). Toposome was found to have little effect on DMPS-d(54) chain orientational order in both the gel and liquid-crystalline phases. Timescales for DMPS-d(54) reorientation were investigated using quadropole echo decay. Echo decay times were sensitive to toposome in the liquid-crystalline phase but not in the gel phase. Additional information about the perturbation of bilayer motions by toposome was obtained by analyzing its effect on the decay of Carr-Purcell-Meiboom-Gill echo trains. Collectively, these results suggest that toposome interacts peripherally with DMPS bilayers and that it increases the amplitude of lipid reorientation, possibly through local enhancement of bilayer curvature.