Sensing isothermal changes in the lateral pressure in model membranes using di-pyrenyl phosphatidylcholine

In this work we present data from a homologous series of di-pyrenyl phospha tidylcholine (dipyPC) probes which can sense lateral pressure variations in the chain region of the amphiphilic membrane (lateral pressures are tangen tial to the interface). The dipyPC has pyrene moieties attached to the ends of equal length acyl chains on a phosphatidylcholine molecule. Ultraviolet stimulation produces both monomer and excimer fluorescence from pyrene. At low dilutions of dipyPC in model membranes the excimer signal is entirely intra-molecular and since it depends on the frequency with which the pyrene moieties are brought into close proximity, the relative intensity of the e xcimer to monomer signal, eta, is a measure of the pressure. We synthesised or purchased dipyPC probes with the pyrene moieties attached to acyl chain s having 4, 6, 8 and 10 carbon atoms and then measured eta in fully hydrate d bilayers composed of dioleoylphosphatidylcholine and dioleoylphosphatidyl ethanolamine (DOPC and DOPE respectively). Although the resolution of our m easurements of lateral pressure as a function of distance into the monolaye r was limited, we did observe a dip in the excimer signal in the region of the DOPC/DOPE cis double bond. As we isothermally increased the DOPE compos ition, and hence the desire for interfacial curvature, we observed, as expe cted, that the net excimer signal increased. However this net increase was apparently brought about by a transfer of pressure from the region around t he glycerol backbone to the region near the chain ends, with the lateral pr essure dropping above the cis double bond but increasing at a greater rate beyond the double bond.