The effect of potential-sensitive molecular probes on the thermal phase transition in dimyristoylphosphatidylcholine preparations.

Differential scanning calorimetry (DSC) has been employed to determine the effect of five commonly employed extrinsic potential-sensitive probes on phase transitions of multilamellar suspensions of L-alpha-dimyristoylphosphatidylcholine (DMPC). At mol% values of less than five, the effect of these probes on the excess heat capacity curve in the vicinity of the gel to liquid crystal phase transition can be described by an equation based on the formation of ideal solutions in both phases. Even at up to 4 mol%, these dyes only moderately reduce the enthalpy change associated with this transition, but cause a marked decrease in the size of the cooperative unit parameter. The excess heat capacity profile for diS-C3-(5) is represented by the ideal solution equation, even at 12 mol%, whereas the suspensions with the other probes present at this level have profiles covering large temperature ranges. Multiple peaks appear at the higher levels for the negatively charged oxonols V and VI, and merocyanine 540, a result consistent with the presence of well-defined microdomains or even phase separation. The enthalpy change associated with the transition near 15 degrees C involving packing in the headgroup region is decreased significantly, indicating that the probes probably affect the lipid headgroup conformation, even at low levels. The cyanine probe diS-C3-(5) causes the heat capacity profile of small unilamellar vesicles to be transformed very rapidly into one similar to that of the vortexed lipid preparations, presumably by a dye-mediated vesicle fusion process, enhanced by the surface location of this probe. All our results are consistent with diS-C3-(5) being located on the surface of the bilayer in both phases, but a penetration of the other probes into the hydrocarbon region, at least in the liquid crystal phase.