The effect of potential-sensitive molecular probes on the thermal phase transition in dimyristoylphosphatidyl choline preparations (original) (raw)

1988, Biochimica Et Biophysica Acta-biomembranes

Differential scanning calorimetry 0DSC) has been employed to determine the effect of five commonly employed extrinsic potential.sensitive probes on phase transitions of multilamellar suspensions of t-n-di. myristoylphesphatidylcboline (DMPC). At tool% 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 pl'~se transition can be described by an equation based on the formation of ideal solutions in both phases. Even at up to 4 mnl%, 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 capaeity profile for diS-C3-(5) is represented by the ideal solution equation, even at 12 moi%, whereas the suspensions with the other probes present at this level have profiles covering It rge 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 presenee of well-defined microdomains or even pP,~se separation. The enthalpy change associated with the transition near 15°C involving packing in the heedgroup region is decreased significantly, indicating that the probes probably affect the lipid be~up ~o.-nation, 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 ~th phases, but a penetration of the other probes into the hydrocarbon region, at least in the liquid crystal phase.