Fumero J, Bammel B P, Hopkins H P, Smith J C
Department of Chemistry, Georgia State University, Atlanta 30303.
Biochim Biophys Acta. 1988 Oct 6;944(2):164-76. doi: 10.1016/0005-2736(88)90429-4.
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.
差示扫描量热法（DSC）已被用于确定五种常用的外在电位敏感探针对L-α-二肉豆蔻酰磷脂酰胆碱（DMPC）多层悬浮液相变的影响。在摩尔百分比小于5时，这些探针在凝胶到液晶相变附近对过量热容曲线的影响可以用一个基于两相中理想溶液形成的方程来描述。即使在高达4摩尔%时，这些染料也只是适度降低与该转变相关的焓变，但会导致协同单元参数的大小显著减小。即使在12摩尔%时，二硬脂酰基三甲基铵丙烷（diS-C3-(5)）的过量热容曲线也可以用理想溶液方程表示，而在该水平下存在其他探针的悬浮液的曲线覆盖较大的温度范围。对于带负电荷的氧杂萘邻酮V和VI以及部花青540，在较高水平时会出现多个峰，这一结果与明确的微区甚至相分离的存在一致。与15℃附近涉及头部基团区域堆积的转变相关的焓变显著降低，这表明即使在低水平下，探针可能也会影响脂质头部基团的构象。花青探针二硬脂酰基三甲基铵丙烷（diS-C3-(5)）使小单层囊泡的热容曲线非常迅速地转变为与涡旋脂质制剂相似的曲线，推测是通过染料介导的囊泡融合过程，该探针在表面的位置增强了这一过程。我们所有的结果都与二硬脂酰基三甲基铵丙烷（diS-C3-(5)）在两个相中都位于双层表面一致，但其他探针至少在液晶相中会渗透到烃区域。
