Kaasgaard Thomas, Mouritsen Ole G, Jørgensen Kent
Department of Chemistry, Technical University of Denmark, Lyngby, Denmark.
Biochim Biophys Acta. 2003 Sep 2;1615(1-2):77-83. doi: 10.1016/s0005-2736(03)00194-9.
Differential scanning calorimetry (DSC) has been used to study the effects of repeated freezing and thawing on dipalmitoylphosphatidylcholine (DPPC) vesicles. Aqueous suspensions of both multilamellar vesicles (MLVs) and large unilamellar vesicles (LUVs) were cycled between -37 and 8 degrees C, and for each thawing event, the enthalpy of ice-melting was measured. In the case of MLVs, the enthalpy increased each time the vesicles were thawed until a steady state was attained. In contrast, the enthalpies measured for LUV suspensions were independent of the number of previous thawing events. It was concluded that MLVs in terms of freezing characteristics contain two pools of water, namely bulk water and interlamellar water. Interlamellar water does not freeze under the conditions employed in the present study, and the MLVs therefore experience freeze-induced dehydration, which is the reason for the observed increase in ice-melting enthalpy. Furthermore, the thermodynamic results suggest that the osmotic stress resulting from the freeze-induced dehydration changes the lamellarity of the MLVs.
差示扫描量热法（DSC）已被用于研究反复冻融对二棕榈酰磷脂酰胆碱（DPPC）囊泡的影响。多层囊泡（MLV）和大单层囊泡（LUV）的水悬浮液在-37℃和8℃之间循环，并且对于每次解冻事件，测量冰融化的焓。在MLV的情况下，每次囊泡解冻时焓都会增加，直到达到稳定状态。相比之下，LUV悬浮液的焓与之前解冻事件的次数无关。得出的结论是，就冷冻特性而言，MLV含有两类水，即体相水和层间水。在本研究采用的条件下层间水不会冻结，因此MLV经历了冷冻诱导的脱水，这就是观察到冰融化焓增加的原因。此外，热力学结果表明，冷冻诱导脱水产生的渗透压改变了MLV的片层结构。
