HEAT CAPACITY EFFECTS IN LIPIDS DURING UNICELLULAR/MULTICELLULAR PHASE CHANGES

磷脂水分散体性质的研究进展 指示发生高阶相变，涉及 单层液晶态的自发转变 随着环境的增加，大的单层囊泡悬浮 温度；有人认为，单层囊泡 形式是一种批判状态。它的热力学性质 变换主要是从以下属性推断出来的 与分散磷脂平衡的空气-水表面膜 相位。更直接、概念上更简单的分析 这种高阶跃迁的热力学性质可以得到 通过测量热容对温度的依赖关系 脂类分散体。因为这种类型的转换被认为是 与细胞膜的组装密切相关，我们已经开发出 一种极灵敏的差动热导量热计 测定水膜脂类分散体的热容。(它 应该指出的是，试图在 商业化的热量计并不成功。)这台仪器有 与商用差分扫描相比，有一些明显的优势 量热仪，特别是基线重复性和分辨率。在这 变换，要合理地测量热容变化，需要一个 灵敏度为0.0001卡/度的量热计。我们估计了 单层囊泡形成的能量学敏感性 利用双分子层内结合能的多层液晶 大约0.01ergs/cm2。假设温度间隔为1度 对于过渡，这将转化为 订购0.001卡/度。为了确保合理的精度，我们需要一个 灵敏度至少为计算值的百分之一的仪器 过渡能量。此外，由于只有镁含量的膜脂 可供选择，我们仅限于小样品尺寸(1毫升)。

Recent studies of the properties of phospholipid dispersions in water indicate that a higher-order phase transition occurs involving a spontaneous transformation from a unilamellar liquid-crystal state to a suspension of large, unilamellar vesicles upon increasing the ambient temperature; it has been suggested that the unilamellar vesicles that form are a critical state. The thermodynamic properties of this transformation have been inferred primarily from the properties of air-water surface films in equilibrium with the dispersed phospholipid phase. A more direct, and conceptually simpler, analysis of the thermodynamic properties of this higher-order transition may be attained by measurements of the temperature dependence of the heat capacity of the lipid dispersions. Since transformations of this type are believed to be intimately involved in the assembly of cell membranes, we have developed an extremely sensitive differential heat conduction calorimeter for measuring heat capacities of aqueous membrane lipid dispersions. (It should be noted that attempts to measure this transformation in commercial calorimeters have not been successful.) This instrument has certain obvious advantages over the commercial differential scanning calorimeters, notably baseline repeatability and resolution. In this transformation, to measure the heat capacity change sensibly requires a calorimeter with a sensitivity of 0.0001 cal/deg-g. We estimated the sensitivity for the energetics of unilamellar vesicle formation from multilamellar liquid crystals using the intrabilayer cohesive energy of approximately 0.01 ergs/cm2. Assuming a one-degree temperature interval for the transition, this translates into a change of heat capacity on the order of 0.001 cal/deg-g. To assure reasonable precision, we required an instrument with a sensitivity of at least one percent of the calculated transition energy. Moreover, since only mg quantities of membrane lipids are available, we are restricted to small sample sizes (1 cc).