Opatowski E, Lichtenberg D, Kozlov M M
Department of Physiology and Pharmacology, Tel Aviv University, Sackler Faculty of Medicine, Israel.
Biophys J. 1997 Sep;73(3):1458-67. doi: 10.1016/S0006-3495(97)78178-5.
We study the heat associated with the transformation of vesicles into micelles in mixtures of bilayer-forming phospholipids and micelle-forming surfactants. We subdivide the total heat evolution deltaQ(coex) within the range of coexistence of vesicles and micelles into three contributions related to the transition of dN(D)m-b molecules of surfactant and dN(L)m-b molecules of lipid from micelles to vesicles and to the extraction of dN(D)m-w molecules of surfactant from micelles to the aqueous solution, so that deltaQ(coex) = deltaH(D)m-w x dN(D)m-w + deltaH(D)m-b x dN(D)m-b + deltaH(L)m-b x dN(L)m-b where deltaH(D)m-w, deltaH(L)m-b, and deltaH(D)m-b are the respective molar "transfer" enthalpies. We design a method for the evaluation of all three molar enthalpies, from isothermal calorimetric titrations conducted according to two different protocols of titration of lipid-surfactant mixtures. In the first protocol the mixture is titrated with an aqueous solution of pure lipid vesicles, and in the second the mixture is titrated with an aqueous solution of pure surfactant. Titration of the mixed systems by a buffer solution serves to verify the results obtained under these protocols. In addition to the values of molar enthalpies, our method yields the cmc value of the pure surfactant. We apply our method to investigating the heat evolution in mixtures of egg yolk phosphatidylcholine and the nonionic surfactant octylglucoside in a phosphate-buffered saline solution at 28 degrees C. These studies gave the following values: deltaH(D)m-w = -1732 cal/mol, deltaH(L)m-b = -592 cal/mol, deltaH(D)m-b = 645 cal/mol, and cmc = 23.5 mM. We discuss the possible physical insight of these values and the perspectives of applications of the proposed method.
我们研究了在形成双层的磷脂和形成胶束的表面活性剂混合物中，囊泡转变为胶束过程中伴随的热量。我们将囊泡和胶束共存范围内的总热释放量ΔQ(coex)细分为三个部分，这三个部分分别与表面活性剂的dN(D)m-b分子和脂质的dN(L)m-b分子从胶束转变为囊泡，以及表面活性剂的dN(D)m-w分子从胶束萃取到水溶液中有关，因此ΔQ(coex)=ΔH(D)m-w×dN(D)m-w + ΔH(D)m-b×dN(D)m-b + ΔH(L)m-b×dN(L)m-b，其中ΔH(D)m-w、ΔH(L)m-b和ΔH(D)m-b分别是相应的摩尔“转移”焓。我们设计了一种方法，通过根据两种不同的脂质-表面活性剂混合物滴定方案进行等温量热滴定来评估所有这三种摩尔焓。在第一个方案中，混合物用纯脂质囊泡的水溶液进行滴定，在第二个方案中，混合物用纯表面活性剂的水溶液进行滴定。用缓冲溶液滴定混合体系用于验证在这些方案下获得的结果。除了摩尔焓的值外，我们的方法还能得出纯表面活性剂的临界胶束浓度（cmc）值。我们将我们的方法应用于研究28℃下磷酸盐缓冲盐溶液中蛋黄磷脂酰胆碱和非离子表面活性剂辛基葡糖苷混合物的热释放情况。这些研究得出了以下值：ΔH(D)m-w = -1732 cal/mol，ΔH(L)m-b = -592 cal/mol，ΔH(D)m-b = 645 cal/mol，cmc = 23.5 mM。我们讨论了这些值可能的物理意义以及所提出方法的应用前景。
