一些无机离子对DSPE脂双层相变的影响

刘 云 娜
（中国科学院化学研究所 北京 100080）

摘要 采用示差扫描量热法研究了在无机离子存在条件下DSPE脂双层的相转变过程，给出了相变的热力学参数。实验结果表明：无机离子对DSPE膜的相行为和生物功能有明显的影响。这对于从分子水平上研究生物膜的结构与功能关系具有重要意义。
关键词 二硬脂酰磷脂酰乙醇胺(DSPE)脂双层 相转变 无机离子 示差扫描量热法
Key words DSPE bilayer, phase transition, inorganic ion, DSC

Effect of the inorganic ions on the phase transition of DSPE bilayer

Liu Yunna
(Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China)

  Differential Scanning Calorimetry ( DSC ) is a very useful thermodynamic technique which allows us to study the phase behavior of lipid membranes in the various conditions ^[1-3]. The inorganic ions play a important role in maintaining physiological function of biological membrane. The effect of inorganic ions on the gel-to-liquid crystalline transition of DSPE bilayer was investigated by differential scanning calorimetry. The values of both T_m and ΔT_1/2 of DSPE membranes in the presence of 0.4 mol/L NaCl and 0.4 mol/L CaCl₂ are increased to pure system, but the average cooperative melting unit (n) of the transition is decreased. These results indicate that inorganic ions increased the stability and fluidity of DSPE membrane and decreased the degree of cooperative interaction between phospholipid molecules within the bilayer. It showed that these ions affected biological function of DSPE membrane. The effect of inorganic ions on the phase transiton may be caused by the interaction between these ions and lipid.
1 Experimental
1.1 Materials
  The DSPE was purchased from Sigma Chemical Corporation. The purity of the sample is better than 99.0%. NaCl and CaCl₂ are A.R. reagent. Distilled de-ionized water was used in the experiment.
1.2 Sample preparation
  DSPE was dissolved in chloroform (99+% pure, Beijing Huagongchang). The solvent was then removed using N₂ stream and samples were dried under high vacuum for 2h. A sample of known mass and 40 m L of 0.4mol/L inorganic salt solution were sealed in a stainless steel capsule. The reference pan contained 40 m L of water. The suspension is vibrated for 3-4 min at a temperature 5-10℃ above T_m. These samples were stored at - 15℃ until used.
1.3 Instruments and methods
  DSC thermograms were obtained on a Perkin-Elmer DSC-2C instrument (with “Intracooler II”). The balance was a Model AD-2Z Electromagnetic Ultramicrobalance from the P-E Corporation. The temperature and energy calibrations were performed with pure indium and zinc. The heating rate was 2.5 K/min.
2 Results and discussion
2.1 DSC experiments
  Typical DSC curves are presented in Fig. 1 for pure DSPE suspension and DSPE membranes containing 0.4 mol/L inorganic salts. Only a sing peak of endothermic transition was observed on the thermogramp.

Fig.1 The thermograms of DSPE bilayers (a. Pure DSPE bilayer; b. DSPE bilayer containing 0.4 mol/L NaCl; c. DSPE bilayer containing 0.4 mol/L CaCl₂)
  As can be seen in Fig.1, the position of the peaks are different, i.e. they have different temperature of phase transition. This showed that inorganic ions affected phase transition temperature of DSPE membrane.
  Thermodynamic parameters associated with the gel-to liquild crystalline transition of DSPE bilayers in the presence of the haloid are shown in Table 1.

* Each Data represents an average of three or four runs
ΔH , phase transition enthalpy; , the amplitude of the heat capacity maximum; n, average cooperative unit

  As shown in Table 1, the T_m and D T_1/2 are 348.8 K and 0.1 K, respectively for pure DSPE suspension. After the addition of 0.4 mol/L haloid, the values of both T_m and ΔT_1/2 are increased. The effect is stronger in the order of 0.4 mol/L CaCl₂>0.4 mol/L NaCl. It shows that the effect of two-valence cation on phase transition is greater than that of one-valence cation at identical anion. These results demonstrate that inorganic ions increased the stability and fluidity of DSPE membrane. The effect of inorganic ions on phase behavior may be caused by the interaction between these ions and liquid.
2.2 The size of the cooperative unit
  For a cooperative system, the average cooperative melting unit (n) may be estimated as follows^[4].

Where R is the gas constant

  The n in three DSPE bilayers are also listed in Table 1. The values of n are lower in two haloid-containing bilayers than that in pure DSPE bilayer. It shows that inorganic ions decreased the degree of cooperative interaction between phospholipid molecules within the bilayer. This decrease in liqid cooperativity means that the average gel and liquid crystal cluster sizes in the transition region ^[5.6] are smaller in the presence of the inorganic ions. This may be due to the perturbing effect of the halide molecules on the bilayer.
3 Conclusion
  Our experimental results showed that these inorganic ions affect the phase behaviour and biological function of DSPE membrane.
4 References
[1] Engelke M, Jessel R. Wiechmann A, Diethl H A, Biophysical Chemistry, 1997, 67:127 -138.
[2] Mavromoustakos T, Theodoropoulou E, Yang D P, Biochimca et Biophysica Acta, 1997, 1328: 65-73.
[3] Wisniewska A, Nishimoto Y, Hyde J.S et al., Biochim. Biophys. Acta, 1996, 1278: 68-72.
[4] Wang G, Chen C H, Archives of Biochemistry and Biophysics, 1993, 301:330-335.
[5] Tsong T Y, Greenberg M, Kanehisa M I, Biochemistry, 1977, 16:3115-3121.
[6] Freire E, Biltonen R, Biopolymers, 1978, 17:481-496.

Received 5 Oct.1998
The project supported by the National Natural Science Foundation of China No.29573097
Abbreviation: DSPE, 1, 2-distearoylphosphatidylethanolamine; DSC, differential scanning calorimetry; T_m, phase transition temperature; T_1/2, the width of the heat capacity curve at half heigh