Laboratório de Biocalorimetria, Instituto de Bioquímica Médica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil.
Langmuir. 2011 Jul 5;27(13):8248-56. doi: 10.1021/la1037525. Epub 2011 Jun 9.
Surface tension and isothermal titration calorimetry (ITC) were used to determine the critical micelle concentration (cmc) of the zwitterionic amidosulfobetaine surfactants ASB-14 and ASB-16 (linear-alkylamidopropyldimethylammoniopropanosulfonates) at 25 °C. The cmc and the heat of micellization were determined from 15 to 75 °C by ITC for both surfactants. The increase in temperature caused significant changes in the enthalpy and in the entropy of micellization, with small changes in the standard Gibbs energy (ΔG(mic)), which is consistent to an enthalpy−entropy compensation with a compensatory temperature of 311 K (ASB-14) and 314 K (ASB-16). In the studied temperature range, the heat capacity of micellization (ΔC(p)(mic)) was essentially constant. The experimental ΔC(p)(mic) was lower than that expected if only hydrophobic interactions were considered, suggesting that polar interactions at the head groups are of significant importance in the thermodynamics of micelle formation by these surfactants. Indeed, a NMR NOESY spectrum showed NOEs that are improbable to occur within the same monomer, resulting from interactions at the polar head groups involving more than one monomer. The ITC and NMR results indicate a tilt in the polar headgroup favoring the polar interactions. We have also observed COSY correlations typical of dipolar interactions that could be recovered with the partial alignment of the molecule in solution, which results in an anisotropic tumbling. The anisotropy suggested an ellipsoidal shape of the micelles, which results in a positive magnetic susceptibility, and ultimately in orientation induced by the magnetic field. Such an ellipsoidal shape was confirmed from results obtained by SAXS experiments that revealed aggregation numbers of 108 and 168 for ASB-14 and ASB-16 micelles, respectively. This study characterizes an interesting micelle system that can be used in the study of membrane proteins by solution NMR spectroscopy.
表面张力和等温热滴定法（ITC）用于在 25°C 下确定两性离子酰胺基磺基甜菜碱表面活性剂 ASB-14 和 ASB-16（直链烷基酰胺丙基二甲基氨丙磺酸盐）的临界胶束浓度（cmc）。通过 ITC 从 15°C 到 75°C 确定了两种表面活性剂的 cmc 和胶束化热。温度升高导致胶束化焓和熵发生显著变化，标准吉布斯自由能（ΔG(mic)）变化较小，这与补偿温度为 311 K（ASB-14）和 314 K（ASB-16）的焓熵补偿一致。在所研究的温度范围内，胶束化热容（ΔC(p)(mic)）基本保持不变。实验得到的 ΔC(p)(mic)低于仅考虑疏水相互作用时的预期值，表明在这些表面活性剂形成胶束的热力学中，头基的极性相互作用非常重要。事实上，NMR NOESY 谱显示出在同一单体中不太可能发生的 NOE，这是由于在极性头基中涉及多个单体的相互作用引起的。ITC 和 NMR 结果表明，极性头基发生倾斜，有利于极性相互作用。我们还观察到 COSY 相关，这是由于分子在溶液中的部分取向导致的偶极相互作用的典型相关性，这导致各向异性的旋转。各向异性表明胶束呈椭球形，导致磁化率为正，最终导致磁场诱导的取向。这种椭球形形状通过 SAXS 实验结果得到证实，结果表明 ASB-14 和 ASB-16 胶束的聚集数分别为 108 和 168。这项研究表征了一个有趣的胶束体系，可用于通过溶液 NMR 光谱研究膜蛋白。
