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Abstract：
OBJECTIVE: Pseudomonas aeruginosa dirhamnolipid (diRL) has been shown to form aggregates of different size and structure, under various conditions. Due to the presence of a carboxyl group in the molecule, it is expected that pH would strongly affect this aggregation behaviour. In addition, preliminary observations of temperature-induced changes in the states of aggregation of diRL supported the need of further investigation.
METHODS: A systematic experimental study, using differential scanning calorimetry (DSC), small-angle Xray diffraction (SAXD), and Fourier-transform infrared spectroscopy (FTIR), has been carried out to characterize pH and temperature driven changes in the aggregation behavior of diRL biosurfactant. Molecular dynamics (MD) simulations, supported by the experimental results, allowed depicting molecular details on formation of diRL membranes and other aggregated structures under various physicochemical conditions.
RESULTS: DiRL could adopt fairly organized multilayered structures (membranes) at low pH and temperature, which became highly disordered upon increasing either of these parameters. The effect of pH on the gauche/all-trans conformer ratio of the diRL acyl chains was not of significance, whereas temperature-induced effects were observed. For the first time it is described that diRL underwent an endothermic thermotropic transition with Tc = 34 °C as observed by DSC, at pH 4.5 (protonated diRL), but not at pH 7.4 (unprotonated diRL). FTIR confirmed these findings, showing a significant additional disordering of the all-trans acyl chains upon increasing temperature around that same value in the protonated form, an effect not observed for the dissociated form of the biosurfactant. In addition, at pH 7.4, changing temperature did not modify the hydration state of the polar moiety of diRL, whereas at pH 4.5 a significant decrease in the hydration state around 34 °C took place. SAXD data showed that protonated diRL formed multilayered structures at 20 °C, which converted into poorly correlated layers at 50 °C. MD simulations supported these findings, showing that the membrane-like structures formed by protonated diRL at 20 °C became unstable at higher temperatures, tending to form other structures, which could be micelles or other type of layered structures, whereas the negatively charged form of diRL organized in micelle-type aggregates in the whole range of temperature under study.
METHODS: A systematic experimental study, using differential scanning calorimetry (DSC), small-angle Xray diffraction (SAXD), and Fourier-transform infrared spectroscopy (FTIR), has been carried out to characterize pH and temperature driven changes in the aggregation behavior of diRL biosurfactant. Molecular dynamics (MD) simulations, supported by the experimental results, allowed depicting molecular details on formation of diRL membranes and other aggregated structures under various physicochemical conditions.
RESULTS: DiRL could adopt fairly organized multilayered structures (membranes) at low pH and temperature, which became highly disordered upon increasing either of these parameters. The effect of pH on the gauche/all-trans conformer ratio of the diRL acyl chains was not of significance, whereas temperature-induced effects were observed. For the first time it is described that diRL underwent an endothermic thermotropic transition with Tc = 34 °C as observed by DSC, at pH 4.5 (protonated diRL), but not at pH 7.4 (unprotonated diRL). FTIR confirmed these findings, showing a significant additional disordering of the all-trans acyl chains upon increasing temperature around that same value in the protonated form, an effect not observed for the dissociated form of the biosurfactant. In addition, at pH 7.4, changing temperature did not modify the hydration state of the polar moiety of diRL, whereas at pH 4.5 a significant decrease in the hydration state around 34 °C took place. SAXD data showed that protonated diRL formed multilayered structures at 20 °C, which converted into poorly correlated layers at 50 °C. MD simulations supported these findings, showing that the membrane-like structures formed by protonated diRL at 20 °C became unstable at higher temperatures, tending to form other structures, which could be micelles or other type of layered structures, whereas the negatively charged form of diRL organized in micelle-type aggregates in the whole range of temperature under study.
摘要：
目的：铜绿假单胞菌二鼠李糖脂（diRL）已显示形成不同大小和结构的聚集体，在各种条件下。由于分子中存在羧基，预计pH会强烈影响这种聚集行为。此外,对温度引起的diRL聚集状态变化的初步观察支持了进一步研究的需要。
方法：系统的实验研究，使用差示扫描量热法（DSC），小角度X射线衍射(SAXD)，和傅里叶变换红外光谱（FTIR），已经进行了表征pH和温度驱动的diRL生物表面活性剂聚集行为的变化。分子动力学（MD）模拟，在实验结果的支持下，允许描述在各种物理化学条件下diRL膜和其他聚集结构形成的分子细节。
结果：DiRL可以在低pH和温度下采用相当有序的多层结构（膜），在增加这些参数中的任何一个时，它们变得高度无序。pH对diRL酰基链的纱布/全反式构象比的影响并不显著，而观察到温度诱导的影响。首次描述了diRL经历了DSC观察到的Tc=34°C的吸热热致转变，在pH4.5（质子化的diRL），但不在pH7.4（未质子化的diRL）。FTIR证实了这些发现，在质子化形式的相同值附近升高温度时，显示出全反式酰基链的显着额外无序，对于生物表面活性剂的解离形式没有观察到的效果。此外,在pH7.4时，改变温度不会改变diRL极性部分的水合状态，而在pH4.5时，水合状态在34°C左右显着降低。SAXD数据表明，质子化的diRL在20°C下形成多层结构，在50°C时转化为相关性较差的层。MD模拟支持这些发现，表明质子化的diRL在20°C下形成的膜状结构在较高温度下变得不稳定，倾向于形成其他结构，可以是胶束或其他类型的层状结构，而在研究的整个温度范围内，带负电荷的diRL形式组织在胶束型聚集体中。
方法：系统的实验研究，使用差示扫描量热法（DSC），小角度X射线衍射(SAXD)，和傅里叶变换红外光谱（FTIR），已经进行了表征pH和温度驱动的diRL生物表面活性剂聚集行为的变化。分子动力学（MD）模拟，在实验结果的支持下，允许描述在各种物理化学条件下diRL膜和其他聚集结构形成的分子细节。
结果：DiRL可以在低pH和温度下采用相当有序的多层结构（膜），在增加这些参数中的任何一个时，它们变得高度无序。pH对diRL酰基链的纱布/全反式构象比的影响并不显著，而观察到温度诱导的影响。首次描述了diRL经历了DSC观察到的Tc=34°C的吸热热致转变，在pH4.5（质子化的diRL），但不在pH7.4（未质子化的diRL）。FTIR证实了这些发现，在质子化形式的相同值附近升高温度时，显示出全反式酰基链的显着额外无序，对于生物表面活性剂的解离形式没有观察到的效果。此外,在pH7.4时，改变温度不会改变diRL极性部分的水合状态，而在pH4.5时，水合状态在34°C左右显着降低。SAXD数据表明，质子化的diRL在20°C下形成多层结构，在50°C时转化为相关性较差的层。MD模拟支持这些发现，表明质子化的diRL在20°C下形成的膜状结构在较高温度下变得不稳定，倾向于形成其他结构，可以是胶束或其他类型的层状结构，而在研究的整个温度范围内，带负电荷的diRL形式组织在胶束型聚集体中。
