“Charge inversion” is a phenomenon in which multivalent counterions overcompensate for interfacial charges and invert the sign of the net charge near a surface. This phenomenon is believed to be relevant to biologically important processes such as DNA condensation, and hence it has attracted much attention. We investigated the polar orientation of interfacial water molecules at two different negatively charged interfaces in the absence and presence of La³⁺ using heterodyne-detected vibrational sum frequency generation spectroscopy, which can directly determine the up/down orientation of interfacial molecules. It was found that the orientations of water molecules at a bio-relevant phospholipid interface change from the hydrogen-up to the hydrogen-down with the addition of 10 µM La³⁺. This change of water orientation indicates that the net charge at the phospholipid interface is inverted by adsorption of La³⁺ to the phosphate headgroup. By contrast, at an alkylsulfate interface, the majority of the interfacial water molecules remain hydrogen-up orientated even in the presence of 25 mM La³⁺, indicating that the sulfate headgroup is still solvated by up-oriented water. The observed headgroup specificity suggests that charge inversion at the phospholipid interface originates primarily from the chemical interaction between the phosphate and La³⁺ ion.

中文翻译：

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界面水分子极性取向揭示电荷反演的分子机理：外差检测的振动和频率产生研究

“电荷反转”是多价抗衡离子过度补偿界面电荷并使表面附近的净电荷的符号反转的现象。据信这种现象与生物学上重要的过程如DNA缩合有关，因此引起了人们的广泛关注。我们使用外差法检测的振动和频率产生光谱技术研究了在不存在和存在La ³⁺的情况下界面水分子在两个不同的带负电荷界面上的极性取向，该光谱可直接确定界面分子的上/下取向。结果发现，添加10 µ M La时，与生物相关的磷脂界面上水分子的方向从氢向上改变为氢向下。³⁺。水取向的这种变化表明磷脂界面处的净电荷通过将La ³⁺吸附到磷酸根基上而反转。相反，在烷基硫酸盐界面处，即使在存在25 mM La ³⁺的情况下，大多数界面水分子仍保持氢向上取向，表明硫酸盐头基仍被向上取向的水溶剂化。观察到的头基特异性表明，磷脂界面处的电荷反转主要源自磷酸盐和La ³⁺离子之间的化学相互作用。