Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan.
J Chem Phys. 2018 Jul 14;149(2):024703. doi: 10.1063/1.5024310.
"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 La 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 La. This change of water orientation indicates that the net charge at the phospholipid interface is inverted by adsorption of La 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 La, 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 La ion.
“电荷反转”是一种多价抗衡离子过度补偿界面电荷并反转表面附近净电荷符号的现象。这种现象被认为与生物重要过程有关，如 DNA 凝聚，因此引起了广泛关注。我们使用异频探测振动和频产生光谱法研究了在不存在和存在 La 的情况下两个不同带负电荷的界面处界面水分子的极性取向，该方法可以直接确定界面分子的上下取向。结果发现，随着 10 µM La 的加入，生物相关磷脂界面上水分子的取向从氢向上变为氢向下。这种水分子取向的变化表明，La 吸附到磷酸盐头部基团使磷脂界面的净电荷反转。相比之下，在烷基硫酸盐界面上，即使存在 25 mM La，大多数界面水分子仍保持氢向上取向，表明硫酸盐头部基团仍被向上取向的水分子溶剂化。观察到的头部基团特异性表明，磷脂界面的电荷反转主要源于磷酸根和 La 离子之间的化学相互作用。
