钙/钡混合溶液中钙离子对带电磷脂酸表面的优先亲合力：X射线反射率和荧光研究
Preferential affinity of calcium ions to charged phosphatidic-acid surface from a mixed calcium/barium solution: X-ray reflectivity and fluorescence studies
摘要：
通过X射线反射率和近全反射荧光实验，研究了二价离子（$\mathrm{Ca^{2+}}$和$\mathrm{Ba^{2+}}$）从水溶液到带电磷脂酸（PA）表面的亲和力。磷脂（1，2-二肉豆蔻酰-sn-甘油-3-磷酸，DMPA），在空气/水界面处作为单层铺展，用于形成和控制界面处的电荷密度。我们发现，对于纯盐的溶液（即，$\mathrm{CaCl_{2}}$和$\mathrm{BaCl_{2}}$），界面处每个DMPA的结合离子数在超过$\mathrm{10^{-3}M}$的浓度下饱和。对于1：1的$\mathrm{Ca^{2+}/Ba^{2+}}$混合溶液，我们发现界面处的结合$\mathrm{Ca^{2+}/Ba^{2+}}$比为4：1。如果决定PA附近电荷积累的唯一性质是离子电荷，则界面处混合$\mathrm{Ca^{2+}/Ba^{2+}}$的浓度将等于本体的浓度。我们的研究结果表明，PA对Ca的特异性亲和力明显高于Ba。我们提供了一些关于这个问题的讨论，以及对生物系统的一些影响。虽然我们的结果表明相对于表面电荷，即，电荷反转，过量的抗衡电荷，反射率和荧光的分析没有揭示过量的共离子（即，$\mathrm{Cl^{-}}$或$\mathrm{I}^{-}$）。
英文摘要：
X-ray reflectivity and fluorescence near total reflection experiments were performed to examine the affinities of divalent ions ($\mathrm{Ca^{2+}}$ and $\mathrm{Ba^{2+}}$) from aqueous solution to a charged phosphatidic-acid (PA) surface. A phospholipid (1,2-Dimyristoyl-sn-Glycero-3-Phosphate, DMPA), spread as a monolayer at the air/water interface, was used to form and control the charge density at the interface. We find that for solutions of the pure salts (i.e., $\mathrm{CaCl_{2}}$ and $\mathrm{BaCl_{2}}$), the number of bound ions per DMPA at the interface is saturated at concentrations that exceed $\mathrm{10^{-3}M}$. For a 1:1 $\mathrm{Ca^{2+}/Ba^{2+}}$ mixed solutions, we find that the bound $\mathrm{Ca^{2+}/Ba^{2+}}$ ratio at the interface is 4:1. If the only property determining charge accumulation near PA were the ionic charges, the concentration of mixed $\mathrm{Ca^{2+}/Ba^{2+}}$ at the interface would equal that of the bulk. Our results show a clear specific affinity of PA for Ca compared to Ba. We provide some discussion on this issues as well as some implications for biological systems. Although our results indicate an excess of counterion charge with respect to the surface charge, that is, charge inversion, the analysis of both reflectivity and fluorescence do not reveal excess of co-ions (namely, $\mathrm{Cl^{-}}$ or $\mathrm{I}^{-}$).
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