Pasquale L, Winiski A, Oliva C, Vaio G, McLaughlin S
J Gen Physiol. 1986 Dec;88(6):697-718. doi: 10.1085/jgp.88.6.697.
For a large smooth particle with charges at the surface, the electrophoretic mobility is proportional to the zeta potential, which is related to the charge density by the Gouy-Chapman theory of the diffuse double layer. This classical model adequately describes the dependence of the electrophoretic mobility of phospholipid vesicles on charge density and salt concentration, but it is not applicable to most biological cells, for which new theoretical models have been developed. We tested these new models experimentally by measuring the effect of UO2++ on the electrophoretic mobility of model membranes and human erythrocytes in 0.15 M NaCl at pH 5. We used UO2++ for these studies because it should adsorb specifically to the bilayer surface of the erythrocyte and should not change the density of fixed charges in the glycocalyx. Our experiments demonstrate that it forms high-affinity complexes with the phosphate groups of several phospholipids in a bilayer but does not bind significantly to sialic acid residues. As observed previously, UO2++ adsorbs strongly to egg phosphatidylcholine (PC) vesicles: 0.1 mM UO2++ changes the zeta potential of PC vesicles from 0 to +40 mV. It also has a large effect on the electrophoretic mobility of vesicles formed from mixtures of PC and the negative phospholipid phosphatidylserine (PS): 0.1 mM UO2++ changes the zeta potential of PC/PS vesicles (10 mol % PS) from -13 to +37 mV. In contrast, UO2++ has only a small effect on the electrophoretic mobility of either vesicles formed from mixtures of PC and the negative ganglioside GM1 or erythrocytes: 0.1 mM UO2++ changes the apparent zeta potential of PC/GM1 vesicles (17 mol % GM1) from -11 to +5 mV and the apparent zeta potential of erythrocytes from -12 to -4 mV. The new theoretical models suggest why UO2++ has a small effect on PC/GM1 vesicles and erythrocytes. First, large groups (e.g., sugar moieties) protruding from the surface of the PC/GM1 vesicles and erythrocytes exert hydrodynamic drag. Second, charges at the surface of a particle (e.g., adsorbed UO2++) exert a smaller effect on the mobility than charges located some distance from the surface (e.g., sialic acid residues).
对于表面带有电荷的大的光滑粒子，其电泳迁移率与zeta电位成正比，根据扩散双电层的 Gouy-Chapman理论，zeta电位与电荷密度相关。这个经典模型充分描述了磷脂囊泡电泳迁移率对电荷密度和盐浓度的依赖性，但它不适用于大多数生物细胞，针对这些细胞已经开发了新的理论模型。我们通过测量UO2++对pH为5的0.15 M NaCl中模型膜和人红细胞电泳迁移率的影响，对这些新模型进行了实验测试。我们在这些研究中使用UO2++，是因为它应该特异性吸附到红细胞的双层表面，并且不会改变糖萼中固定电荷的密度。我们的实验表明，它与双层中几种磷脂的磷酸基团形成高亲和力复合物，但与唾液酸残基没有明显结合。如先前观察到的，UO2++强烈吸附到卵磷脂（PC）囊泡上：0.1 mM UO2++使PC囊泡的zeta电位从0变为+40 mV。它对由PC和负电荷磷脂磷脂酰丝氨酸（PS）混合物形成的囊泡的电泳迁移率也有很大影响：0.1 mM UO2++使PC/PS囊泡（10 mol% PS）的zeta电位从-13变为+37 mV。相比之下，UO2++对由PC和负电荷神经节苷脂GM1混合物形成的囊泡或红细胞的电泳迁移率只有很小的影响：0.1 mM UO2++使PC/GM1囊泡（17 mol% GM1）的表观zeta电位从-11变为+5 mV，使红细胞的表观zeta电位从-12变为-4 mV。新的理论模型解释了为什么UO2++对PC/GM1囊泡和红细胞影响较小。首先，从PC/GM1囊泡和红细胞表面突出的大基团（如糖部分）产生流体动力学阻力。其次，粒子表面的电荷（如吸附的UO2++）对迁移率的影响小于距离表面一定距离处的电荷（如唾液酸残基）。
