Shah D O, Schulman J H
J Lipid Res. 1967 May;8(3):227-33.
Surface potentials of mixed monolayers of dicetyl phosphate and eicosanyl trimethylammonium bromide (1:1) were the same on subsolutions of 0.02 M NaCl or 0.01 M CaCl(2), which indicated that ionic phosphate does not interact with Ca(++) in the presence of a neighboring trimethylammonium group. Surface potential-pH plots of dicetyl phosphate, and of dipalmitoyl, egg, and dioleoyl lecithins showed that as the pH of the subsolution is decreased the phosphate groups in the monolayer are neutralized in the order: dicetyl phosphate > dipalmitoyl lecithin > egg lecithin > dioleoyl lecithin. The binding of cations (Na(+), Ca(++)) to the phosphate group of lecithin also showed the same order. The binding of Ca(++)) to egg phosphatidic acid monolayers, as measured by the increase in surface potential, is considerably greater than that to egg lecithin. These results suggest that there is an internal salt linkage between the phosphate and trimethylammonium groups on the same lecithin molecule. An increase in unsaturation of fatty acyl chains increases the intermolecular spacing, which reduces the ionic repulsion between polar groups, and hence strengthens the internal salt linkage. The results support the concept of a vertical rather than coplanar orientation of the phosphoryl choline group with respect to the interface. A position has been proposed for Ca(++) in the dipole lattice of lecithin from a consideration of the surface potential measurements.
磷酸二鲸蜡酯与二十烷基三甲基溴化铵（1:1）混合单分子层在0.02 M NaCl或0.01 M CaCl₂的亚溶液上的表面电位相同，这表明在相邻的三甲基铵基团存在的情况下，离子型磷酸盐不与Ca²⁺相互作用。磷酸二鲸蜡酯以及二棕榈酰卵磷脂、卵黄卵磷脂和二油酰卵磷脂的表面电位-pH图表明，随着亚溶液pH值的降低，单分子层中的磷酸基团按以下顺序被中和：磷酸二鲸蜡酯＞二棕榈酰卵磷脂＞卵黄卵磷脂＞二油酰卵磷脂。阳离子（Na⁺、Ca²⁺）与卵磷脂磷酸基团的结合也呈现相同顺序。通过表面电位增加来衡量，Ca²⁺与卵黄磷脂酸单分子层的结合远大于与卵黄卵磷脂的结合。这些结果表明，在同一卵磷脂分子上的磷酸基团和三甲基铵基团之间存在内部盐键。脂肪酰链不饱和度的增加会增大分子间间距，从而降低极性基团之间的离子排斥力，进而加强内部盐键。这些结果支持了磷酰胆碱基团相对于界面呈垂直而非共面取向的概念。基于表面电位测量结果，已提出了Ca²⁺在卵磷脂偶极晶格中的位置。
