Equilibria of Phosphatidylcholine − Ca²⁺Ions in Monolayer at the Air/Water Interface

Abstract

The effect of Ca(2+) ion interaction with monolayers of phosphatidylcholine (lecithin, L) was investigated at the air/water interface. We present surface tension measurements of lecithin monolayers obtained using a Langmuir method as a function of Ca(2+) ion concentration. The measurements were carried out at 22 degrees C using a Teflon trough and a Nima 9000 tensiometer. The interactions between lecithin and Ca(2+) ions result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants and area occupied by one molecule of LCa(+) and L(2)Ca complexes. The stability constants, K(1) = 1.92 x 10(3) m(2) mol(-1) and K(2) = 5.35 x 10(5) m(2) mol(-1) were calculated by inserting the experimental data. The value of area occupied by one LCa(+) complex is 65 A(2) molecule(-1), while the area occupied by an L(2)Ca complex is 117 A(2) molecule(-1).

The Equilibrium of Phosphatidylcholine–Amino Acid System in Monolayer at the Air/water Interface

“…The surface area for the phosphatidylcholine molecule is 59 ± 0.6 Å 2 molecule -1 . This value is agreed with that reported in the literature [21,23,24].…”

Effect of cation enrichment on dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface

“…The condensation effect becomes more enhanced with increasing concentration, as observed by the decrease in the surface pressure of the LE-LC phase as the Ca 2+ concentration increases. Results presented here contradict previously published studies including work from our group and others, in which an expansion of the PC monolayer occurred on CaCl 2 solutions [15,40,[53][54][55]. However, large discrepancies exist between reported results.…”

“…Surface pressure-area isotherms show that Na + , K + , and Mg 2+ cause a monolayer expansion that increases with salt concentration, consistent with previous literature [40,49]. The phase behavior of Ca 2+ contradicts previous reports [15,40,55], demonstrating that purification of salt solutions is necessary as impurities cause contradictory results even for stated salt purities >99%. Imaging of the surface morphology reveals that increasing salt concentration leads to disruption of lipid packing, resulting in a monolayer that resembles an expanded state.…”

“…In order for the molecular area to decrease, two possible scenarios can occur: (1) Ca 2+ acts as a bridge between two neighboring lipids, forming a 2:1 DPPC:Ca 2+ complex; (2) Ca 2+ causes the reorientation of the PC headgroup. Binding constant calculations and MD simulations have both found that formation of 2:1 complexes are more probable than 1:1 complexes [46,55]. Yet, recent surface potential measurements suggest that reorientation of the PC headgroup occurs in highly concentrated Ca 2+ solutions (2 M) [34].…”

The Equilibria of Lipid–K+ Ions in Monolayer at the Air/Water Interface

“…We find that a K + ion affects the PC and Ch monolayers, like calcium ions, which perturb the lipid organization significantly (Sovago et al 2007; Petelska et al 2010). The effect of calcium on lipid organization suggests that the Ca 2+ ion interacts with the head group, most likely the phosphate group.…”