Thermodynamics of monolayers formed by mixtures of phosphatidylcholine/phosphatidylserine

Abstract

In this work we obtain the thermodynamic properties of mixed (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) PC and (1-stearoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (sodium salt)) PS monolayers. Measurements of compressibility (isotherms, bulk modulus, and excess area per molecule) and surface potential show that the properties of monolayers at the air-water interface depend on the concentration of ions (Na(+) and K(+)) and the proportion of PS in the mixture. The dependence on PS arises because the molecule is originally bound to a Na(+) counterion; by increasing the concentration of ions the entropy changes, creating a favorable system for the bound counterions of PS to join the bulk, leaving a negatively charged molecule. This change leads to an increase in electrostatic repulsions which is reflected by the increase in area per molecule versus surface pressure and a higher surface potential. The results lead to the conclusion that this mixture of phospholipids follows a non ideal behavior and can help to understand the thermodynamic behavior of membranes made of binary mixtures of a zwitterionic and an anionic phospholipid with a bound counterion.

Figure 1

The figure shows the phosphatidylcholine (top) and phosphatidylserine (bottom) headgroup chemical structure and charges, including the Na⁺ counterion position.

Figure 2

Compression isotherms of PC:PS mixtures. Surface pressure vs. area per molecule plots of PC:PS mixtures on ultrapure water. Standard deviation bars from three independent experiments are smaller than symbols

Figure 3

Compression isotherms of PC with different concentrations of ions. Comparison between isotherms of PC with ultrapure water, 50 mM KCl and NaCl at the subphase. Standard deviation bars from three independent experiments are smaller than symbols

Figure 4

PS monolayer with different concentrations of ions. a) Comparison between isotherms of PS with 50 mM NaCl and KCl, b) Area per molecule at a constant pressure of 15 mN/m (as indicated by the horizontal line in a)) versus concentration of NaCl, KCl and CaCl₂ at the subphase. Standard deviation bars from three independent experiments are smaller than symbols

Figure 5

Bulk modulus (C⁻¹) of pure PC and PS monolayers on ultrapure water, as well as PS monolayer on 50 mM NaCl, as a function of area per molecule.

Figure 6

Surface potential of a PS monolayer with different concentrations of NaCl. Standard deviation bars from three independent experiments are smaller than symbols

Figure 7

PC:PS mixtures. a) Isotherms of PC:PS mixtures and b) Surface potential of PC:PS mixtures (200 Ų/molecule) on 50 mM NaCl. Standard deviation bars from three independent experiments are smaller than symbols

Figure 8

Phosphatidylserine molecule and Na⁺ counterion. Phosphatidylserine molecules with a bound counterion have less area per molecule (left) than those where the counterion has joined the subphase (right). This is due to electrostatic repulsion between negatively charged phosphatidylserine headgroups.

Figure 9

Deviation from ideality. a) Ideal area per molecule and experimental area per molecule (calculated at a surface pressure of 25 mN/m) as a function of concentration b) Experimental and theoretical bulk modulus of PC:PS monolayers, calculated at 100 Ų/molecule.