Phase transitions and spatially ordered counterion association in ionic-lipid membranes: A statistical model

Abstract

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayeror lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types-each one associated, respectively, with the polar-headgroup and the acyl-chain states-which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Lattice solution model for order-disorder transitions in membranes and Langmuir monolayers

“…Lateral pressure, Despite the utility of the model for many purposes, it may be unsuitable under certain circumstances, such as in the case of charged lipid membranes [19], whose thermodynamic properties depend strongly on charge surface density.…”

“…As a consequence, a true discontinuity in density is not displayed by the model at the order-disorder transition of the chains. This aspect represents an important limitation in the case of competing interactions, such as for ionic lipid layers [19][20][21]. For dissociating lipids, electrostatic repulsion competes with the hydrophobic effect, and a delicate equilibrium is established between chain order, charge and molecular surface density.…”

“…For dissociating lipids, electrostatic repulsion competes with the hydrophobic effect, and a delicate equilibrium is established between chain order, charge and molecular surface density. In this case, precise description of the local lipid density is essential in order to appropriately rationalize thermal, electrical and structural properties of the experimental system [19,22].…”

Charge-regulation phase transition on surface lattices of titratable sites adjacent to electrolyte solutions: An analog of the Ising antiferromagnet in a magnetic field

“…In recent work, Tamashiro et al [14,15] have considered a model in some ways similar to the one we consider here, which also showed a charge-regulation patterning transition. Their model, for phase transitions in ionic-lipid membranes, is more complicated because of the possibility of having gel and fluid states in addition to charged (“associated”) and uncharged (“dissociated”) states for the lipid.…”

Sistemas carregados: modelos de simulação

“…Este trabalho faz parte de um projeto de colaboração entre teóricos e experimentais para estudar membranas lipídicas em soluções iônicas [1,2,3,4,5]. Membranas lipídicas são constituídas por moléculas compostas por uma cabeça polar e uma cadeia carbônica apolar.…”