Thermodynamic and Textural Characterization of DPPG Phospholipid Monolayers

Abstract

The phase properties of DPPG (dipalmitoyl-phosphatidylglycerol, sodium salt) monolayers are investigated at different temperatures 25 °C ≤ T ≤ 34 °C where the surface pressure−area (Π−A) isotherms have large two-phase coexistence regions. The studies are based on the thermodynamic and textural characterization of the monolayers. The main phase transitions obtained from the Π−A isotherms are in complete agreement with the Brewster angle microscopy results. In equilibrium, compact domains are formed which are never really circular. The domains differ from each other in the azimuthal tilt, but they have no inner texture. Over the entire region of the gaseous to the condensed state, the experimental Π−A isotherms are well described by an equation of state recently derived on the basis of the generalized Volmer's equation and the quasi-chemical equilibrium model of 2D aggregation. The possible dissociation effect on the monolayer properties of the ionic DPPG can be largely ignored, as clarified by extending the theoretical approach under consideration of the dissociation degree of the monolayer substance. The standard thermodynamic characteristics of the 2D aggregation are calculated for different temperatures. The analysis of these results shows that, unlike the micelle formation, within the Langmuir monolayer the aggregation of the amphiphilic molecules to condensed phase results only in an entropy decrease of the system due to the ordering of the amphiphilic molecules.

Progress in characterization of Langmuir monolayers by consideration of compressibility

“…[14], was then used to obtain Frumkin's, van der Waals' and Volmer's [28] equations of state. The equation of state for monolayers in the fluid (G or LE) state is represented by the Volmer-type equation [14,27,29,30]:…”

“…where Π is the surface pressure, k is the Boltzmann constant, T is temperature, ω is the partial molecular area for monomers (or the limiting area of molecule in the gaseous state), A is the area per molecule, Π coh is the cohesion pressure, which accounts for the intermolecular interaction under the condition of a constant value of the enthalpy activity coefficient in the monolayer [14], and the n value accounts for the association (n b 1, and 1/n is the aggregation number of small aggregates [27]) or dissociation (n N 1, and n is the number of kinetically independent units: fragments or ions in a molecule [29,31]) degrees of amphiphilic molecules in the monolayer. For example, for some insoluble proteins in a liquid-expanded monolayer state, a value of n = 20-100 was obtained in Ref.…”

“…Also, the condition which describes the equilibrium between the monomers and aggregates was formulated considering the free surface existing in the monolayer. The following equation of state valid for the bimodal distribution (large clusters and monomers) was derived [14,27,29,30]:…”

“…(5)-(7)) obtained by a combination of the generalised Volmer's equation (Eq. (4)) and the quasi-chemical monomers/aggregates equilibrium model [14,27,29,30,33].…”

Physicochemical techniques in description of interactions in model and native plant membranes under stressful conditions and in physiological processes

“…Wu et al (2005) demonstrated that calcium ions modify the interaction between PSI and PG, mainly by neutralizing the phosphate group of PG, which resulted in a reorganization of the molecules in the PSI complex. Testing temperature dependence of the structural properties of DPPG monolayers showed the probability of formation of the lipid aggregates at lower temperatures, close to phase transition temperature (Vollhardt et al 2000). This observation may be important for understanding the mechanisms of membrane changes in physiological processes occurring at low temperatures (stress, vernalization).…”