Mesoscopic Structure in the Chain-Melting Regime of Anionic Phospholipid Vesicles: DMPG

Abstract

In a range of low ionic strength, aqueous dispersions of the anionic phospholipid DMPG (dimyristoylphosphatidylglycerol) have a transparent intermediate phase (IP, between T(m)(on) congruent with 20 degrees C and T(m)(off) congruent with 30 degrees C) between the turbid gel and fluid membrane phases, evidenced in turbidity data. Small angle x-ray scattering results on DMPG dispersions show that, besides the bilayer peak present in all phases, a peak corresponding to a mesoscopic structure at approximately 400 A is detected only in IP. The dependence of this peak position on DMPG concentration suggests a correlation in the bilayer plane, consistent with the stability of vesicles in IP. Moreover, observation of giant DMPG vesicles with phase contrast light microscopy show that vesicles "disappear" upon cooling below T(m)(off) and "reappear" after reheating. This further proves that although vesicles cannot be visualized in IP, their overall structure is maintained. We propose that the IP in the melting regime corresponds to unilamellar vesicles with perforations, a model which is consistent with all described experimental observations. Furthermore, the opening of pores across the membrane tuned by ionic strength, temperature, and lipid composition is likely to have biological relevance and could be used in applications for controlled release from nanocompartments.

Rheological characterization of thermal phase behavior of anionic lipid DMPG dispersions

“…SAXS patterns in the gel and fluid phases show an isotropic ring suggesting that DMPG forms spherical vesicles as expected [3,7,17]. Broad peak is a characteristic of single bilayer membranes [3,18]. In the intermediate phase at T = 22 and 24°C, on the other hand, anisotropic rheo-SAXS patterns are obviously observed.…”

“…However, electron spin resonance (ESR) experiments demonstrated that the lipid monomers were not exchanged between vesicles, which did not support the existence of the sponge phase [11]. Other proposed model raised by SAXS data analysis suggests that new lamellar phase with pores or porous spherical vesicle is the most plausible structure [3,7,10,12,13]. Appearance of such intermediate structure during the chain-melting transition motivated us to characterize the phase behavior of DMPG systems by the use of a rheo-physical method, a *Current address: Division of Applied Physics, Graduate School of Engineering, Hokkaido University, N13W8, Sapporo 060-8628, Japan E-mail: sfujii@eng.hokudai.ac.jp combination of rheology and small angle X-ray scattering.…”

“…Rheo-SAXS measurements under shear flow of γ = 10 s -1 were performed only for the systems with DMPG concentration of 50 mM. SAXS patterns in the gel and fluid phases show an isotropic ring suggesting that DMPG forms spherical vesicles as expected [3,7,17]. Broad peak is a characteristic of single bilayer membranes [3,18].…”

“…DMPG dispersions exhibit an anomalous melting (gel / fluid transition) behavior over a wide ranges of the temperatures, instead of a definite melting transition [2][3][4]. In the gel and fluid phases, DMPG forms spherical vesicles with low viscosity.…”

“…However, in the chain-melting "regime", the system shows specific viscous property, which clearly suggests the existence of an intermediate structure between gel and fluid phases [2,4,5]. The phase behavior of DMPG dispersions in the quiescent state have been intensively studied by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) measurements [2][3][4][5][6][7][8][9][10]. Absence of the definite chain-melting transition is attributed to continuous melting of DMPG monomers [7].…”