Thermal Phase Behavior of DMPG:  The Exclusion of Continuous Network and Dense Aggregates

Abstract

1,2-Dimyristoyl-sn-glycero-3-phospho-rac-glycerol has been suggested to form at intermediate temperatures and at high concentrations in low-salt solutions as a continuous sponge phase (Heimburg, T.; Biltonen, R. L. Biochemistry 1994, 33, 9477-9488). In the present study, the changes in signals seen for a range of fluorescent probes during phase transformations of this phospholipid indicate continuous melting and a change in lipid packing, in accordance with previous reports. However, in accordance with Lamy-Freund and Riske (Lamy-Freund, M. T.; Riske, K. A. Chem. Phys. Lipids 2003, 122, 19-32), no enhancement of lipid mixing within the putative sponge phase region was seen, suggesting a lack of a connected lipid surface. Accordingly, a typical sponge phase cannot account for the properties of the intermediate phase. The low scattering intensities of the latter have also been taken as evidence for disaggregation. While dynamic light scattering and data for membranes containing poly(ethylene glycol)-ylated lipids could lend credence to disaggregation, the most likely explanation for the scattering data would appear to be a shape transition without significant changes in neither vesicle aggregation nor bilayer connectivity. An abrupt change in light scattering and signals from some of the fluorescent probes used reveals a new transition at Tt approximately 43 degrees C, with the formation of a more ordered interface.

Rheological characterization of thermal phase behavior of anionic lipid DMPG dispersions

“…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.…”

“…In any case, conformation change of a head group and acyl chain in DMPG monomer due to the chain-melting transition would play a role on variable morphology [6,8,15,16]. Complex endotherm profiles with multi-peaks in the same DMPG system indicate that the chain-melting continuously proceeds, and thus a chain packing in the membrane also changes continuously [2,7]. In the intermediate phase, therefore, coexistence of the fluid membrane with high curvature and rigid domains would be expected [6].…”

“…2. By eyes, the intermediate region is rather transparent compared to the gel and fluid phases where spherical vesicles with μm-scale yields turbidity [2,4,7,8]. Transparency in the intermediate phase indicates change in surface properties associated with the reflective index of the bilayer membrane or mesoscale transformation of the structure such as a rupture of the vesicle and a ripple phase.…”

“…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].…”

“…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]. In spite of that the existence of the intermediate structure was well accepted, structural character was a controversial issue for a decade.…”